Thinking in Java, 3rd ed. Revision 4.0

Operator precedence defines how an expression evaluates when several operators are present. Java has specific rules that determine the order of evaluation. The easiest one to remember is that multiplication and division happen before addition and subtraction. Programmers often forget the other precedence rules, so you should use parentheses to make the order of evaluation explicit. For example:

a = x + y - 2/2 + z;

has a very different meaning from the same statement with a particular grouping of parentheses:

a = x + (y - 2)/(2 + z);

Assignment is performed with the operator =. It means "take the value of the right-hand side (often called the rvalue) and copy it into the left-hand side (often called the lvalue)." An rvalue is any constant, variable, or expression that can produce a value, but an lvalue must be a distinct, named variable. (That is, there must be a physical space to store the value.) For instance, you can assign a constant value to a variable:

a = 4;

but you cannot assign anything to constant value-it cannot be an lvalue. (You can't say 4 = a;.)

Assignment of primitives is quite straightforward. Since the primitive holds the actual value and not a reference to an object, when you assign primitives, you copy the contents from one place to another. For example, if you say a = b for primitives, then the contents of b are copied into a. If you then go on to modify a, b is naturally unaffected by this modification. As a programmer, this is what you've come to expect for most situations.

When you assign objects, however, things change. Whenever you manipulate an object, what you're manipulating is the reference, so when you assign "from one object to another," you're actually copying a reference from one place to another. This means that if you say c = d for objects, you end up with both c and d pointing to the object that, originally, only d pointed to. Here's an example that demonstrates this behavior:

//: c03:Assignment.java
// Assignment with objects is a bit tricky.
import com.bruceeckel.simpletest.*;
 
class Number {
	int i;
}
 
public class Assignment {
	static Test monitor = new Test();
	public static void main(String[] args) {
		Number n1 = new Number();
		Number n2 = new Number();
		n1.i = 9;
		n2.i = 47;
		System.out.println("1: n1.i: " + n1.i +
			", n2.i: " + n2.i);
		n1 = n2;
		System.out.println("2: n1.i: " + n1.i +
			", n2.i: " + n2.i);
		n1.i = 27;
		System.out.println("3: n1.i: " + n1.i +
			", n2.i: " + n2.i);
		monitor.expect(new String[] {
			"1: n1.i: 9, n2.i: 47",
			"2: n1.i: 47, n2.i: 47",
			"3: n1.i: 27, n2.i: 27"
		});
	}
} ///:~

First, notice that something new has been added. The line:

import com.bruceeckel.simpletest.*;

imports the "simpletest" library that has been created to test the code in this book, and is explained in Chapter 15. At the beginning of the Assignment class, you see the line:

static Test monitor = new Test();

This creates an instance of the simpletest class Test, called monitor. Finally, at the end of main( ), you see the statement:

monitor.expect(new String[] {
	"1: n1.i: 9, n2.i: 47",
	"2: n1.i: 47, n2.i: 47",
	"3: n1.i: 27, n2.i: 27"
});

This is the expected output of the program, expressed as an array of String objects. When the program is run, it not only prints out the output, but it compares it to this array to verify that the array is correct. Thus, when you see a program in this book that uses simpletest, you will also see an expect( ) call that will show you what the output of the program is. This way, you see validated output from the program.

The Number class is simple, and two instances of it (n1 and n2) are created within main( ). The i value within each Number is given a different value, and then n2 is assigned to n1, and n1 is changed. In many programming languages you would expect n1 and n2 to be independent at all times, but because you've assigned a reference, you'll see the output in the expect( ) statement. Changing the n1 object appears to change the n2 object as well! This is because both n1 and n2 contain the same reference, which is pointing to the same object. (The original reference that was in n1, that pointed to the object holding a value of 9, was overwritten during the assignment and effectively lost; its object will be cleaned up by the garbage collector.)

This phenomenon is often called aliasing, and it's a fundamental way that Java works with objects. But what if you don't want aliasing to occur in this case? You could forego the assignment and say:

n1.i = n2.i;

This retains the two separate objects instead of tossing one and tying n1 and n2 to the same object, but you'll soon realize that manipulating the fields within objects is messy and goes against good object-oriented design principles. This is a nontrivial topic, so it is left for Appendix A, which is devoted to aliasing. In the meantime, you should keep in mind that assignment for objects can add surprises.

3-1-2-1. Aliasing during method calls▲

//: c03:PassObject.java
// Passing objects to methods may not be what
// you're used to.
import com.bruceeckel.simpletest.*;
 
class Letter {
	char c;
}
 
public class PassObject {
	static Test monitor = new Test();
	static void f(Letter y) {
		y.c = 'z';
	}
	public static void main(String[] args) {
		Letter x = new Letter();
		x.c = 'a';
		System.out.println("1: x.c: " + x.c);
		f(x);
		System.out.println("2: x.c: " + x.c);
		monitor.expect(new String[] {
			"1: x.c: a",
			"2: x.c: z"
		});
	}
} ///:~

y.c = 'z';

The basic mathematical operators are the same as the ones available in most programming languages: addition (+), subtraction (-), division (/), multiplication (*) and modulus (%, which produces the remainder from integer division). Integer division truncates, rather than rounds, the result.

Java also uses a shorthand notation to perform an operation and an assignment at the same time. This is denoted by an operator followed by an equal sign, and is consistent with all the operators in the language (whenever it makes sense). For example, to add 4 to the variable x and assign the resultto x, use: x += 4.

This example shows the use of the mathematical operators:

//: c03:MathOps.java
// Demonstrates the mathematical operators.
import com.bruceeckel.simpletest.*;
import java.util.*;
 
public class MathOps {
	static Test monitor = new Test();
	// Shorthand to print a string and an int:
	static void printInt(String s, int i) {
		System.out.println(s + " = " + i);
	}
	// Shorthand to print a string and a float:
	static void printFloat(String s, float f) {
		System.out.println(s + " = " + f);
	}
	public static void main(String[] args) {
	    // Create a random number generator,
	    // seeds with current time by default:
	    Random rand = new Random();
	    int i, j, k;
	    // Choose value from 1 to 100:
	    j = rand.nextInt(100) + 1;
	    k = rand.nextInt(100) + 1;
	    printInt("j", j);  printInt("k", k);
	    i = j + k; printInt("j + k", i);
	    i = j - k; printInt("j - k", i);
	    i = k / j; printInt("k / j", i);
	    i = k * j; printInt("k * j", i);
	    i = k % j; printInt("k % j", i);
	    j %= k; printInt("j %= k", j);
	    // Floating-point number tests:
	    float u,v,w;  // applies to doubles, too
	    v = rand.nextFloat();
	    w = rand.nextFloat();
	    printFloat("v", v); printFloat("w", w);
	    u = v + w; printFloat("v + w", u);
	    u = v - w; printFloat("v - w", u);
	    u = v * w; printFloat("v * w", u);
	    u = v / w; printFloat("v / w", u);
	    // the following also works for
	    // char, byte, short, int, long,
	    // and double:
	    u += v; printFloat("u += v", u);
	    u -= v; printFloat("u -= v", u);
	    u *= v; printFloat("u *= v", u);
	    u /= v; printFloat("u /= v", u);
	    monitor.expect(new String[] {
			"%% j = -?\\d+",
			"%% k = -?\\d+",
			"%% j \\+ k = -?\\d+",
			"%% j - k = -?\\d+",
			"%% k / j = -?\\d+",
			"%% k \\* j = -?\\d+",
			"%% k % j = -?\\d+",
			"%% j %= k = -?\\d+",
			"%% v = -?\\d+\\.\\d+(E-?\\d)?",
			"%% w = -?\\d+\\.\\d+(E-?\\d)?",
			"%% v \\+ w = -?\\d+\\.\\d+(E-?\\d)??",
			"%% v - w = -?\\d+\\.\\d+(E-?\\d)??",
			"%% v \\* w = -?\\d+\\.\\d+(E-?\\d)??",
			"%% v / w = -?\\d+\\.\\d+(E-?\\d)??",
			"%% u \\+= v = -?\\d+\\.\\d+(E-?\\d)??",
			"%% u -= v = -?\\d+\\.\\d+(E-?\\d)??",
			"%% u \\*= v = -?\\d+\\.\\d+(E-?\\d)??",
			"%% u /= v = -?\\d+\\.\\d+(E-?\\d)??"
		});
	}
} ///:~

The first thing you will see are some shorthand methods for printing: the printInt( ) prints a String followed by an int and the printFloat( ) prints a String followed by a float.

To generate numbers, the program first creates a Random object. Because no arguments are passed during creation, Java uses the current time as a seed for the random number generator. The program generates a number of different types of random numbers with the Random object simply by calling the methods: nextInt( ) and nextFloat( ) (you can also call nextLong( ) or nextDouble( )).

The modulus operator, when used with the result of the random number generator, limits the result to an upper bound of the operand minus 1 (99 in this case).

3-1-3-1. Regular expressions▲

-?

-?\\d+

%% j = -?\\d+

3-1-3-2. Unary minus and plus operators▲

x = -a;

x = a * -b;

x = a * (-b);

Java, like C, is full of shortcuts. Shortcuts can make code much easier to type, and either easier or harder to read.

Two of the nicer shortcuts are the increment and decrement operators (often referred to as the auto-increment and auto-decrement operators). The decrement operator is -- and means "decrease by one unit." The increment operator is ++ and means "increase by one unit." If a is an int, for example, the expression ++a is equivalent to (a = a + 1). Increment and decrement operators not only modify the variable, but also produce the value of the variable as a result.

There are two versions of each type of operator, often called the prefix and postfix versions. Pre-increment means the ++ operator appears before the variable or expression, and post-increment means the ++ operator appears after the variable or expression. Similarly, pre-decrement means the -- operator appears before the variable or expression, and post-decrement means the -- operator appears after the variable or expression. For pre-increment and pre-decrement, (i.e., ++a or --a), the operation is performed and the value is produced. For post-increment and post-decrement (i.e. a++ or a--), the value is produced, then the operation is performed. As an example:

//: c03:AutoInc.java
// Demonstrates the ++ and -- operators.
import com.bruceeckel.simpletest.*;
 
public class AutoInc {
	static Test monitor = new Test();
	public static void main(String[] args) {
		int i = 1;
		System.out.println("i : " + i);
		System.out.println("++i : " + ++i); // Pre-increment
		System.out.println("i++ : " + i++); // Post-increment
		System.out.println("i : " + i);
		System.out.println("--i : " + --i); // Pre-decrement
		System.out.println("i-- : " + i--); // Post-decrement
		System.out.println("i : " + i);
		monitor.expect(new String[] {
			"i : 1",
			"++i : 2",
			"i++ : 2",
			"i : 3",
			"--i : 2",
			"i-- : 2",
			"i : 1"
		});
	}
} ///:~

You can see that for the prefix form, you get the value after the operation has been performed, but with the postfix form, you get the value before the operation is performed. These are the only operators (other than those involving assignment) that have side effects. (That is, they change the operand rather than using just its value.)

The increment operator is one explanation for the name C++, implying "one step beyond C." In an early Java speech, Bill Joy (one of the Java creators), said that "Java=C++--" (C plus plus minus minus), suggesting that Java is C++ with the unnecessary hard parts removed, and therefore a much simpler language. As you progress in this book, you'll see that many parts are simpler, and yet Java isn't that much easier than C++.

Relational operators generate a boolean result. They evaluate the relationship between the values of the operands. A relational expression produces true if the relationship is true, and false if the relationship is untrue. The relational operators are less than (<), greater than (>), less than or equal to (<=), greater than or equal to (>=), equivalent (==) and not equivalent (!=). Equivalence and nonequivalence work with all built-in data types, but the other comparisons won't work with type boolean.

3-1-5-1. Testing object equivalence▲

//: c03:Equivalence.java
import com.bruceeckel.simpletest.*;
 
public class Equivalence {
	static Test monitor = new Test();
	public static void main(String[] args) {
		Integer n1 = new Integer(47);
		Integer n2 = new Integer(47);
		System.out.println(n1 == n2);
		System.out.println(n1 != n2);
		monitor.expect(new String[] {
			"false",
			"true"
		});
	}
} ///:~

/
/: c03:EqualsMethod.java
import com.bruceeckel.simpletest.*;
 
public class EqualsMethod {
	static Test monitor = new Test();
	public static void main(String[] args) {
		Integer n1 = new Integer(47);
		Integer n2 = new Integer(47);
		System.out.println(n1.equals(n2));
		monitor.expect(new String[] {
			"true"
		});
	}
} ///:~

//: c03:EqualsMethod2.java
import com.bruceeckel.simpletest.*;
 
class Value {
	int i;
}
 
public class EqualsMethod2 {
	static Test monitor = new Test();
	public static void main(String[] args) {
		Value v1 = new Value();
		Value v2 = new Value();
		v1.i = v2.i = 100;
		System.out.println(v1.equals(v2));
		monitor.expect(new String[] {
			"false"
		});
	}
} ///:~

Each of the logical operators AND (&&), OR (||) and NOT (!) produces a boolean value of true or false based on the logical relationship of its arguments. This example uses the relational and logical operators:

//: c03:Bool.java
// Relational and logical operators.
import com.bruceeckel.simpletest.*;
import java.util.*;
 
public class Bool {
	static Test monitor = new Test();
	public static void main(String[] args) {
		Random rand = new Random();
		int i = rand.nextInt(100);
		int j = rand.nextInt(100);
		System.out.println("i = " + i);
		System.out.println("j = " + j);
		System.out.println("i > j is " + (i > j));
		System.out.println("i < j is " + (i < j));
		System.out.println("i >= j is " + (i >= j));
		System.out.println("i <= j is " + (i <= j));
		System.out.println("i == j is " + (i == j));
		System.out.println("i != j is " + (i != j));
		// Treating an int as a boolean is not legal Java:
//! System.out.println("i && j is " + (i && j));
//! System.out.println("i || j is " + (i || j));
//! System.out.println("!i is " + !i);
		System.out.println("(i < 10) && (j < 10) is "
			+ ((i < 10) && (j < 10)) );
		System.out.println("(i < 10) || (j < 10) is "
			+ ((i < 10) || (j < 10)) );
		monitor.expect(new String[] {
			"%% i = -?\\d+",
			"%% j = -?\\d+",
			"%% i > j is (true|false)",
			"%% i < j is (true|false)",
			"%% i >= j is (true|false)",
			"%% i <= j is (true|false)",
			"%% i == j is (true|false)",
			"%% i != j is (true|false)",
			"%% \\(i < 10\\) && \\(j < 10\\) is (true|false)",
			"%% \\(i < 10\\) \\|\\| \\(j < 10\\) is (true|false)"
		});
	}
} ///:~

In the regular expressions in the expect( ) statement, parentheses have the effect of grouping an expression, and the vertical bar '|' means OR. So:

(true|false)

Means that this part of the string may be either 'true' or 'false'. Because these characters are special in regular expressions, they must be escaped with a '\\' if you want them to appear as ordinary characters in the expression.

You can apply AND, OR, or NOT to boolean values only. You can't use a non-boolean as if it were a boolean in a logical expression as you can in C and C++. You can see the failed attempts at doing this commented out with a //! comment marker. The subsequent expressions, however, produce boolean values using relational comparisons, then use logical operations on the results.

Note that a boolean value is automatically converted to an appropriate text form if it's used where a String is expected.

You can replace the definition for int in the preceding program with any other primitive data type except boolean. Be aware, however, that the comparison of floating-point numbers is very strict. A number that is the tiniest fraction different from another number is still "not equal." A number that is the tiniest bit above zero is still nonzero.

3-1-6-1. Short-circuiting▲

//: c03:ShortCircuit.java
// Demonstrates short-circuiting behavior.
// with logical operators.
import com.bruceeckel.simpletest.*;
 
public class ShortCircuit {
	static Test monitor = new Test();
	static boolean test1(int val) {
		System.out.println("test1(" + val + ")");
		System.out.println("result: " + (val < 1));
		return val < 1;
	}
	static boolean test2(int val) {
		System.out.println("test2(" + val + ")");
		System.out.println("result: " + (val < 2));
		return val < 2;
	}
	static boolean test3(int val) {
		System.out.println("test3(" + val + ")");
		System.out.println("result: " + (val < 3));
		return val < 3;
	}
	public static void main(String[] args) {
		if(test1(0) && test2(2) && test3(2))
			System.out.println("expression is true");
		else
			System.out.println("expression is false");
		monitor.expect(new String[] {
			"test1(0)",
			"result: true",
			"test2(2)",
			"result: false",
			"expression is false"
		});
	}
} ///:~

if(test1(0) && test2(2) && test3(2))

The bitwise operators allow you to manipulate individual bits in an integral primitive data type. Bitwise operators perform Boolean algebra on the corresponding bits in the two arguments to produce the result.

The bitwise operators come from C's low-level orientation, where you often manipulate hardware directly and must set the bits in hardware registers. Java was originally designed to be embedded in TV set-top boxes, so this low-level orientation still made sense. However, you probably won't use the bitwise operators much.

The bitwise AND operator (&) produces a one in the output bit if both input bits are one, otherwise it produces a zero. The bitwise OR operator (|) produces a one in the output bit if either input bit is a one and produces a zero only if both input bits are zero. The bitwise EXCLUSIVE OR, or XOR (^), produces a one in the output bit if one or the other input bit is a one, but not both. The bitwise NOT (~, also called the ones complement operator) is a unary operator; it takes only one argument. (All other bitwise operators are binary operators.) Bitwise NOT produces the opposite of the input bit-a one if the input bit is zero, a zero if the input bit is one.

The bitwise operators and logical operators use the same characters, so it is helpful to have a mnemonic device to help you remember the meanings: because bits are "small," there is only one character in the bitwise operators.

Bitwise operators can be combined with the = sign to unite the operation and assignment: &=, |= and ^= are all legitimate. (Since ~ is a unary operator, it cannot be combined with the = sign.)

The boolean type is treated as a one-bit value, so it is somewhat different. You can perform a bitwise AND, OR, and XOR, but you can't perform a bitwise NOT (presumably to prevent confusion with the logical NOT). For booleans, the bitwise operators have the same effect as the logical operators except that they do not short circuit. Also, bitwise operations on booleans include an XOR logical operator that is not included under the list of "logical" operators. You're prevented from using booleans in shift expressions, which are described next.

The shift operators also manipulate bits. They can be used solely with primitive, integral types. The left-shift operator (<<) produces the operand to the left of the operator shifted to the left by the number of bits specified after the operator (inserting zeroes at the lower-order bits). The signed right-shift operator (>>) produces the operand to the left of the operator shifted to the right by the number of bits specified after the operator. The signed right shift >> uses sign extension: if the value is positive, zeroes are inserted at the higher-order bits; if the value is negative, ones are inserted at the higher-order bits. Java has also added the unsigned right shift >>>, whichuses zero extension: regardless of the sign, zeroes are inserted at the higher-order bits. This operator does not exist in C or C++.

If you shift a char, byte, or short, it will be promoted to int before the shift takes place, and the result will be an int. Only the five low-order bits of the right-hand side will be used. This prevents you from shifting more than the number of bits in an int. If you're operating on a long, you'll get a long result. Only the six low-order bits of the right-hand side will be used, so you can't shift more than the number of bits in a long.

Shifts can be combined with the equal sign (<<= or >>= or >>>=). The lvalue is replaced by the lvalue shifted by the rvalue. There is a problem, however, with the unsigned right shift combined with assignment. If you use it with byte or short, you don't get the correct results. Instead, these are promoted to int and right shifted, but then truncated as they are assigned back into their variables, so you get -1 in those cases. The following example demonstrates this:

//: c03:URShift.java
// Test of unsigned right shift.
import com.bruceeckel.simpletest.*;
 
public class URShift {
	static Test monitor = new Test();
	public static void main(String[] args) {
		int i = -1;
		System.out.println(i >>>= 10);
		long l = -1;
		System.out.println(l >>>= 10);
		short s = -1;
		System.out.println(s >>>= 10);
		byte b = -1;
		System.out.println(b >>>= 10);
		b = -1;
		System.out.println(b>>>10);
		monitor.expect(new String[] {
			"4194303",
			"18014398509481983",
			"-1",
			"-1",
			"4194303"
		});
	}
} ///:~

In the last shift, the resulting value is not assigned back into b, but is printed directly, so the correct behavior occurs.

Here's an example that demonstrates the use of all the operators involving bits:

//: c03:BitManipulation.java
// Using the bitwise operators.
import com.bruceeckel.simpletest.*;
import java.util.*;
 
public class BitManipulation {
	static Test monitor = new Test();
	public static void main(String[] args) {
		Random rand = new Random();
		int i = rand.nextInt();
		int j = rand.nextInt();
		printBinaryInt("-1", -1);
		printBinaryInt("+1", +1);
		int maxpos = 2147483647;
		printBinaryInt("maxpos", maxpos);
		int maxneg = -2147483648;
		printBinaryInt("maxneg", maxneg);
		printBinaryInt("i", i);
		printBinaryInt("~i", ~i);
		printBinaryInt("-i", -i);
		printBinaryInt("j", j);
		printBinaryInt("i & j", i & j);
		printBinaryInt("i | j", i | j);
		printBinaryInt("i ^ j", i ^ j);
		printBinaryInt("i << 5", i << 5);
		printBinaryInt("i >> 5", i >> 5);
		printBinaryInt("(~i) >> 5", (~i) >> 5);
		printBinaryInt("i >>> 5", i >>> 5);
		printBinaryInt("(~i) >>> 5", (~i) >>> 5);
 
		long l = rand.nextLong();
		long m = rand.nextLong();
		printBinaryLong("-1L", -1L);
		printBinaryLong("+1L", +1L);
		long ll = 9223372036854775807L;
		printBinaryLong("maxpos", ll);
		long lln = -9223372036854775808L;
		printBinaryLong("maxneg", lln);
		printBinaryLong("l", l);
		printBinaryLong("~l", ~l);
		printBinaryLong("-l", -l);
		printBinaryLong("m", m);
		printBinaryLong("l & m", l & m);
		printBinaryLong("l | m", l | m);
		printBinaryLong("l ^ m", l ^ m);
		printBinaryLong("l << 5", l << 5);
		printBinaryLong("l >> 5", l >> 5);
		printBinaryLong("(~l) >> 5", (~l) >> 5);
		printBinaryLong("l >>> 5", l >>> 5);
		printBinaryLong("(~l) >>> 5", (~l) >>> 5);
		monitor.expect("BitManipulation.out");
	}
	static void printBinaryInt(String s, int i) {
		System.out.println(
			s + ", int: " + i + ", binary: ");
		System.out.print("   ");
		for(int j = 31; j >= 0; j--)
			if(((1 << j) &  i) != 0)
				System.out.print("1");
			else
				System.out.print("0");
		System.out.println();
	}
	static void printBinaryLong(String s, long l) {
		System.out.println(
			s + ", long: " + l + ", binary: ");
		System.out.print("   ");
		for(int i = 63; i >= 0; i--)
			if(((1L << i) & l) != 0)
				System.out.print("1");
			else
				System.out.print("0");
		System.out.println();
	}
} ///:~

The two methods at the end, printBinaryInt( ) and printBinaryLong( ), take an int or a long, respectively, and print it out in binary format along with a descriptive string. You can ignore the implementation of these for now.

You'll note the use of System.out.print( ) instead of System.out.println( ). The print( ) method does not emit a newline, so it allows you to output a line in pieces.

In this case, the expect( ) statement takes a file name, from which it reads the expected lines (which may or may not include regular expressions). This is useful in situations where the output is too long or inappropriate to include in the book. The files ending with ".out" are part of the code distribution, available for download from www.BruceEckel.com, so you can open the file and look at it to see what the output should be (or simply run the program yourself).

As well as demonstrating the effect of all the bitwise operators for int and long, this example also shows the minimum, maximum, +1, and -1 values for int and long so you can see what they look like. Note that the high bit represents the sign: 0 means positive and 1 means negative. The output for the int portion looks like this:

-1, int: -1, binary: 
	11111111111111111111111111111111
+1, int: 1, binary: 
	00000000000000000000000000000001
maxpos, int: 2147483647, binary: 
	01111111111111111111111111111111
maxneg, int: -2147483648, binary: 
	10000000000000000000000000000000
i, int: 59081716, binary: 
	00000011100001011000001111110100
~i, int: -59081717, binary: 
	11111100011110100111110000001011
-i, int: -59081716, binary: 
	11111100011110100111110000001100
j, int: 198850956, binary: 
	00001011110110100011100110001100
i & j, int: 58720644, binary: 
	00000011100000000000000110000100
i | j, int: 199212028, binary: 
	00001011110111111011101111111100
i ^ j, int: 140491384, binary: 
	00001000010111111011101001111000
i << 5, int: 1890614912, binary: 
	01110000101100000111111010000000
i >> 5, int: 1846303, binary: 
	00000000000111000010110000011111
(~i) >> 5, int: -1846304, binary: 
	11111111111000111101001111100000
i >>> 5, int: 1846303, binary: 
	00000000000111000010110000011111
(~i) >>> 5, int: 132371424, binary: 
	00000111111000111101001111100000

The binary representation of the numbers is referred to as signed two's complement.

This operator is unusual because it has three operands. It is truly an operator because it produces a value, unlike the ordinary if-else statement that you'll see in the next section of this chapter. The expression is of the form:

boolean-exp ? value0 : value1

If boolean-exp evaluates to true, value0 is evaluated, and its result becomes the value produced by the operator. If boolean-exp is false, value1 is evaluated and its result becomes the value produced by the operator.

Of course, you could use an ordinary if-else statement (described later), but the ternary operator is much terser. Although C (where this operator originated) prides itself on being a terse language, and the ternary operator might have been introduced partly for efficiency, you should be somewhat wary of using it on an everyday basis-it's easy to produce unreadable code.

The conditional operator can be used for its side effects or for the value it produces, but in general you want the value, since that's what makes the operator distinct from the if-else. Here's an example:

static int ternary(int i) {
	return i < 10 ? i * 100 : i * 10;
}

You can see that this code is more compact than what you'd need to write without the ternary operator:

static int alternative(int i) {
	if (i < 10)
		return i * 100;
	else
		return i * 10;
}

The second form is easier to understand, and doesn't require a lot more typing. So be sure to ponder your reasons when choosing the ternary operator-it's generally warranted when you're setting a variable to one of two values.

The comma is used in C and C++ not only as a separator in function argument lists, but also as an operator for sequential evaluation. The sole place that the comma operator is used in Java is in for loops, which will be described later in this chapter.

There's one special usage of an operator in Java: the + operator can be used to concatenate strings, as you've already seen. It seems a natural use of the + even though it doesn't fit with the traditional way that + is used. This capability seemed like a good idea in C++, so operator overloading was added to C++ to allow the C++ programmer to add meanings to almost any operator. Unfortunately, operator overloading combined with some of the other restrictions in C++ turns out to be a fairly complicated feature for programmers to design into their classes. Although operator overloading would have been much simpler to implement in Java than it was in C++, this feature was still considered too complex, so Java programmers cannot implement their own overloaded operators like C++ programmers can.

The use of the String + has some interesting behavior. If an expression begins with a String, then all operands that follow must be Strings (remember that the compiler will turn a quoted sequence of characters into a String):

int x = 0, y = 1, z = 2;
String sString = "x, y, z ";
System.out.println(sString + x + y + z);

Here, the Java compiler will convert x, y, and z into their String representations instead of adding them together first. And if you say:

System.out.println(x + sString);

Java will turn x into a String.

One of the pitfalls when using operators is attempting to leave out the parentheses when you are even the least bit uncertain about how an expression will evaluate. This is still true in Java.

An extremely common error in C and C++ looks like this:

while(x = y) {
	// ....
}

The programmer was clearly trying to test for equivalence (==) rather than do an assignment. In C and C++ the result of this assignment will always be true if y is nonzero, and you'll probably get an infinite loop. In Java, the result of this expression is not a boolean, but the compiler expects a boolean and won't convert from an int, so it will conveniently give you a compile-time error and catch the problem before you ever try to run the program. So the pitfall never happens in Java. (The only time you won't get a compile-time error is when x and y are boolean, in which case x = y is a legal expression, and in the preceding example, probably an error.)

A similar problem in C and C++ is using bitwise AND and OR instead of the logical versions. Bitwise AND and OR use one of the characters (& or |) while logical AND and OR use two (&& and ||). Just as with = and ==, it's easy to type just one character instead of two. In Java, the compiler again prevents this, because it won't let you cavalierly use one type where it doesn't belong.

The word cast is used in the sense of "casting into a mold." Java will automatically change one type of data into another when appropriate. For instance, if you assign an integral value to a floating-point variable, the compiler will automatically convert the int to a float. Casting allows you to make this type conversion explicit, or to force it when it wouldn't normally happen.

To perform a cast, put the desired data type (including all modifiers) inside parentheses to the left of any value. Here's an example:

void casts() {
	int i = 200;
	long l = (long)i;
	long l2 = (long)200;
}

As you can see, it's possible to perform a cast on a numeric value as well as on a variable. In both casts shown here, however, the cast is superfluous, since the compiler will automatically promote an int value to a long when necessary. However, you are allowed to use superfluous casts to make a point or to make your code more clear. In other situations, a cast may be essential just to get the code to compile.

In C and C++, casting can cause some headaches. In Java, casting is safe, with the exception that when you perform a so-called narrowing conversion (that is, when you go from a data type that can hold more information to one that doesn't hold as much), you run the risk of losing information.Here the compiler forces you to do a cast, in effect saying "this can be a dangerous thing to do-if you want me to do it anyway you must make the cast explicit." With a widening conversion an explicit cast is not needed, because the new type will more than hold the information from the old type so that no information is ever lost.

Java allows you to cast any primitive type to any other primitive type, except for boolean, which doesn't allow any casting at all. Class types do not allow casting. To convert one to the other, there must be special methods. (String is a special case, and you'll find out later in this book that objects can be cast within a family of types; an Oak can be cast to a Tree and vice-versa, but not to a foreign type such as a Rock.)

3-1-13-1. Literals▲

//: c03:Literals.java
 
public class Literals {
	char c = 0xffff; // max char hex value
	byte b = 0x7f; // max byte hex value
	short s = 0x7fff; // max short hex value
	int i1 = 0x2f; // Hexadecimal (lowercase)
	int i2 = 0X2F; // Hexadecimal (uppercase)
	int i3 = 0177; // Octal (leading zero)
	// Hex and Oct also work with long.
	long n1 = 200L; // long suffix
	long n2 = 200l; // long suffix (but can be confusing)
	long n3 = 200;
	//! long l6(200); // not allowed
	float f1 = 1;
	float f2 = 1F; // float suffix
	float f3 = 1f; // float suffix
	float f4 = 1e-45f; // 10 to the power
	float f5 = 1e+9f; // float suffix
	double d1 = 1d; // double suffix
	double d2 = 1D; // double suffix
	double d3 = 47e47d; // 10 to the power
} ///:~

long n3 = 200;

float f4 = 1e-47f; // 10 to the power

3-1-13-2. Promotion▲

In C and C++, the sizeof( ) operator satisfies a specific need: it tells you the number of bytes allocated for data items. The most compelling need for sizeof( ) in C and C++ is portability. Different data types might be different sizes on different machines, so the programmer must find out how big those types are when performing operations that are sensitive to size. For example, one computer might store integers in 32 bits, whereas another might store integers as 16 bits. Programs could store larger values in integers on the first machine. As you might imagine, portability is a huge headache for C and C++ programmers.

Java does not need a sizeof( ) operator for this purpose, because all the data types are the same size on all machines. You do not need to think about portability on this level-it is designed into the language.

Upon hearing me complain about the complexity of remembering operator precedence during one of my seminars, a student suggested a mnemonic that is simultaneously a commentary: "Ulcer Addicts Really Like C A lot."

Of course, with the shift and bitwise operators distributed around the table it is not a perfect mnemonic, but for non-bit operations it works.

The following example shows which primitive data types can be used with particular operators. Basically, it is the same example repeated over and over, but using different primitive data types. The file will compile without error because the lines that would cause errors are commented out with a //!.

//: c03:AllOps.java
// Tests all the operators on all the primitive data types
// to show which ones are accepted by the Java compiler.
 
public class AllOps {
	// To accept the results of a boolean test:
	void f(boolean b) {}
	void boolTest(boolean x, boolean y) {
		// Arithmetic operators:
		//! x = x * y;
		//! x = x / y;
		//! x = x % y;
		//! x = x + y;
		//! x = x - y;
		//! x++;
		//! x--;
		//! x = +y;
		//! x = -y;
		// Relational and logical:
		//! f(x > y);
		//! f(x >= y);
		//! f(x < y);
		//! f(x <= y);
		f(x == y);
		f(x != y);
		f(!y);
		x = x && y;
		x = x || y;
		// Bitwise operators:
		//! x = ~y;
		x = x & y;
		x = x | y;
		x = x ^ y;
		//! x = x << 1;
		//! x = x >> 1;
		//! x = x >>> 1;
		// Compound assignment:
		//! x += y;
		//! x -= y;
		//! x *= y;
		//! x /= y;
		//! x %= y;
		//! x <<= 1;
		//! x >>= 1;
		//! x >>>= 1;
		x &= y;
		x ^= y;
		x |= y;
		// Casting:
		//! char c = (char)x;
		//! byte B = (byte)x;
		//! short s = (short)x;
		//! int i = (int)x;
		//! long l = (long)x;
		//! float f = (float)x;
		//! double d = (double)x;
	}
	void charTest(char x, char y) {
		// Arithmetic operators:
		x = (char)(x * y);
		x = (char)(x / y);
		x = (char)(x % y);
		x = (char)(x + y);
		x = (char)(x - y);
		x++;
		x--;
		x = (char)+y;
		x = (char)-y;
		// Relational and logical:
		f(x > y);
		f(x >= y);
		f(x < y);
		f(x <= y);
		f(x == y);
		f(x != y);
		//! f(!x);
		//! f(x && y);
		//! f(x || y);
		// Bitwise operators:
		x= (char)~y;
		x = (char)(x & y);
		x  = (char)(x | y);
		x = (char)(x ^ y);
		x = (char)(x << 1);
		x = (char)(x >> 1);
		x = (char)(x >>> 1);
		// Compound assignment:
		x += y;
		x -= y;
		x *= y;
		x /= y;
		x %= y;
		x <<= 1;
		x >>= 1;
		x >>>= 1;
		x &= y;
		x ^= y;
		x |= y;
		// Casting:
		//! boolean b = (boolean)x;
		byte B = (byte)x;
		short s = (short)x;
		int i = (int)x;
		long l = (long)x;
		float f = (float)x;
		double d = (double)x;
	}
	void byteTest(byte x, byte y) {
		// Arithmetic operators:
		x = (byte)(x* y);
		x = (byte)(x / y);
		x = (byte)(x % y);
		x = (byte)(x + y);
		x = (byte)(x - y);
		x++;
		x--;
		x = (byte)+ y;
		x = (byte)- y;
		// Relational and logical:
		f(x > y);
		f(x >= y);
		f(x < y);
		f(x <= y);
		f(x == y);
		f(x != y);
		//! f(!x);
		//! f(x && y);
		//! f(x || y);
		// Bitwise operators:
		x = (byte)~y;
		x = (byte)(x & y);
		x = (byte)(x | y);
		x = (byte)(x ^ y);
		x = (byte)(x << 1);
		x = (byte)(x >> 1);
		x = (byte)(x >>> 1);
		// Compound assignment:
		x += y;
		x -= y;
		x *= y;
		x /= y;
		x %= y;
		x <<= 1;
		x >>= 1;
		x >>>= 1;
		x &= y;
		x ^= y;
		x |= y;
		// Casting:
		//! boolean b = (boolean)x;
		char c = (char)x;
		short s = (short)x;
		int i = (int)x;
		long l = (long)x;
		float f = (float)x;
		double d = (double)x;
	}
	void shortTest(short x, short y) {
		// Arithmetic operators:
		x = (short)(x * y);
		x = (short)(x / y);
		x = (short)(x % y);
		x = (short)(x + y);
		x = (short)(x - y);
		x++;
		x--;
		x = (short)+y;
		x = (short)-y;
		// Relational and logical:
		f(x > y);
		f(x >= y);
		f(x < y);
		f(x <= y);
		f(x == y);
		f(x != y);
		//! f(!x);
		//! f(x && y);
		//! f(x || y);
		// Bitwise operators:
		x = (short)~y;
		x = (short)(x & y);
		x = (short)(x | y);
		x = (short)(x ^ y);
		x = (short)(x << 1);
		x = (short)(x >> 1);
		x = (short)(x >>> 1);
		// Compound assignment:
		x += y;
		x -= y;
		x *= y;
		x /= y;
		x %= y;
		x <<= 1;
		x >>= 1;
		x >>>= 1;
		x &= y;
		x ^= y;
		x |= y;
		// Casting:
		//! boolean b = (boolean)x;
		char c = (char)x;
		byte B = (byte)x;
		int i = (int)x;
		long l = (long)x;
		float f = (float)x;
		double d = (double)x;
	}
	void intTest(int x, int y) {
		// Arithmetic operators:
		x = x * y;
		x = x / y;
		x = x % y;
		x = x + y;
		x = x - y;
		x++;
		x--;
		x = +y;
		x = -y;
		// Relational and logical:
		f(x > y);
		f(x >= y);
		f(x < y);
		f(x <= y);
		f(x == y);
		f(x != y);
		//! f(!x);
		//! f(x && y);
		//! f(x || y);
		// Bitwise operators:
		x = ~y;
		x = x & y;
		x = x | y;
		x = x ^ y;
		x = x << 1;
		x = x >> 1;
		x = x >>> 1;
		// Compound assignment:
		x += y;
		x -= y;
		x *= y;
		x /= y;
		x %= y;
		x <<= 1;
		x >>= 1;
		x >>>= 1;
		x &= y;
		x ^= y;
		x |= y;
		// Casting:
		//! boolean b = (boolean)x;
		char c = (char)x;
		byte B = (byte)x;
		short s = (short)x;
		long l = (long)x;
		float f = (float)x;
		double d = (double)x;
	}
	void longTest(long x, long y) {
		// Arithmetic operators:
		x = x * y;
		x = x / y;
		x = x % y;
		x = x + y;
		x = x - y;
		x++;
		x--;
		x = +y;
		x = -y;
		// Relational and logical:
		f(x > y);
		f(x >= y);
		f(x < y);
		f(x <= y);
		f(x == y);
		f(x != y);
		//! f(!x);
		//! f(x && y);
		//! f(x || y);
		// Bitwise operators:
		x = ~y;
		x = x & y;
		x = x | y;
		x = x ^ y;
		x = x << 1;
		x = x >> 1;
		x = x >>> 1;
		// Compound assignment:
		x += y;
		x -= y;
		x *= y;
		x /= y;
		x %= y;
		x <<= 1;
		x >>= 1;
		x >>>= 1;
		x &= y;
		x ^= y;
		x |= y;
		// Casting:
		//! boolean b = (boolean)x;
		char c = (char)x;
		byte B = (byte)x;
		short s = (short)x;
		int i = (int)x;
		float f = (float)x;
		double d = (double)x;
	}
	void floatTest(float x, float y) {
		// Arithmetic operators:
		x = x * y;
		x = x / y;
		x = x % y;
		x = x + y;
		x = x - y;
		x++;
		x--;
		x = +y;
		x = -y;
		// Relational and logical:
		f(x > y);
		f(x >= y);
		f(x < y);
		f(x <= y);
		f(x == y);
		f(x != y);
		//! f(!x);
		//! f(x && y);
		//! f(x || y);
		// Bitwise operators:
		//! x = ~y;
		//! x = x & y;
		//! x = x | y;
		//! x = x ^ y;
		//! x = x << 1;
		//! x = x >> 1;
		//! x = x >>> 1;
		// Compound assignment:
		x += y;
		x -= y;
		x *= y;
		x /= y;
		x %= y;
		//! x <<= 1;
		//! x >>= 1;
		//! x >>>= 1;
		//! x &= y;
		//! x ^= y;
		//! x |= y;
		// Casting:
		//! boolean b = (boolean)x;
		char c = (char)x;
		byte B = (byte)x;
		short s = (short)x;
		int i = (int)x;
		long l = (long)x;
		double d = (double)x;
	}
	void doubleTest(double x, double y) {
		// Arithmetic operators:
		x = x * y;
		x = x / y;
		x = x % y;
		x = x + y;
		x = x - y;
		x++;
		x--;
		x = +y;
		x = -y;
		// Relational and logical:
		f(x > y);
		f(x >= y);
		f(x < y);
		f(x <= y);
		f(x == y);
		f(x != y);
		//! f(!x);
		//! f(x && y);
		//! f(x || y);
		// Bitwise operators:
		//! x = ~y;
		//! x = x & y;
		//! x = x | y;
		//! x = x ^ y;
		//! x = x << 1;
		//! x = x >> 1;
		//! x = x >>> 1;
		// Compound assignment:
		x += y;
		x -= y;
		x *= y;
		x /= y;
		x %= y;
		//! x <<= 1;
		//! x >>= 1;
		//! x >>>= 1;
		//! x &= y;
		//! x ^= y;
		//! x |= y;
		// Casting:
		//! boolean b = (boolean)x;
		char c = (char)x;
		byte B = (byte)x;
		short s = (short)x;
		int i = (int)x;
		long l = (long)x;
		float f = (float)x;
	}
} ///:~

Note that boolean is quite limited. You can assign to it the values true and false, and you can test it for truth or falsehood, but you cannot add booleans or perform any other type of operation on them.

In char, byte, and short, you can see the effect of promotion with the arithmetic operators. Each arithmetic operation on any of those types produces an int result, which must be explicitly cast back to the original type (a narrowing conversion that might lose information) to assign back to that type. With int values, however, you do not need to cast, because everything is already an int. Don't be lulled into thinking everything is safe, though. If you multiply two ints that are big enough, you'll overflow the result. The following example demonstrates this:

//: c03:Overflow.java
// Surprise! Java lets you overflow.
import com.bruceeckel.simpletest.*;
 
public class Overflow {
	static Test monitor = new Test();
	public static void main(String[] args) {
		int big = 0x7fffffff; // max int value
		System.out.println("big = " + big);
		int bigger = big * 4;
		System.out.println("bigger = " + bigger);
		monitor.expect(new String[] {
			"big = 2147483647",
			"bigger = -4"
		});
	}
} ///:~

You get no errors or warnings from the compiler, and no exceptions at run time. Java is good, but it's not that good.

Compound assignments do not require casts for char, byte, or short, even though they are performing promotions that have the same results as the direct arithmetic operations. On the other hand, the lack of the cast certainly simplifies the code.

You can see that, with the exception of boolean, any primitive type can be cast to any other primitive type. Again, you must be aware of the effect of a narrowing conversion when casting to a smaller type, otherwise you might unknowingly lose information during the cast.

All conditional statements use the truth or falsehood of a conditional expression to determine the execution path. An example of a conditional expression is A == B. This uses the conditional operator == to see if the value of A is equivalent to the value of B. The expression returns true or false. Any of the relational operators you've seen earlier in this chapter can be used to produce a conditional statement. Note that Java doesn't allow you to use a number as a boolean, even though it's allowed in C and C++ (where truth is nonzero and falsehood is zero). If you want to use a non-boolean in a boolean test, such as if(a), you must first convert it to a boolean value by using a conditional expression, such as if(a != 0).

The if-else statement is probably the most basic way to control program flow. The else is optional, so you can use if in two forms:

if(Boolean-expression)
	statement

or

if(Boolean-expression)
	statement
else
	statement

The conditional must produce a boolean result. The statement is either a simple statement terminated by a semicolon, or a compound statement, which is a group of simple statements enclosed in braces. Any time the word "statement" is used, it always implies that the statement can be simple or compound.

As an example of if-else, here is a test( ) method that will tell you whether a guess is above, below, or equivalent to a target number:

//: c03:IfElse.java
import com.bruceeckel.simpletest.*;
 
public class IfElse {
	static Test monitor = new Test();
	static int test(int testval, int target) {
		int result = 0;
		if(testval > target)
			result = +1;
		else if(testval < target)
			result = -1;
		else
			result = 0; // Match
		return result;
	}
	public static void main(String[] args) {
		System.out.println(test(10, 5));
		System.out.println(test(5, 10));
		System.out.println(test(5, 5));
		monitor.expect(new String[] {
			"1",
			"-1",
			"0"
		});
	}
} ///:~

It is conventional to indent the body of a control flow statement so the reader can easily determine where it begins and ends.

The return keyword has two purposes: It specifies what value a method will return (if it doesn't have a void return value) and it causes that value to be returned immediately. The preceding test( ) method can be rewritten to take advantage of this:

//: c03:IfElse2.java
import com.bruceeckel.simpletest.*;
 
public class IfElse2 {
	static Test monitor = new Test();
	static int test(int testval, int target) {
		if(testval > target)
			return +1;
		else if(testval < target)
			return -1;
		else
			return 0; // Match
	}
	public static void main(String[] args) {
		System.out.println(test(10, 5));
		System.out.println(test(5, 10));
		System.out.println(test(5, 5));
		monitor.expect(new String[] {
			"1",
			"-1",
			"0"
		});
	}
} ///:~

There's no need for else, because the method will not continue after executing a return.

Looping is controlled by while, do-while and for, which are sometimes classified as iteration statements. A statement repeats until the controlling Boolean-expression evaluates to false. The form for a while loop is

while(Boolean-expression)
	statement

The Boolean-expression is evaluated once at the beginning of the loop and again before each further iteration of the statement.

Here's a simple example that generates random numbers until a particular condition is met:

//: c03:WhileTest.java
// Demonstrates the while loop.
import com.bruceeckel.simpletest.*;
 
public class WhileTest {
	static Test monitor = new Test();
	public static void main(String[] args) {
		double r = 0;
		while(r < 0.99d) {
			r = Math.random();
			System.out.println(r);
			monitor.expect(new String[] {
				"%% \\d\\.\\d+E?-?\\d*"
			}, Test.AT_LEAST);
		}
	}
} ///:~

This uses the static method random( ) in the Math library, which generates a double value between 0 and 1. (It includes 0, but not 1.) The conditional expression for the while says "keep doing this loop until the number is 0.99 or greater." Each time you run this program, you'll get a different-sized list of numbers.

In the expect( ) statement, you see the Test.AT_LEAST flag following the expected list of strings. The expect( ) statement can include several different flags to modify its behavior; this one says that expect( ) should see at least the lines shown, but others may also appear (which it ignores). Here, it says "you should see at least one double value."

The form for do-while is

do
	statement
while(Boolean-expression);

The sole difference between while and do-while is that the statement of the do-while always executes at least once, even if the expression evaluates to false the first time. In a while, if the conditional is false the first time the statement never executes. In practice, do-while is less common than while.

A for loop performs initialization before the first iteration. Then it performs conditional testing and, at the end of each iteration, some form of "stepping." The form of the for loop is:

for(initialization; Boolean-expression; step)
	statement

Any of the expressions initialization, Boolean-expression or step can be empty. The expression is tested before each iteration, and as soon as it evaluates to false, execution will continue at the line following the for statement. At the end of each loop, the step executes.

for loops are usually used for "counting" tasks:

//: c03:ListCharacters.java
// Demonstrates "for" loop by listing
// all the lowercase ASCII letters.
import com.bruceeckel.simpletest.*;
 
public class ListCharacters {
	static Test monitor = new Test();
	public static void main(String[] args) {
		for(int i = 0; i < 128; i++)
			if(Character.isLowerCase((char)i))
				System.out.println("value: " + i +
					" character: " + (char)i);
		monitor.expect(new String[] {
			"value: 97 character: a",
			"value: 98 character: b",
			"value: 99 character: c",
			"value: 100 character: d",
			"value: 101 character: e",
			"value: 102 character: f",
			"value: 103 character: g",
			"value: 104 character: h",
			"value: 105 character: i",
			"value: 106 character: j",
			"value: 107 character: k",
			"value: 108 character: l",
			"value: 109 character: m",
			"value: 110 character: n",
			"value: 111 character: o",
			"value: 112 character: p",
			"value: 113 character: q",
			"value: 114 character: r",
			"value: 115 character: s",
			"value: 116 character: t",
			"value: 117 character: u",
			"value: 118 character: v",
			"value: 119 character: w",
			"value: 120 character: x",
			"value: 121 character: y",
			"value: 122 character: z"
		});
	}
} ///:~

Note that the variable i is defined at the point where it is used, inside the control expression of the for loop, rather than at the beginning of the block denoted by the open curly brace. The scope of i is the expression controlled by the for.

This program also uses the java.lang.Character "wrapper" class, which not only wraps the primitive char type in an object, but also provides other utilities. Here, the static isLowerCase( ) method is used to detect whether the character in question is a lower-case letter.

Traditional procedural languages like C require that all variables be defined at the beginning of a block so that when the compiler creates a block, it can allocate space for those variables. In Java and C++, you can spread your variable declarations throughout the block, defining them at the point that you need them. This allows a more natural coding style and makes code easier to understand.

You can define multiple variables within a for statement, but they must be of the same type:

for(int i = 0, j = 1; i < 10 && j != 11; i++, j++)
	// body of for loop

The int definition in the for statement covers both i and j. The ability to define variables in the control expression is limited to the for loop. You cannot use this approach with any of the other selection or iteration statements.

3-2-6-1. The comma operator▲

//: c03:CommaOperator.java
import com.bruceeckel.simpletest.*;
 
public class CommaOperator {
	static Test monitor = new Test();
	public static void main(String[] args) {
		for(int i = 1, j = i + 10; i < 5;
			i++, j = i * 2) {
			System.out.println("i= " + i + " j= " + j);
		}
		monitor.expect(new String[] {
			"i= 1 j= 11",
			"i= 2 j= 4",
			"i= 3 j= 6",
			"i= 4 j= 8"
		});
	}
} ///:~

You can also control the flow of the loop inside the body of any of the iteration statements by using break and continue. break quits the loop without executing the rest of the statements in the loop. continue stops the execution of the current iteration and goes back to the beginning of the loop to begin the next iteration.

This program shows examples of break and continue within for and while loops:

//: c03:BreakAndContinue.java
// Demonstrates break and continue keywords.
import com.bruceeckel.simpletest.*;
 
public class BreakAndContinue {
	static Test monitor = new Test();
	public static void main(String[] args) {
		for(int i = 0; i < 100; i++) {
			if(i == 74) break; // Out of for loop
			if(i % 9 != 0) continue; // Next iteration
			System.out.println(i);
		}
		int i = 0;
		// An "infinite loop":
		while(true) {
			i++;
			int j = i * 27;
			if(j == 1269) break; // Out of loop
			if(i % 10 != 0) continue; // Top of loop
			System.out.println(i);
		}
		monitor.expect(new String[] {
			"0",
			"9",
			"18",
			"27",
			"36",
			"45",
			"54",
			"63",
			"72",
			"10",
			"20",
			"30",
			"40"
		});
	}
} ///:~

In the for loop, the value of i never gets to 100 because the break statement breaks out of the loop when i is 74. Normally, you'd use a break like this only if you didn't know when the terminating condition was going to occur. The continue statement causes execution to go back to the top of the iteration loop (thus incrementing i) whenever i is not evenly divisible by 9. When it is, the value is printed.

The second portion shows an "infinite loop" that would, in theory, continue forever. However, inside the loop there is a break statement that will break out of the loop. In addition, you'll see that the continue moves back to the top of the loop without completing the remainder. (Thus printing happens in the second loop only when the value of i is divisible by 10.) In the output, The value 0 is printed, because 0 % 9 produces 0.

A second form of the infinite loop is for(;;). The compiler treats both while(true) and for(;;) in the same way, so whichever one you use is a matter of programming taste.

3-2-7-1. The infamous "goto"▲

label1:

label1: 
outer-iteration {
	inner-iteration {
		//...
		break; // 1
		//...
		continue;  // 2
		//...
		continue label1; // 3
		//...
		break label1;  // 4
	}
}

//: c03:LabeledFor.java
// Java's "labeled for" loop.
import com.bruceeckel.simpletest.*;
 
public class LabeledFor {
	static Test monitor = new Test();
	public static void main(String[] args) {
		int i = 0;
		outer: // Can't have statements here
		for(; true ;) { // infinite loop
			inner: // Can't have statements here
			for(; i < 10; i++) {
				System.out.println("i = " + i);
				if(i == 2) {
					System.out.println("continue");
					continue;
				}
				if(i == 3) {
					System.out.println("break");
					i++; // Otherwise i never
						// gets incremented.
					break;
				}
				if(i == 7) {
					System.out.println("continue outer");
					i++; // Otherwise i never
					   // gets incremented.
					continue outer;
				}
				if(i == 8) {
					System.out.println("break outer");
					break outer;
				}
				for(int k = 0; k < 5; k++) {
					if(k == 3) {
						System.out.println("continue inner");
						continue inner;
					}
				}
			}
		}
		// Can't break or continue to labels here
		monitor.expect(new String[] {
			"i = 0",
			"continue inner",
			"i = 1",
			"continue inner",
			"i = 2",
			"continue",
			"i = 3",
			"break",
			"i = 4",
			"continue inner",
			"i = 5",
			"continue inner",
			"i = 6",
			"continue inner",
			"i = 7",
			"continue outer",
			"i = 8",
			"break outer"
		});
	}
} ///:~

//: c03:LabeledWhile.java
// Java's "labeled while" loop.
import com.bruceeckel.simpletest.*;
 
public class LabeledWhile {
	static Test monitor = new Test();
	public static void main(String[] args) {
		int i = 0;
		outer:
		while(true) {
			System.out.println("Outer while loop");
			while(true) {
				i++;
				System.out.println("i = " + i);
				if(i == 1) {
					System.out.println("continue");
					continue;
				}
				if(i == 3) {
					System.out.println("continue outer");
					continue outer;
				}
				if(i == 5) {
					System.out.println("break");
					break;
				}
				if(i == 7) {
					System.out.println("break outer");
					break outer;
				}
			}
		}
		monitor.expect(new String[] {
			"Outer while loop",
			"i = 1",
			"continue",
			"i = 2",
			"i = 3",
			"continue outer",
			"Outer while loop",
			"i = 4",
			"i = 5",
			"break",
			"Outer while loop",
			"i = 6",
			"i = 7",
			"break outer"
		});
	}
} ///:~

The switch is sometimes classified as a selection statement. The switch statement selects from among pieces of code based on the value of an integral expression. Its form is:

switch(integral-selector) {
	case integral-value1 : statement; break; 
	case integral-value2 : statement; break;
	case integral-value3 : statement; break;
	case integral-value4 : statement; break;
	case integral-value5 : statement; break;
	// ...
	default: statement;
}

Integral-selector is an expression that produces an integral value. The switch compares the result of integral-selector to each integral-value. If it finds a match, the corresponding statement (simple or compound) executes. If no match occurs, the default statement executes.

You will notice in the preceding definition that each case ends with a break, which causes execution to jump to the end of the switch body. This is the conventional way to build a switch statement, but the break is optional. If it is missing, the code for the following case statements execute until a break is encountered. Although you don't usually want this kind of behavior, it can be useful to an experienced programmer. Note that the last statement, following the default, doesn't have a break because the execution just falls through to where the break would have taken it anyway. You could put a break at the end of the default statement with no harm if you considered it important for style's sake.

The switch statement is a clean way to implement multiway selection (i.e., selecting from among a number of different execution paths), but it requires a selector that evaluates to an integral value, such as int or char. If you want to use, for example, a string or a floating-point number as a selector, it won't work in a switch statement. For non-integral types, you must use a series of if statements.

Here's an example that creates letters randomly and determines whether they're vowels or consonants:

//: c03:VowelsAndConsonants.java
// Demonstrates the switch statement.
import com.bruceeckel.simpletest.*;
 
public class VowelsAndConsonants {
	static Test monitor = new Test();
	public static void main(String[] args) {
		for(int i = 0; i < 100; i++) {
			char c = (char)(Math.random() * 26 + 'a');
			System.out.print(c + ": ");
			switch(c) {
				case 'a':
				case 'e':
				case 'i':
				case 'o':
				case 'u': System.out.println("vowel");
				          break;
				case 'y':
				case 'w': System.out.println("Sometimes a vowel");
				          break;
				default:  System.out.println("consonant");
			}
			monitor.expect(new String[] {
				"%% [aeiou]: vowel|[yw]: Sometimes a vowel|" +
			  	"[^aeiouyw]: consonant"
			}, Test.AT_LEAST);
		}
	}
} ///:~

Since Math.random( ) generates a value between 0 and 1, you need only multiply it by the upper bound of the range of numbers you want to produce (26 for the letters in the alphabet) and add an offset to establish the lower bound.

Although it appears you're switching on a character here, the switch statement is actually using the integral value of the character. The single-quoted characters in the case statements also produce integral values that are used for comparison.

Notice how the cases can be "stacked" on top of each other to provide multiple matches for a particular piece of code. You should also be aware that it's essential to put the break statement at the end of a particular case; otherwise, control will simply drop through and continue processing on the next case.

In the regular expression in this expect( ) statement, the '|' is used to indicate three different possibilities. The '[]' encloses a "set" of characters in a regular expression, so the first part says "one of a, e, i, o, or u, followed by a colon and the word 'vowel'." The second possibility indicates either y or w and: "Sometimes a vowel." The set in the third possibility begins with a '^', which means "not any of the characters in this set," so it indicates anything other than a vowel will match.

3-2-8-1. Calculation details▲

char c = (char)(Math.random() * 26 + 'a');

//: c03:CastingNumbers.java
// What happens when you cast a float
// or double to an integral value?
import com.bruceeckel.simpletest.*;
 
public class CastingNumbers {
	static Test monitor = new Test();
	public static void main(String[] args) {
		double
			above = 0.7,
			below = 0.4;
		System.out.println("above: " + above);
		System.out.println("below: " + below);
		System.out.println("(int)above: " + (int)above);
		System.out.println("(int)below: " + (int)below);
		System.out.println("(char)('a' + above): " +
			(char)('a' + above));
		System.out.println("(char)('a' + below): " +
			(char)('a' + below));
		monitor.expect(new String[] {
			"above: 0.7",
			"below: 0.4",
			"(int)above: 0",
			"(int)below: 0",
			"(char)('a' + above): a",
			"(char)('a' + below): a"
		});
	}
} ///:~

//: c03:RandomBounds.java
// Does Math.random() produce 0.0 and 1.0?
// {RunByHand}
 
public class RandomBounds {
	static void usage() {
		System.out.println("Usage: \n\t" +
			"RandomBounds lower\n\tRandomBounds upper");
		System.exit(1);
	}
	public static void main(String[] args) {
		if(args.length != 1) usage();
		if(args[0].equals("lower")) {
			while(Math.random() != 0.0)
				; // Keep trying
			System.out.println("Produced 0.0!");
		}
		else if(args[0].equals("upper")) {
			while(Math.random() != 1.0)
				; // Keep trying
			System.out.println("Produced 1.0!");
		}
		else
			usage();
	}
} ///:~

java RandomBounds lower

java RandomBounds upper