Thinking in Java, 3rd ed. Revision 4.0

It's useful to visualize some aspects of how things are laid out while the program is running-in particular how memory is arranged. There are six different places to store data:

• Registers. This is the fastest storage because it exists in a place different from that of other storage: inside the processor. However, the number of registers is severely limited, so registers are allocated by the compiler according to its needs. You don't have direct control, nor do you see any evidence in your programs that registers even exist.
• The stack. This lives in the general random-access memory (RAM) area, but has direct support from the processor via its stack pointer. The stack pointer is moved down to create new memory and moved up to release that memory. This is an extremely fast and efficient way to allocate storage, second only to registers. The Java compiler must know, while it is creating the program, the exact size and lifetime of all the data that is stored on the stack, because it must generate the code to move the stack pointer up and down. This constraint places limits on the flexibility of your programs, so while some Java storage exists on the stack-in particular, object references-Java objects themselves are not placed on the stack.
• The heap. This is a general-purpose pool of memory (also in the RAM area) where all Java objects live. The nice thing about the heap is that, unlike the stack, the compiler doesn't need to know how much storage it needs to allocate from the heap or how long that storage must stay on the heap. Thus, there's a great deal of flexibility in using storage on the heap. Whenever you need to create an object, you simply write the code to create it by using new,and the storage is allocated on the heap when that code is executed. Of course there's a price you pay for this flexibility. It takes more time to allocate heap storage than it does to allocate stack storage (if you even could create objects on the stack in Java, as you can in C++).
• Static storage. "Static" is used here in the sense of "in a fixed location" (although it's also in RAM). Static storage contains data that is available for the entire time a program is running. You can use the static keyword to specify that a particular element of an object is static, but Java objects themselves are never placed in static storage.
• Constant storage. Constant values are often placed directly in the program code, which is safe since they can never change. Sometimes constants are cordoned off by themselves so that they can be optionally placed in read-only memory (ROM), in embedded systems.
• Non-RAM storage. If data lives completely outside a program, it can exist while the program is not running, outside the control of the program. The two primary examples of this are streamed objects, in which objects are turned into streams of bytes, generally to be sent to another machine, and persistent objects, in which the objects are placed on disk so they will hold their state even when the program is terminated. The trick with these types of storage is turning the objects into something that can exist on the other medium, and yet can be resurrected into a regular RAM-based object when necessary. Java provides support for lightweight persistence, and future versions of Java might provide more complete solutions for persistence.

One group of types, which you'll use quite often in your programming, gets special treatment. You can think of these as "primitive" types. The reason for the special treatment is that to create an object with new-especially a small, simple variable-isn't very efficient, because new places objects on the heap. For these types Java falls back on the approach taken by C and C++. That is, instead of creating the variable by using new, an "automatic" variable is created that is not a reference. The variable holds the value, and it's placed on the stack, so it's much more efficient.

Java determines the size of each primitive type. These sizes don't change from one machine architecture to another as they do in most languages. This size invariance is one reason Java programs are portable.

All numeric types are signed, so don't look for unsigned types.

The size of the boolean type is not explicitly specified; it is only defined to be able to take the literal values true or false.

The "wrapper"classes for the primitive data types allow you to make a nonprimitive object on the heap to represent that primitive type. For example:

char c = 'x';
Character C = new Character(c);

Or you could also use:

Character C = new Character('x');

The reasons for doing this will be shown in a later chapter.

2-2-2-1. High-precision numbers▲

Virtually all programming languages support arrays. Using arrays in C and C++ is perilous because those arrays are only blocks of memory. If a program accesses the array outside of its memory block or uses the memory before initialization (common programming errors), there will be unpredictable results.

One of the primary goals of Java is safety, so many of the problems that plague programmers in C and C++ are not repeated in Java. A Java array is guaranteed to be initialized and cannot be accessed outside of its range. The range checking comes at the price of having a small amount of memory overhead on each array as well as verifying the index at run time, but the assumption is that the safety and increased productivity is worth the expense.

When you create an array of objects, you are really creating an array of references, and each of those references is automatically initialized to a special value with its own keyword: null. When Java sees null, it recognizes that the reference in question isn't pointing to an object.You must assign an object to each reference before you use it, and if you try to use a reference that's still null, the problem will be reported at run time. Thus, typical array errors are prevented in Java.

You can also create an array of primitives. Again, the compiler guarantees initialization because it zeroes the memory for that array.

Arrays will be covered in detail in later chapters.

Most procedural languages have the concept of scope. This determines both the visibility and lifetime of the names defined within that scope. In C, C++, and Java, scope is determined by the placement of curly braces {}. So for example:

{
  int x = 12;
  // Only x available
  {
    int q = 96;
    // Both x & q available
  }
  // Only x available
  // q "out of scope"
}

A variable defined within a scope is available only to the end of that scope.

Any text after a '//' to the end of a line is a comment.

Indentation makes Java code easier to read. Since Java is a free-form language, the extra spaces, tabs, and carriage returns do not affect the resulting program.

Note that you cannot do the following, even though it is legal in C and C++:

{
	int x = 12;
	{
		int x = 96; // Illegal
	}
}

The compiler will announce that the variable x has already been defined. Thus the C and C++ ability to "hide" a variable in a larger scope is not allowed, because the Java designers thought that it led to confusing programs.

Java objects do not have the same lifetimes as primitives. When you create a Java object using new, it hangs around past the end of the scope. Thus if you use:

{
	String s = new String("a string");
} // End of scope

the reference s vanishes at the end of the scope. However, the String object that s was pointing to is still occupying memory. In this bit of code, there is no way to access the object, because the only reference to it is out of scope. In later chapters you'll see how the reference to the object can be passed around and duplicated during the course of a program.

It turns out that because objects created with new stay around for as long as you want them, a whole slew of C++ programming problems simply vanish in Java. The hardest problems seem to occur in C++ because you don't get any help from the language in making sure that the objects are available when they're needed. And more important, in C++ you must make sure that you destroy the objects when you're done with them.

That brings up an interesting question. If Java leaves the objects lying around, what keeps them from filling up memory and halting your program? This is exactly the kind of problem that would occur in C++. This is where a bit of magic happens. Java has a garbage collector, which looks at all the objects that were created with new and figures out which ones are not being referenced anymore. Then it releases the memory for those objects, so the memory can be used for new objects. This means that you never need to worry about reclaiming memory yourself. You simply create objects, and when you no longer need them, they will go away by themselves. This eliminates a certain class of programming problem: the so-called "memory leak," in which a programmer forgets to release memory.

When you define a class (and all you do in Java is define classes, make objects of those classes, and send messages to those objects), you can put two types of elements in your class: fields (sometimes called data members), and methods (sometimes called member functions). A field is an object of any type that you can communicate with via its reference. It can also be one of the primitive types (which isn't a reference). If it is a reference to an object, you must initialize that reference to connect it to an actual object (using new, as seen earlier) in a special method called a constructor (described fully in Chapter 4). If it is a primitive type, you can initialize it directly at the point of definition in the class. (As you'll see later, references can also be initialized at the point of definition.)

Each object keeps its own storage for its fields; the fields are not shared among objects. Here is an example of a class with some fields:

class DataOnly {
	int i;
	float f;
	boolean b;
}

This class doesn't do anything, but you can create an object:

DataOnly d = new DataOnly();

You can assign values to the fields, but you must first know how to refer to a member of an object. This is accomplished by stating the name of the object reference, followed by a period (dot), followed by the name of the member inside the object:

objectReference.member

For example:

d.i = 47;
d.f = 1.1f; // 'f' after number indicates float constant
d.b = false;

It is also possible that your object might contain other objects that contain data you'd like to modify. For this, you just keep "connecting the dots." For example:

myPlane.leftTank.capacity = 100;

The DataOnly class cannot do much of anything except hold data, because it has no methods. To understand how those work, you must first understand arguments and return values, which will be described shortly.

2-4-1-1. Default values for primitive members▲

int x;

The method argument list specifies what information you pass into the method. As you might guess, this information-like everything else in Java-takes the form of objects. So, what you must specify in the argument list are the types of the objects to pass in and the name to use for each one. As in any situation in Java where you seem to be handing objects around, you are actually passing references. (12) The type of the reference must be correct, however. If the argument is supposed to be a String, you must pass in a String or the compiler will give an error.

Consider a method that takes a String as its argument. Here is the definition, which must be placed within a class definition for it to be compiled:

int storage(String s) {
	return s.length() * 2;
}

This method tells you how many bytes are required to hold the information in a particular String. (Each char in a String is 16 bits, or two bytes, long, to support Unicode characters.) The argument is of type String and is called s. Once s is passed into the method, you can treat it just like any other object. (You can send messages to it.) Here, the length( ) method is called, which is one of the methods for Strings; it returns the number of characters in a string.

You can also see the use of the return keyword, which does two things. First, it means "leave the method, I'm done." Second, if the method produces a value, that value is placed right after the return statement. In this case, the return value is produced by evaluating the expression s.length( ) * 2.

You can return any type you want, but if you don't want to return anything at all, you do so by indicating that the method returns void. Here are some examples:

boolean flag() { return true; }
float naturalLogBase() { return 2.718f; }
void nothing() { return; }
void nothing2() {}

When the return type is void, then the return keyword is used only to exit the method, and is therefore unnecessary when you reach the end of the method. You can return from a method at any point, but if you've given a non-void return type, then the compiler will force you (with error messages) to return the appropriate type of value regardless of where you return.

At this point, it can look like a program is just a bunch of objects with methods that take other objects as arguments and send messages to those other objects. That is indeed much of what goes on, but in the following chapter you'll learn how to do the detailed low-level work by making decisions within a method. For this chapter, sending messages will suffice.

A problem in any programming language is the control of names. If you use a name in one module of the program, and another programmer uses the same name in another module, how do you distinguish one name from another and prevent the two names from "clashing?" In C this is a particular problem because a program is often an unmanageable sea of names. C++ classes (on which Java classes are based) nest functions within classes so they cannot clash with function names nested within other classes. However, C++ still allows global data and global functions, so clashing is still possible. To solve this problem, C++ introduced namespaces using additional keywords.

Java was able to avoid all of this by taking a fresh approach. To produce an unambiguous name for a library, the specifier used is not unlike an Internet domain name. In fact, the Java creators want you to use your Internet domain name in reverse since those are guaranteed to be unique. Since my domain name is BruceEckel.com, my utility library of foibles would be named com.bruceeckel.utility.foibles. After your reversed domain name, the dots are intended to represent subdirectories.

In Java 1.0 and Java 1.1 the domain extensions com, edu, org, net, etc., were capitalized by convention, so the library would appear: COM.bruceeckel.utility.foibles. Partway through the development of Java 2, however, it was discovered that this caused problems, so now the entire package name is lowercase.

This mechanism means that all of your files automatically live in their own namespaces, and each class within a file must have a unique identifier. So you do not need to learn special language features to solve this problem-the language takes care of it for you.

Whenever you want to use a predefined class in your program, the compiler must know how to locate it. Of course, the class might already exist in the same source code file that it's being called from. In that case, you simply use the class-even if the class doesn't get defined until later in the file (Java eliminates the "forward referencing" problem, so you don't need to think about it).

What about a class that exists in some other file? You might think that the compiler should be smart enough to simply go and find it, but there is a problem. Imagine that you want to use a class with a particular name, but more than one definition for that class exists (presumably these are different definitions). Or worse, imagine that you're writing a program, and as you're building it you add a new class to your library that conflicts with the name of an existing class.

To solve this problem, you must eliminate all potential ambiguities. This is accomplished by telling the Java compiler exactly what classes you want by using the import keyword. import tells the compiler to bring in a package, which is a library of classes. (In other languages, a library could consist of functions and data as well as classes, but remember that all code in Java must be written inside a class.)

Most of the time you'll be using components from the standard Java libraries that come with your compiler. With these, you don't need to worry about long, reversed domain names; you just say, for example:

import java.util.ArrayList;

to tell the compiler that you want to use Java's ArrayList class. However, util contains a number of classes and you might want to use several of them without declaring them all explicitly. This is easily accomplished by using '*' to indicate a wild card:

import java.util.*;

It is more common to import a collection of classes in this manner than to import classes individually.

Ordinarily, when you create a class you are describing how objects of that class look and how they will behave. You don't actually get anything until you create an object of that class with new, and at that point data storage is created and methods become available.

But there are two situations in which this approach is not sufficient. One is if you want to have only one piece of storage for a particular piece of data, regardless of how many objects are created, or even if no objects are created. The other is if you need a method that isn't associated with any particular object of this class. That is, you need a method that you can call even if no objects are created. You can achieve both of these effects with the static keyword. When you say something is static, it means that data or method is not tied to any particular object instance of that class. So even if you've never created an object of that class you can call a static method or access a piece of static data. With ordinary, non-static data and methods, you must create an object and use that object to access the data or method, since non-static data and methods must know the particular object they are working with. Of course, since static methods don't need any objects to be created before they are used, they cannot directly access non-static members or methods by simply calling those other members without referring to a named object (since non-static members and methods must be tied to a particular object).

Some object-oriented languages use the terms class data and class methods, meaning that the data and methods exist only for the class as a whole, and not for any particular objects of the class. Sometimes the Java literature uses these terms too.

To make a field or method static, you simply place the keyword before the definition. For example, the following produces a static field and initializes it:

class StaticTest {
	static int i = 47;
}

Now even if you make two StaticTest objects, there will still be only one piece of storage for StaticTest.i. Both objects will share the same i.Consider:

StaticTest st1 = new StaticTest();
StaticTest st2 = new StaticTest();

At this point, both st1.i and st2.i have the same value of 47 since they refer to the same piece of memory.

There are two ways to refer to a static variable. As the preceeding example indicates, you can name it via an object, by saying, for example, st2.i. You can also refer to it directly through its class name, something you cannot do with a non-static member. (This is the preferred way to refer to a static variable since it emphasizes that variable's static nature.)

StaticTest.i++;

The ++ operator increments the variable. At this point, both st1.i and st2.i will have the value 48.

Similar logic applies to static methods. You can refer to a static method either through an object as you can with any method, or with the special additional syntax ClassName.method( ). You define a static method in a similar way:

class StaticFun {
	static void incr() { StaticTest.i++; }
}

You can see that the StaticFun method incr( ) increments the static data i using the ++ operator. You can call incr( ) in the typical way, through an object:

StaticFun sf = new StaticFun();
sf.incr();

Or, because incr( ) is a static method, you can call it directly through its class:

StaticFun.incr();

Although static, when applied to a field, definitely changes the way the data is created (one for each class versus the non-static one for each object), when applied to a method it's not so dramatic. An important use of static for methods is to allow you to call that method without creating an object. This is essential, as we will see, in defining the main( ) method that is the entry point for running an application.

Like any method, a static method can create or use named objects of its type, so a static method is often used as a "shepherd" for a flock of instances of its own type.

To compile and run this program, and all the other programs in this book, you must first have a Java programming environment. There are a number of third-party development environments, but in this book we will assume that you are using the Java Developer's Kit (JDK) from Sun, which is free. If you are using another development system, (14) you will need to look in the documentation for that system to determine how to compile and run programs.

Get on the Internet and go to java.sun.com. There you will find information and links that will lead you through the process of downloading and installing the JDK for your particular platform.

Once the JDK is installed, and you've set up your computer's path information so that it will find javac and java, download and unpack the source code for this book (you can find it at www.BruceEckel.com). This will create a subdirectory for each chapter in this book. Move to subdirectory c02 and type:

javac HelloDate.java

This command should produce no response. If you get any kind of an error message, it means you haven't installed the JDK properly and you need to investigate those problems.

On the other hand, if you just get your command prompt back, you can type:

java HelloDate

and you'll get the message and the date as output.

This is the process you can use to compile and run each of the programs in this book. However, you will see that the source code for this book also has a file called build.xml in each chapter, and this contains "ant" commands for automatically building the files for that chapter. Buildfiles and Ant (including where to download it) are described more fully in Chapter 15, but once you have Ant installed (from http://jakarta.apache.org/ant) you can just type 'ant' at the command prompt to compile and run the programs in each chapter. If you haven't installed Ant yet, you can just type the javac and java commands by hand.

One of the better ideas in Java is that writing code isn't the only important activity-documenting it is at least as important. Possibly the biggest problem with documenting code has been maintaining that documentation. If the documentation and the code are separate, it becomes a hassle to change the documentation every time you change the code. The solution seems simple: link the code to the documentation. The easiest way to do this is to put everything in the same file. To complete the picture, however, you need a special comment syntax to mark the documentation and a tool to extract those comments and put them in a useful form. This is what Java has done.

The tool to extract the comments is called javadoc, and it is part of the JDK installation. It uses some of the technology from the Java compiler to look for special comment tags that you put in your programs. It not only extracts the information marked by these tags, but it also pulls out the class name or method name that adjoins the comment. This way you can get away with the minimal amount of work to generate decent program documentation.

The output of javadoc is an HTML file that you can view with your Web browser. Thus, javadoc allows you to create and maintain a single source file and automatically generate useful documentation. Because of javadoc we have a standard for creating documentation, and it's easy enough that we can expect or even demand documentation with all Java libraries.

In addition, you can write your own javadoc handlers, called doclets, if you want to perform special operations on the information processed by javadoc (output in a different format, for example). Doclets are introduced in Chapter 15.

What follows is only an introduction and overview of the basics of javadoc. A thorough description can be found in the JDK documentation downloadable from java.sun.com (note that this documentation doesn't come packed with the JDK; you have to do a separate download to get it). When you unpack the documentation, look in the "tooldocs" subdirectory (or follow the "tooldocs" link).

All of the javadoc commands occur only within /** comments. The comments end with */ as usual. There are two primary ways to use javadoc: embed HTML or use "doc tags." Standalone doc tags are commands that start with a '@' and are placed at the beginning of a comment line. (A leading '*', however, is ignored.) Inline doc tags can appear anywhere within a javadoc comment and also start with a '@' but are surrounded by curly braces.

There are three "types" of comment documentation, which correspond to the element the comment precedes: class, variable, or method. That is, a class comment appears right before the definition of a class; a variable comment appears right in front of the definition of a variable, and a method comment appears right in front of the definition of a method. As a simple example:

/** A class comment */
public class DocTest {
	/** A variable comment */	
	public int i;
	/** A method comment */
	public void f() {}
}

Note that javadoc will process comment documentation for only public and protected members. Comments for private and package-access members (see Chapter 5) are ignored, and you'll see no output. (However, you can use the -private flag to include private members as well.) This makes sense, since only public and protected members are available outside the file, which is the client programmer's perspective. However, all class comments are included in the output.

The output for the preceding code is an HTML file that has the same standard format as all the rest of the Java documentation, so users will be comfortable with the format and can easily navigate your classes. It's worth entering the preceding code, sending it through javadoc, and viewing the resulting HTML file to see the results.

Javadoc passes HTML commands through to the generated HTML document. This allows you full use of HTML; however, the primary motive is to let you format code, such as:

/**
 * <pre>
 * System.out.println(new Date());
 * </pre>
 */

You can also use HTML just as you would in any other Web document to format the regular text in your descriptions:

/**
 * You can <em>even</em> insert a list:
 * <ol>
 * <li> Item one
 * <li> Item two
 * <li> Item three
 * </ol>
 */

Note that within the documentation comment, asterisks at the beginning of a line are thrown away by javadoc, along with leading spaces. Javadoc reformats everything so that it conforms to the standard documentation appearance. Don't use headings such as <h1> or <hr> as embedded HTML, because javadoc inserts its own headings and yours will interfere with them.

All types of comment documentation-class, variable, and method-can support embedded HTML.

Here are some of the javadoc tags available for code documentation. Before trying to do anything serious using javadoc, you should consult the javadoc reference in the downloadable JDK documentation to get full coverage of the way to use javadoc.

2-8-4-1. @see: referring to other classes▲

@see classname
@see fully-qualified-classname
@see fully-qualified-classname#method-name

2-8-4-2. {@link package.class#member label}▲

2-8-4-3. {@docRoot}▲

2-8-4-4. {@inheritDoc} ▲

2-8-4-5. @version▲

@version version-information

2-8-4-6. @author▲

@author author-information

2-8-4-7. @since▲

2-8-4-8. @param▲

@param parameter-name description

2-8-4-9. @return▲

@return description

2-8-4-10. @throws▲

@throws fully-qualified-class-name description

2-8-4-11. @deprecated▲

Here is the first Java program again, this time with documentation comments added:

//: c02:HelloDate.java
import java.util.*;
 
/** The first Thinking in Java example program.
 * Displays a string and today's date.
 * @author Bruce Eckel
 * @author www.BruceEckel.com
 * @version 2.0
*/
public class HelloDate {
	/** Sole entry point to class & application
	 * @param args array of string arguments
	 * @return No return value
	 * @exception exceptions No exceptions thrown
	 */
	public static void main(String[] args) {
		System.out.println("Hello, it's: ");
		System.out.println(new Date());
	}
} ///:~

The first line of the file uses my own technique of putting a '//:' as a special marker for the comment line containing the source file name. That line contains the path information to the file (in this case, c02 indicates Chapter 2) followed by the file name. (15) The last line also finishes with a comment, and this one ('///:~') indicates the end of the source code listing, which allows it to be automatically updated into the text of this book after being checked with a compiler and executed.