A Closer Look at Methods and Classes - Introducing final

(Page 9 of 12 )

A variable can be declared as final. Doing so prevents its contents from being modified. This means that you must initialize a final variable when it is declared. (In this usage, final is similar to const in C/C++/C#.) For example:

final int FILE_NEW = 1;
final int FILE_OPEN = 2;
final int FILE_SAVE = 3;
final int FILE_SAVEAS = 4;
final int FILE_QUIT = 5;

Subsequent parts of your program can now use FILE_OPEN, etc., as if they were constants, without fear that a value has been changed.

It is a common coding convention to choose all uppercase identifiers for final variables. Variables declared as final do not occupy memory on a per-instance basis. Thus, a final variable is essentially a constant.

The keyword final can also be applied to methods, but its meaning is substantially different than when it is applied to variables. This second usage of final is described in the next chapter, when inheritance is described.

Arrays Revisited

Arrays were introduced earlier in this book, before classes had been discussed. Now that you know about classes, an important point can be made about arrays: they are implemented as objects. Because of this, there is a special array attribute that you will want to take advantage of. Specifically, the size of an array—that is, the number of elements that an array can hold—is found in its length instance variable. All arrays have this variable, and it will always hold the size of the array. Here is a program that demonstrates this property:

// This program demonstrates the length array member.
class Length {
  public static void main(String args[]) {
    int a1[] = new int[10];
    int a2[] = {3, 5, 7, 1, 8, 99, 44, -10};
    int a3[] = {4, 3, 2, 1};
    System.out.println("length of a1 is " + a1.length);
    System.out.println("length of a2 is " + a2.length);
    System.out.println("length of a3 is " + a3.length);
  }
}

This program displays the following output:

length of a1 is 10
length of a2 is 8
length of a3 is 4

As you can see, the size of each array is displayed. Keep in mind that the value of length has nothing to do with the number of elements that are actually in use. It only reflects the number of elements that the array is designed to hold.

You can put the length member to good use in many situations. For example, here is an improved version of the Stack class. As you might recall, the earlier versions of this class always created a ten-element stack. The following version lets you create stacks of any size. The value of stck.length is used to prevent the stack from overflowing.

// Improved Stack class that uses the length array member.
class Stack {
  private int stck[];
  private int tos;
  // allocate and initialize stack
  Stack(int size) {
    stck = new int[size];
    tos = -1;
  }
  // Push an item onto the stack
  void push(int item) {
    if(tos==stck.length-1) // use length member 
      System.out.println("Stack is full.");
    else
      stck[++tos] = item;
  }
  // Pop an item from the stack
  int pop() {
    if(tos < 0) {
      System.out.println("Stack underflow.");
      return 0;
    }
    else
      return stck[tos--];
  }
}
class TestStack2 {
  public static void main(String args[]) {
    Stack mystack1 = new Stack(5);
    Stack mystack2 = new Stack(8);
    // push some numbers onto the stack
    for(int i=0; i<5; i++) mystack1.push(i);
    for(int i=0; i<8; i++) mystack2.push(i);
    // pop those numbers off the stack
    System.out.println("Stack in mystack1:");
    for(int i=0; i<5; i++)
      System.out.println(mystack1.pop());
    System.out.println("Stack in mystack2:");
    for(int i=0; i<8; i++)
      System.out.println(mystack2.pop());
  }
}

Notice that the program creates two stacks: one five elements deep and the other eight elements deep. As you can see, the fact that arrays maintain their own length information makes it easy to create stacks of any size.

Next: Introducing Nested and Inner Classes >>
 

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This article is excerpted from chapter 7 of Java: the Complete Reference, J2SE 5 Edition, written by Herbert Schildt (McGraw-Hill/Osborne, 2004; ISBN: 0072230738). Check it out at your favorite bookstore. Buy this book now.