java中final的用法[Java编程]

//: FinalData.java
// The effect of final on fields

class Value {
  int i = 1;
}

public class FinalData {
  // Can be compile-time constants
  final int i1 = 9;
  static final int I2 = 99;
  // Typical public constant:
  public static final int I3 = 39;
  // Cannot be compile-time constants:
  final int i4 = (int)(Math.random()*20);
  static final int i5 = (int)(Math.random()*20);
  
  Value v1 = new Value();
  final Value v2 = new Value();
  static final Value v3 = new Value();
  //! final Value v4; // Pre-Java 1.1 Error: 
                      // no initializer
  // Arrays:
  final int[] a = { 1, 2, 3, 4, 5, 6 };

  public void print(String id) {
    System.out.println(
      id + ": " + "i4 = " + i4 + 
      ", i5 = " + i5);
  }
  public static void main(String[] args) {
    FinalData fd1 = new FinalData();
    //! fd1.i1++; // Error: can't change value
    fd1.v2.i++; // Object isn't constant!
    fd1.v1 = new Value(); // OK -- not final
    for(int i = 0; i < fd1.a.length; i++)
      fd1.a[i]++; // Object isn't constant!
    //! fd1.v2 = new Value(); // Error: Can't 
    //! fd1.v3 = new Value(); // change handle
    //! fd1.a = new int[3];

    fd1.print("fd1");
    System.out.println("Creating new FinalData");
    FinalData fd2 = new FinalData();
    fd1.print("fd1");
    fd2.print("fd2");
  }
} ///:~

fd1: i4 = 15, i5 = 9
Creating new FinalData
fd1: i4 = 15, i5 = 9
fd2: i4 = 10, i5 = 9

//: BlankFinal.java
// "Blank" final data members

class Poppet { }

class BlankFinal {
  final int i = 0; // Initialized final
  final int j; // Blank final
  final Poppet p; // Blank final handle
  // Blank finals MUST be initialized
  // in the constructor:
  BlankFinal() {
    j = 1; // Initialize blank final
    p = new Poppet();
  }
  BlankFinal(int x) {
    j = x; // Initialize blank final
    p = new Poppet();
  }
  public static void main(String[] args) {
    BlankFinal bf = new BlankFinal();
  }
} ///:~

//: FinalArguments.java
// Using "final" with method arguments

class Gizmo {
  public void spin() {}
}

public class FinalArguments {
  void with(final Gizmo g) {
    //! g = new Gizmo(); // Illegal -- g is final
    g.spin();
  }
  void without(Gizmo g) {
    g = new Gizmo(); // OK -- g not final
    g.spin();
  }
  // void f(final int i) { i++; } // Can't change
  // You can only read from a final primitive:
  int g(final int i) { return i + 1; }
  public static void main(String[] args) {
    FinalArguments bf = new FinalArguments();
    bf.without(null);
    bf.with(null);
  }
} ///:~