JAVA菜鸟入门(8) Java的Final关键字

java final 和C++的const功能类似，用来表达常量，还能用在class或者method上，提供不同的用法。

1. Java Final Variable 

Once a final variable has been assigned, it always contains the same value.但是 static final和private final是有区别的。

这个区别一般体现在类中，在类中，被写为private static final v.s. private final.

(1).什么情况下用private static final，什么情况下用 private final

如果你有一个类，它的instance不管运行多少次，你的意图是它的variable不变化

你那么就该用private static final

如果你有一个类，在它的instance每次运行的生命周期中，你的意图是它的variable不变化

你那么就该用private final

(2).如何用private static final

第一种 正确

class A {
     private static final int x；
     static {
          x = 5;
     }
}

第二种正确

class A {
     private static final int x = 5;
...
}

(3). 如何用private final

第一种 正确

class A {
     private final int x;
     public A() {
          x = 5;
     }
}

第二种 正确

class A {
     private final int x = 5;
    ...
}

(4) 如果final中存的不是primitive type，是instance of a class, 那么。

If a final variable holds a reference to an object, then the state of the object may be changed by operations on the object(e.g. add an item, delete an item, depending on whether the object is mutable or not), but the variable will always refer to the same object

2. final method

If you make any method as final, you cannot override it.

class Bike{
    final void run(){
        System.out.println("running");
    }
}
class Honda extends Bike{
    //Output:Compile Time Error
    void run(){System.out.println("running safely with 100kmph");} 
    public static void main(String args[]){
        Honda honda= new Honda();
        honda.run();
    }
}

3. final class

If you make any class as final, you cannot extend it.

Example of final class

final class Bike{}
//:Compile Time Error
class Honda1 extends Bike{
    void run(){System.out.println("running safely with 100kmph");}
    public static void main(String args[]){
    Honda1 honda= new Honda();
    honda.run();
    }
}

4. final parameter

If you declare any parameter as final, you cannot change the value of it.

class Bike11{
     int cube(final int n){
     n=n+2;//can't be changed as n is final
     n*n*n;
}
public static void main(String args[]){
     Bike11 b=new Bike11();
     b.cube(5);
     }
}

5. 如果要在inner class中使用外层class的变量

When an anonymous inner class is defined within the body of a method, all variables declared final in the scope of that method are accessible from within the inner class. For scalar values, once it has been assigned, the value of the final variable cannot change. For object values, the reference cannot change.

import javax.swing.*;
public class FooGUI {
    public static void main(String[] args) {
        //initialize GUI components
        final JFrame jf = new JFrame("Hello world!"); //allows jf to be accessed from inner class body
        jf.add(new JButton("Click me"));
 

        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                jf.pack(); //this would be a compile-time error if jf were not final
                jf.setLocationRelativeTo(null);
                jf.setVisible(true);
            }
        });
    }
}
 

参考资料:

1 http://www.javatpoint.com/final-keyword

2 http://stackoverflow.com/questions/5093744/initialize-a-static-final-field-in-the-constructor

3 http://en.wikipedia.org/wiki/Final_(Java)