Java:The final Keyword

 看不懂

import java.util.*;

class Value {
    int i;

    public Value(int i) {
        this.i = i;
    }
}

public class FinalData {

    private static Random rand = new Random(47);
    private String id;

    public FinalData(String id) {
        this.id = id;
    }
    //can be compile-time constants:
    private final int valueOne = 9;
    private static final int VALUE_TWO = 99;
    //Typical public constant:
    public static final int VALUE_THREE = 39;
    //cannot be compile-time constants
    private final int i4 = rand.nextInt(20);
    static final int INT_5 = rand.nextInt(20);
    private Value v1 = new Value(11);
    private final Value v2 = new Value(22);
    private static final Value VAL_3 = new Value(33);
    private final int[] a = { 1, 2, 3, 4, 5, 6 };

    public String toString() {
        return id + ": i4 = " + i4 + ", INT_5 = " + INT_5;
    }

    public static void main(String[] args) {
        FinalData fd1 = new FinalData("fd1");
        //!fd1.valueOne++; //Error:cannot change value
        fd1.v2.i++;//Object is not constant
        fd1.v1 = new Value(9);//OK--not final
        for (int i = 0; i < fd1.a.length; i++) {
            fd1.a[i]++; //Object is not constant
        }
        //!fd1.v2=new Value(0); //Error:cannot
        //!fd1.VAL_3=new Value(1);//change reference
        //!fd1.a=new int[3];
        System.out.println(fd1);
        System.out.println("creating new FinalData");
        FinalData fd2 = new FinalData("fd2");
        System.out.println(fd1);
        System.out.println(fd2);
    }
}

输出

fd1: i4 = 15, INT_5 = 18
creating new FinalData
fd1: i4 = 15, INT_5 = 18
fd2: i4 = 13, INT_5 = 18

 Since valueOne and VALUE_TWO are final primitives with compile-time values , they can both the used as compile-time constants and are not different in any importent way. VALUE_THREE is the more typical way you'll see such constants defined:public so they're usable outside the package, static to emphasize that there's only one, and final to say that it'sa constant. Note that final static primitives with constant initial values(that is,compile-time constants)are named with all capitals by convention, with words separeted by underscores.(This is just like C constants, which is where the convention originated.)

Just because something is final doesn't mean that its value is known at compile time, This is demonstrated by initializing i4 and INT_5 at run time using randomly generated numbers. This portion of the example also shows the difference between making a final value static or non-static. This difference shows up only when the values are initialized at run time ,since the compile-time values are treated the same by the compiler.(And presumably optimized out of existence.)The difference is shown when you run the program. Note that the values of i4 for fd1 and fd2 are unique. but the value for INT_5 is not changed by creating the second FinalData object. That's because it's static and is initialized once loading and not each time a new object is created.

The variables v1 throught VAL_3 demonstrated the meaning of a final reference. As you can see in main(),just because v2 is final doesn't mean that you can't change its value. Because it's  a reference, final means that you cannot rebind v2 to a  new object.You can also see that the same meaning holds true for an array, which is just another kind of refeence.(There is no way that I know of to make  the array references themselves final.) Making references final seems less useful than making primitives final.