Final 数据
对于原始类型，final使数值恒定不变，而用于对象引用，final使引用恒定不变。一旦引用被初始化指向一个对象，就无法对它进行改变以指向另一个对象。然而，对象其自身却是可以被修改的，Java并未提供使任何对象恒定不变的途径。（但你可以自己编写类以取得使对象恒定不变的效果。）这一限制同样适用数组，它也是对象。下面是一个用以说明final数据成员的示例：
//: c06:FinalData.java// The effect of final on fields.import com.bruceeckel.simpletest.*;import java.util.*;class Value {int i; // Package accesspublic Value(int i) {this.i = i;}}public class FinalData {private static Test monitor = new Test();private static Random rand = new Random();private String id;public FinalData(String id) {this.id = id;}// Can be compile-time constants:private final int VAL_ONE = 9;private static final int VAL_TWO = 99;// Typical public constant:public static final int VAL_THREE = 39;// Cannot be compile-time constants:private final int i4 = rand.nextInt(20);static final int i5 = rand.nextInt(20);private Value v1 = new Value(11);private final Value v2 = new Value(22);private static final Value v3 = new Value(33);// Arrays:private final int[] a = { 1, 2, 3, 4, 5, 6 };public String toString() {return id + ": " + "i4 = " + i4 + ", i5 = " + i5;}public static void main(String[] args) {FinalData fd1 = new FinalData("fd1");// ! fd1.VAL_ONE++; // Error: can't change valuefd1.v2.i++; // Object isn't constant!fd1.v1 = new Value(9); // OK -- not finalfor (int i = 0; i < fd1.a.length; i++)fd1.a++; // Object isn't constant!// ! fd1.v2 = new Value(0); // Error: Can't// ! fd1.v3 = new Value(1); // change reference// ! fd1.a = new int;System.out.println(fd1);System.out.println("Creating new FinalData");FinalData fd2 = new FinalData("fd2");System.out.println(fd1);System.out.println(fd2);monitor.expect(new String[] { "%% fd1: i4 = \\d+, i5 = \\d+","Creating new FinalData", "%% fd1: i4 = \\d+, i5 = \\d+","%% fd2: i4 = \\d+, i5 = \\d+" });}} // /:~
//: c06:BlankFinal.java// "Blank" final fields.class Poppet {private int i;Poppet(int ii) {i = ii;}}public class BlankFinal {private final int i = 0; // Initialized finalprivate final int j; // Blank finalprivate final Poppet p; // Blank final reference// Blank finals MUST be initialized in the constructor:public BlankFinal() {j = 1; // Initialize blank finalp = new Poppet(1); // Initialize blank final reference}public BlankFinal(int x) {j = x; // Initialize blank finalp = new Poppet(x); // Initialize blank final reference}public static void main(String[] args) {new BlankFinal();new BlankFinal(47);}} // /:~
你被强制在数据成员的定义处或者是每个构造器中用表达式对final进行赋值。这正是final数据成员在使用前总是被初始化的原因所在。
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