Java实现Immutable Class要点

Java中很多class都是immutable，像String，Integer等，它们通常用来作为Map的key.

那么在实现自定义的Immutable的Class的时候，应该注意哪些要点呢？

a)Class 应该定义成final，避免被继承。

b)所有的成员变量应该被定义成final。

c)不要提供可以改变类状态(成员变量)的方法。【get 方法不要把类里的成员变量让外部客服端引用,当需要访问成员变量时，返回成员变量的copy】

d)构造函数不要引用外部可变对象。如果需要引用外部可以变量，应该在构造函数里进行defensive copy。

 Wrong way to write a constructor:
 public final class MyImmutable {
 private final int[] myArray;
 public MyImmutable(int[] anArray) {
 this.myArray = anArray; // wrong
 }
 public String toString() {
 StringBuffer sb = new StringBuffer("Numbers are: ");
 for (int i = 0; i < myArray.length; i++) {
 sb.append(myArray[i] + " ");
 }
 return sb.toString();
 }
 }
 // the caller could change the array after calling the
 constructor.
 int[] array = {1,2};
 MyImmutable myImmutableRef = new MyImmutable(array) ;
 System.out.println("Before constructing " + myImmutableRef);
 array[1] = 5; // change (i.e. mutate) the element
 System.out.println("After constructing " + myImmutableRef);
 Out put:
 Before constructing Numbers are: 1 2
 After constructing Numbers are: 1 5



 Right way to write an immutable class
 Right way is to copy the array before assigning in the constructor.

 public final class MyImmutable {
 private final int[] myArray;
 public MyImmutable(int[] anArray) {
 this.myArray = anArray.clone(); // defensive copy
 }
 public String toString() {
 StringBuffer sb = new StringBuffer("Numbers are: ");
 for (int i = 0; i < myArray.length; i++) {
 sb.append(myArray[i] + " ");
 }
 return sb.toString();
 }
 }
 // the caller cannot change the array after calling the constructor.
 int[] array = {1,2};
 MyImmutable myImmutableRef = new MyImmutable(array) ;
 System.out.println("Before constructing " + myImmutableRef);
 array[1] = 5; // change (i.e. mutate) the element
 System.out.println("After constructing " + myImmutableRef);
 Out put:
 Before constructing Numbers are: 1 2
 After constructing Numbers are: 1 2