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