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。www.

[java] view plaincopyprint?
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