final 修饰的变量可以变

final修饰的引用类型，引用不可以改变，但是它所指向的对象的内容可以改变。

public class P {
	public static void main(String[] args)throws Exception {
		final B a =new B();
		System.out.println(a.o.hashCode());
		c2(a);
		
	}
	public static void c2(B a)throws Exception{
		Field f = a.getClass().getDeclaredField("o");
		f.setAccessible(true);
		System.out.println(f.get(a).hashCode());
		f.set(a, new Object());
		System.out.println(a.o.hashCode());
	}
}
class B{
	final Object o =new Object();
}
	
   结果：

10039797
10039797
18378667

大家怎么看？

评论

6 楼 m635674608 2013-01-14  

reason2003 写道

m635674608 写道

reason2003 写道

reason2003 写道

final B a =new B();

这里面的a 指向的是堆里面的 B 对象的实例，还不是指向 O 对象实例。
final 的意思应该是 a 的指向不能改变

B类中的final o属性，和P类型中的final a属性不是一个意思么？我这里值改了B中的final o属性，要改P中的 final a属性页是一样的啊

File的set(Object obj, Object value)方法描述

If the underlying field is final, the method throws an IllegalAccessException unless setAccessible(true) has succeeded for this Field object and the field is non-static. Setting a final field in this way is meaningful only during deserialization or reconstruction of instances of classes with blank final fields, before they are made available for access by other parts of a program. Use in any other context may have unpredictable effects, including cases in which other parts of a program continue to use the original value of this field.



我修改了一下程序，可以看出反射没有改变对象中final的值

public class B {
    final Object o =new String("1");
}

public class P {
    public static void main(String[] args)throws Exception {
        final B a =new B();
        System.out.println("o hashCode is "+a.o.hashCode());

        c2(a);

        final B a1 =new B();
        System.out.println("new a1.o hashCode is "+a1.o.hashCode());
        System.out.println("new a1.o hashCode is "+a1.o.toString());

        final B a2 =new B();
        System.out.println("new o2 hashCode is "+a2.o.hashCode());
        System.out.println("new o2 hashCode is "+a2.o.toString());
    }

    public static void c2(B a)throws Exception{
        Field f = a.getClass().getDeclaredField("o");
        f.setAccessible(true);
        System.out.println("before reflect hashcode is "+f.get(a).hashCode());
        System.out.println("after reflect value is "+f.get(a).toString());
        f.set(a, new String("2"));
        System.out.println("after reflect hashcode is "+a.o.hashCode());
        System.out.println("after reflect value is "+a.o.toString());
    }
}

o hashCode is 49
before reflect hashcode is 49
after reflect value is 1
after reflect hashcode is 50
after reflect value is 2
new a1.o hashCode is 49
new a1.o hashCode is 1
new o2 hashCode is 49
new o2 hashCode is 1

before reflect hashcode is 49 
after reflect value is 1 
after reflect hashcode is 50 
after reflect value is 2 

这不是说明已经该了吗？？？

5 楼 reason2003 2013-01-14  

m635674608 写道

reason2003 写道

reason2003 写道

final B a =new B();

这里面的a 指向的是堆里面的 B 对象的实例，还不是指向 O 对象实例。
final 的意思应该是 a 的指向不能改变

B类中的final o属性，和P类型中的final a属性不是一个意思么？我这里值改了B中的final o属性，要改P中的 final a属性页是一样的啊

File的set(Object obj, Object value)方法描述

If the underlying field is final, the method throws an IllegalAccessException unless setAccessible(true) has succeeded for this Field object and the field is non-static. Setting a final field in this way is meaningful only during deserialization or reconstruction of instances of classes with blank final fields, before they are made available for access by other parts of a program. Use in any other context may have unpredictable effects, including cases in which other parts of a program continue to use the original value of this field.



我修改了一下程序，可以看出反射没有改变对象中final的值

public class B {
    final Object o =new String("1");
}

public class P {
    public static void main(String[] args)throws Exception {
        final B a =new B();
        System.out.println("o hashCode is "+a.o.hashCode());

        c2(a);

        final B a1 =new B();
        System.out.println("new a1.o hashCode is "+a1.o.hashCode());
        System.out.println("new a1.o hashCode is "+a1.o.toString());

        final B a2 =new B();
        System.out.println("new o2 hashCode is "+a2.o.hashCode());
        System.out.println("new o2 hashCode is "+a2.o.toString());
    }

    public static void c2(B a)throws Exception{
        Field f = a.getClass().getDeclaredField("o");
        f.setAccessible(true);
        System.out.println("before reflect hashcode is "+f.get(a).hashCode());
        System.out.println("after reflect value is "+f.get(a).toString());
        f.set(a, new String("2"));
        System.out.println("after reflect hashcode is "+a.o.hashCode());
        System.out.println("after reflect value is "+a.o.toString());
    }
}

o hashCode is 49
before reflect hashcode is 49
after reflect value is 1
after reflect hashcode is 50
after reflect value is 2
new a1.o hashCode is 49
new a1.o hashCode is 1
new o2 hashCode is 49
new o2 hashCode is 1

4 楼 reason2003 2013-01-14  

m635674608 写道

reason2003 写道

reason2003 写道

final B a =new B();

这里面的a 指向的是堆里面的 B 对象的实例，还不是指向 O 对象实例。
final 的意思应该是 a 的指向不能改变

B类中的final o属性，和P类型中的final a属性不是一个意思么？我这里值改了B中的final o属性，要改P中的 final a属性页是一样的啊

AccessibleObject setAccessible方法

Set the accessible flag for this object to the indicated boolean value. A value of true indicates that the reflected object should suppress Java language access checking when it is used. A value of false indicates that the reflected object should enforce Java language access checks.

3 楼 m635674608 2013-01-14  

reason2003 写道

reason2003 写道

final B a =new B();

这里面的a 指向的是堆里面的 B 对象的实例，还不是指向 O 对象实例。
final 的意思应该是 a 的指向不能改变

B类中的final o属性，和P类型中的final a属性不是一个意思么？我这里值改了B中的final o属性，要改P中的 final a属性页是一样的啊

2 楼 reason2003 2013-01-14  

reason2003 写道

final B a =new B();

这里面的a 指向的是堆里面的 B 对象的实例，还不是指向 O 对象实例。
final 的意思应该是 a 的指向不能改变

1 楼 reason2003 2013-01-14  

final B a =new B();