How to access fields declared inside anonymous object without reflection?

Question

It's possible to make anonymous classes have new fields in Java:

class A {
   public static void main(String[] args) {
       Object o = new Object() {
           public int x = 0;
           {
               System.out.println("x: " + x++);
               System.out.println("x: " + x++);
           }
       };
       System.out.println(o.x);
   }
}

But they can't be accessed in this example, because o is of type Object:

$ javac A.java && java A
A.java:10: cannot find symbol
symbol  : variable x
location: class java.lang.Object
       System.out.println(o.x);
                           ^
1 error

Note that o is actually not an Object, it's an instance of a different class extending Object. This class has one public field x. Does this class have a name? How can I make o be the proper type so I can get access to o.x? (Note that it's possible to do this with reflection, but I want to do it statically).

Have you considered declaring the class within the method? At that point you could get at it... — Jon Skeet
– Jon Skeet, Commented May 15, 2013 at 18:17
@Dog: Why do you think it should be possible? The only way of expressing a variable of that type is by giving the type a name. It doesn't surprise me at all that this can't be done, to be honest. — Jon Skeet
– Jon Skeet, Commented May 15, 2013 at 21:04
@Dog: No, there's no "surely" about it. newacct's answer isn't just limited to a field - it's also limited to an instance that you don't want to do anything else with. This is simply not something anonymous inner classes were designed for. — Jon Skeet
– Jon Skeet, Commented May 16, 2013 at 5:49
@Dog: Not at all. That's relying on a compiler implementation detail and it requires recompilation. It's horribly brittle. Just don't do it. Give it up - create a named type instead. You still haven't said anything about why you want to work against the language in this way. — Jon Skeet
– Jon Skeet, Commented May 16, 2013 at 15:30

wchargin · Accepted Answer · 2013-05-10 23:03:00Z

3

This class does have a name. It's A$1. However, you can't access this at compile time (the compiler creates A$1 for you). Thus, you can't access the field without reflection.

If you're in Eclipse, you can use Source menu (AltShiftS) → Convert Anonymous Class to Nested to convert it to a "real" class automatically.

Alternatively, you could have:

class A {
   public static void main(String[] args) {
       I o = new I() {
           public int x = 0;
           {
               System.out.println("x: " + x++);
               System.out.println("x: " + x++);
           }
           public int getX() { return x; }
       };
       System.out.println(o.getX());
   }
   interface I {
       public int getX();
   }
}

EDIT: Here's a really evil way to accomplish this that you should not do:

class A {
   public static void main(String[] args) {
       Object o = new Object() {
           public int x = 0;
           {
               System.out.println("x: " + x++);
               System.out.println("x: " + x++);
           }

           public Object clone() {
               // BAD BAD BAD
               return x;
           }
       };
       try {
           System.out.println(o.clone());
       } catch (CloneNotSupportedException cnse) {
           assert false;
       }
   }
}

edited May 10, 2013 at 23:03

answered May 10, 2013 at 21:26

wchargin

16.1k12 gold badges75 silver badges113 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Louis Wasserman Over a year ago

Yup. There really isn't a way to do it without changing the question.

newacct · Accepted Answer · 2013-05-10 23:49:42Z

3

You could access it directly on the anonymous class creation expression:

class A {
   public static void main(String[] args) {
       System.out.println(new Object() {
           public int x = 0;
           {
               System.out.println("x: " + x++);
               System.out.println("x: " + x++);
           }
       }.x);
   }
}

But then, you can't otherwise use the object created anymore, so it's kinda useless.

answered May 10, 2013 at 23:49

newacct

123k29 gold badges168 silver badges227 bronze badges

Comments

Mikhail · Accepted Answer · 2013-05-15 11:04:20Z

Java is not a C#, if I'am not mistaken such syntax sugar is allowed there. You can have a look at sun.reflect.MagicAccessorImpl. But this class is not a common practice. Used only for auto generated code. If you extend MagicAccessorImpl JVM will not perform field access level check. So you can do thing like this:

// ----- A.java -----
class A {
    private int privateField = 5;
}

// ----- B.java -----
class B extends sun.reflect.MagicAccessorBridge {
    public static void main(String[] args) {
        A a = new A();
        a.privateField = 10;
        System.out.println(a.privateField);
    }
}

// ----- MagicAccessorBridge.java -----
package sun.reflect;

public class MagicAccessorBridge extends MagicAccessorImpl {
    // Since MagicAccessorImpl is package-private, we'll make a public bridge
}

Taken from here

Dude Bro · Accepted Answer · 2013-05-16 03:15:38Z

0

+50

like this

class A {
   public static void main(String[] args) {
       Object o = new Object() {
           public int x = 0;
           {
               System.out.println("x: " + x++);
               System.out.println("x: " + x++);
           }
       };
       //System.out.println(((A$1)o).x);
   }
}

compile once

$ java A.java

uncomment print line

recompile & run

$ java A.java && java A
x: 0
x: 1
2

answered May 16, 2013 at 3:15

Dude Bro

1,1521 gold badge10 silver badges13 bronze badges

3 Comments

Dog Over a year ago

howcome it needs to be compiled twice?

Jon Skeet Over a year ago

@Dog: Because the first time it will create an A$1.class file, and then second time it will consume it, while also recreating it. This is really brittle, and I'd strongly suggest you don't do it.

Natix Over a year ago

Don't try this at home!

Collectives™ on Stack Overflow

How to access fields declared inside anonymous object without reflection?

4 Answers 4

1 Comment

Comments

Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related