访问匿名内部类变量

Question

How to access i from the outer class?

  HashSet<Integer> hs=new HashSet<Integer>(){
        int i=30;
    };

I can do it like this

int k=new HashSet<Integer>(){
    int i=30;
}.i;

But if I get 'i' then I cannot get the instance of hashset.Is there a way to get both?The question just out of curiosity.It doesn't have much practical applications.I just want to know whether it can be done.

Answer 1

Solution 1: Object registration system

The two snippets presented essentially made a choice between storing the reference to the anonymous class instance just created, or to immediately access its custom field. Doing one would seem to forfeit the ability to do the other.

One way to achieve a compromise is to have an object registration system, and use an instance initializer ( JLS 8.6 ) in your anonymous class to self-register to this sytem.

Here's a simple proof-of-concept, using Object[] for a simple registration system:

    final Object[] registrar = { null };
    int k = new HashSet<Integer>(){
        { registrar[0] = this; }
        int i= 666;
    }.i;
    Set<Integer> set = (Set<Integer>) registrar[0];

    System.out.println(k); // 666
    System.out.println(set.getClass().isAnonymousClass()); // true

Essentially we use a 1-element Object[] , and in the instance initializer of our anonymous class, we "register" this to this array. Note that this is just a proof-of-concept; in production code, you'd use a more robust and typesafe object registration system than this.

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Solution 2: Tuples

This approach works nicely if your anonymous class may have multiple "fields". Simply pack them all into a tuple , making sure to include a reference to this in the anonymous class.

Here's a simple proof-of-concept, using Object[] for a tuple:

    Object[] tuple = new HashSet<Integer>(){
        Object[] tuple = { this, 666, 13, "XXX", "OMG ABUZE" };
    }.tuple;
    Set<Integer> set = (Set<Integer>) tuple[0];
    set.add(null);
    System.out.println(set.getClass().isAnonymousClass()); // true
    System.out.println(Arrays.toString(tuple));
    // [[null], 666, 13, XXX, OMG ABUZE]

In production code, you'd use a CustomTuple class that is more object-oriented and typesafe. Note that this is an analogous solution to the "how can my method return 2 values" problem: simply return 1 value that captures all of those information.

Answer 2

Reflection to the rescue, I guess:

HashSet<Integer> hs = new HashSet<Integer>() {
    int i = 30;
};
int i = hs.getClass().getDeclaredField("i").getInt(hs);
System.out.println(i);

PS I feel kind of dirty after writing that.

Answer 3

It makes sense when you think about it.

The expression

new HashSet<Integer>(){
    int i=30;
}

is of type java.util.HashSet , and java.util.HashSet has no field called i , so

new HashSet<Integer>(){
   int i=30;
}.i

will not compile. Put another way, try breaking up the compound expression so the the result of the new is assigned to a variable x , then you call xi . What type could you give x that would allow xi to compile? There is no such type name, it's an anonymous class.

This is a "downside" on anonymous classes - they can only meaningfully override members that exist in the type being extended. If you add new members, they can only be accessed via reflection.

Answer 4

I don't believe you can. You can't declare the type of a variable as an anonymous type ¹ . You can introduce a named class within a method though:

import java.util.*;

public class Test
{
    public static void main(String[] args)
    {
        class Foo extends Hashtable
        {
            int i = 30;
        }

        Foo f = new Foo();
        System.out.println(f.i);
    }
}

It's horrible (really, really horrible) and I don't think I've ever seen it used in production code - but it works.

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¹ C# gets around this using var - the compiler uses the initialization expression to determine the variable's type. Anonymous types in Java are somewhat different to those in C#, but if Java had something like var , I believe your code would work.

Answer 5

Well, to my mind, there is always the reflection way, but it seems a little overkill to me.

And as you set the class to be anonymous way, I cannot see any other way to do it than yours.

Answer 6

Annonymous inner classes weren't designed for that purpose, so from what I remember there's no way you could do that. If you want you can create a "normal" inner class.