What is this referencing?

Question

Suppose I have this class:

public class class1 extends Applet implements Runnable
{
    private String s;
    private URL u;
    ...
}

And a second class:

class TS extends Thread
{
    private final class1 _$97913;
    public TS(class1 paramclass1)
    {
        this._$97913 = paramclass1;
    }
    ...
    public void PostData()
    {
        ...
        class1.access$16(this._$97913, new Socket(class1.access$17(this._$97913), 80);
        ...
    }
    ...
}

Can someone explain how class1.access$16(this._$97913, new Socket(class1.access$17(this._$97913), 80); is referencing private URL u; from class1?

Where does the access$16 come from? What is this called and where can I learn more about it?

Ok, this being the result of decompiled code, is there a way to associate the numbers (access$16, access$17, etc.) to the original variable or class? From what I can see, the only way would be to do so manually (i.e. see what is being referenced where and guess that since 'this' class received a URL, then 'this' must be associated with 'that' variable)?

Answer 1

Is this the result of decompiling java?

It looks like a synthetic method created to allow outer and inner classes access to one another's private fields or methods.

The Java compiler must create synthetic methods on nested classes when their attributes specified with the private modifier are accessed by the enclosing class. The next code sample indicates this situation.

...

As the above screen snapshot indicates, a synthetic method with the name access$100 has been created on the nested class NestedClass to provide its private String to the enclosing class. Note that the synthetic method is only added for the single private attribute of the NestedClass that the enclosing class accesses.

Answer 2

Ok, this being the result of decompiled code, is there a way to associate the numbers (access$16, access$17, etc.) to the original variable or class? From what I can see, the only way would be to do so manually (i.e. see what is being referenced where and guess that since 'this' class received a URL, then 'this' must be associated with 'that' variable)?

The access$x methods are created if you access private methods or variables from a nested class (or the other way around, or from one nested class to another). They are created by the compiler, since the VM does not allow direct access to private variables.

If the decompiler lets these method calls stay in the recreated source code for the using class, it should also let the synthetic methods definitions stay in the recreated source code for the used class. If so, have a look at the class which is the receiver of the method in question (class1 in your case), there should be such a method (access$17). In the code of this method you can see which real method (or variable) is accessed here.

If the decompiler removed the synthetic methods, this is either a bug, or it may be configurable. It could also be that you have to pass it all the classes at once, and then it can put in the right methods/fields everywhere - look at its documentation.

---

If you have the classes before the dot of a method call (and their superclasses, if any), you should have the methods.

From the snippet you posted, there should be a access$16 and access$17 method in class1 (or is class1 a local variable here?).

If it isn't, maybe your decompiler tried to be smarter then he should. You could have a look at the output of javap class1 to see if the methods are there, and javap -c class1 for the whole bytecode. Or use another decompiler.