java anonymous inner class unreachable code

Question

Java compiler complains when you write a code that is unreachable. For example

public void go()
{
    return;
    System.out.println("unreachable");
}

However, when you define a new method in an anonymous class which cannot be reached from anywhere, compiler does not complain. It allows you to do that, why? For example,

class A
{
   public void go()
   {
     System.out.println("reachable - A");
   }
}
class B
{
   public static void main(String [] args)
   {
     A a = new A() {
              public void go()
              {
                System.out.println("reachable - B");
              }
              public void foo()
              {
                System.out.println("unreachable - B");
              }
           };
     a.go(); // valid
     a.foo(); // invalid, compiler error
  }
}

You're missing parenthesis `()` after `new A` and a brace `};` to end the declaration of the anonymous inner class. — Jesper, Jan 21 '11 at 08:11
I just copied it to B.java file and commented the line a.foo(), then compiled it and ran it. Sorry, but I couldn't see any error in the code except a.foo() part which I already mentioned that it causes a compile error. — kioto, Jan 21 '11 at 12:28
Another way of calling the method not yet mentioned, is that it might be run with an updated version of `A` that provides a `foo` method. — Tom Hawtin - tackline, Jan 21 '11 at 16:07

score 5 · Answer 1 · answered Jan 21 '11 at 08:09

5

That's only equivalent to allowing you to create private methods which aren't called anywhere in the class, really. It's not unreachable code in quite the same sense - and it could be accessed by reflection.

It's the kind of thing which might reasonably give a warning (particularly on IDEs which allow very fine-grained tweaking of warnings) but I don't think it should be an error.

answered Jan 21 '11 at 08:09

Jon Skeet

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Thank you, you are also right. One can access that method within another method of the inner class. I see now. – kioto Jan 21 '11 at 12:33

score 5 · Accepted Answer · answered Jan 21 '11 at 08:14

First of all: Eclipse does notify my that foo() is never used locally. It's a warning and not an error, however, for reasons pointed out by the other anserws.

Note that there is a way to reach foo():

new A() {
      public void go()
      {
        System.out.println("reachable - B");
      }
      public void foo()
      {
        System.out.println("unreachable - B");
      }
}.foo();

This works, because the type of the expression new A() {} is not A, but actually the anonymous subclass of A. And that subclass has a public foo method.

Since you can't have a variable with the same type, you can't access foo() this way from a variable.

Thank you. A very nice way to invoke the method. Inner classes have very interesting propertiesç — kioto, Jan 21 '11 at 12:35

Jesper · Answer 3 · 2011-01-21T08:14:45.240

4

In principle you could call the foo() method on a via reflection, so it is not always unreachable.

Try this after a.go();:

Method m = a.getClass().getDeclaredMethod("foo");
m.invoke(a);

edited Jan 21 '11 at 08:14

answered Jan 21 '11 at 08:09

Jesper

202,709
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1

As well as the EL-language in JSP/Facelets. – Thorbjørn Ravn Andersen Jan 21 '11 at 08:10
Thank you. You are right. But I think it doesn't make sense to reach a method of anonymous inner class via reflection. However, as mentioned in other answer, there are other ways to reach it. – kioto Jan 21 '11 at 12:32
@kioto, it doesn't matter if it makes *sense* or not, what matters is that it's *possible*. Joachim's answer is indeed good. – Jesper Jan 24 '11 at 09:39

score 0 · Answer 4 · answered Jan 21 '11 at 12:50

public void go()
{
   foo();
   System.out.println("reachable - B");
}

public void foo()
{
   System.out.println("unreachable - B");
}

Simply another way to reach the foo method it to use it another method of the class. I should have understood this before asking. Sorry.

java anonymous inner class unreachable code

4 Answers4