Question

Below, is some toy code that demonstrates my question. The first one is a version that compiles, and the second is a version that does not compile.

In example 1, in order to create an instance of InnerClass, I must create the instance below the class definition of InnerClass. This makes sense, because above the class definition, InnerClass is not visible. However, lets say for the sake of neatness, I want to create and use an instance of InnerClass at the top of foo(). Is there a way to define InnerClass on the fly before the actual class definition, such that my code could look more like example 2 but would be legal Java?

example 1

public class OuterClass {
    public void foo() {
        class InnerClass {  

            public InnerClass() {
                // do nothing.
            }

        }

        InnerClass in = new InnerClass(); // Defined below, and compiles!

    }
}

example 2

public class OuterClass {
    public void foo() {

        InnerClass in = new InnerClass(); // Defined above, does not compile!

        class InnerClass {  

            public InnerClass() {
                // do nothing.
            }

        }

    }
}

Answer 1

Conceptually, local class declaration is a statement. The class doesn’t exist until the statement is executed. The class also disappears when the scope it’s declared in ends. In general a local class may reference other local scope names, so execution ordering is important.

void f()
{
  if(true) 
      throw new Error();

  class InnerClass
  { 
    ... 
  }
}

In this example, InnerClass declaration statement is never reached, so the class will never exist. Referencing it beforehand would be meaningless.

It’s always a good idea to declare something before using it. In Java classes can have circular dependencies so Java must allow referencing classes out of blue – but don’t do it if it can be avoided.