What is a forward reference in Java?

Question

I have been through this question on the legality of forward references, but I don't know what "forward references" means in Java. Can someone please explain with the help of an example?

Answer 1

This is specifically a compilation error. And its all about ordering of class variable declarations. Let's use some code for illustrative purposes:

public class ForwardReference {        
   public ForwardReference() {
      super();
   }

   public ForwardReference echoReference() {
      return this;
   }

   public void testLegalForwardReference() {
      // Illustration: Legal
      this.x = 5;
   }

   private int x = 0;

   // Illustration: Illegal
   private ForwardReference b = a.reference();
   private ForwardReference a = new ForwardReference();
}

As you can see, Java allows you to reference a class variable in a class method, even if the declaration of the variable comes after the method. This is an example of a (legal) forward reference, and support for this is built into the Java compiler.

What you cannot do though, is declare a class variable 'a' that depends on another class variable 'b' that has not been declared yet. Dependent class variable declarations must appear in reverse order of their dependency.

On a tangent, Most, if not all IDE's will warn you if your code contains illegal reference errors.

Illegal forward references are covered in section 8.3.2.3 of the JLS.