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Language: Java Language
Expertise: Beginner

Mar 21, 1997

Question:
How can code in one frame construct an object whose constructor is declared in another frame? For example, new parent.SomeObject(); does not seem to work.

Why?

Answer:
Good question! As in C++, constructors in Java are not considered methods. They don't return values, and the implicit "this" parameter passed to a constructor is a pointer to the memory area allocated by the call to new immediately preceding the constructor. Thus,

new Robot(x, y, z);

is roughly equivalent to the C code:

   Robot *a;
   a = malloc(sizeof(Robot));
   Robot(a, x, y, z);

 

Assume the class Robot looks like this:

   class Robot {
      Robot(...) { ... } // Robot constructor
      SomeObject(...) { ... } // your SomeObject "constructor"
      // etc.
   }

and here's your declaration of parent:

Robot parent = new Robot(...);

Unfortunately for you, Java regards SomeObject as an ordinary Robot method, not a constructor. Thus, the call

new parent.SomeObject(x, y, z);

would (if it worked at all) pass a pointer to parent as the implicit parameter of SomeObject, not the space allocated by new:

SomeObject(parent, x, y, z);

This picture could become murky in the near future; I hear Sun is considering nested classes in a future release of Java.