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Tip of the Day
Language: Java Language
Expertise: Beginner
Mar 21, 1997



Building the Right Environment to Support AI, Machine Learning and Deep Learning

How Code in Frame Constructs Object Whose Constructor is Declared in Another Fra

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.


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.
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