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

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” parameterpassed to a constructor is a pointer to the memory area allocated by thecall 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 implicitparameter 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 isconsidering nested classes in a future release of Java.

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