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Tip of the Day
Language: Java Language
Expertise: Beginner
Mar 20, 1997

Does Java Have Equivalent to C++ Copy Constructor?

Question:
I want a copy of an instance not a reference, e.g.(Image). I know of the clone method, but not many classes have it. Along those lines, does Java pass objects to methods as references or copies -- as to a constructor?

Answer:
I like this question because it raises several interesting issues.

If x and y are Java objects, the assignment x = y merely causes x and y to refer to the same object. For example, altering a member of y alters the corresponding member of x. In contrast, if x and y are C++ objects, the assignment x = y assigns to x a copy of y made either by the default member-wise copy algorithm, or by a user-defined operator= method. Thus, altering a member of y has no effect on the corresponding member of x.

Copies of a C++ object, x, will also be made when:

  1. x is passed as a parameter,
  2. a variable is initialized to x,
  3. x is returned as a function value.
In each case, the copy is made by either the default member-wise copy algorithm or a user-defined copy constructor (i.e. a constructor that takes x as a parameter).

By contrast, if x is a Java object, x will be passed by reference to any function. This means the function's formal parameter will be merely an alias for x. To put it another way, Java functions (methods) don't get private copies of their object parameters like their C++ counterparts do. (They get private copies of their non-object parameters, though.)

If x is an instance of a class that has a copy constructor, this can be used to initialize variables in the style of C++. For example, assume x and y are instances of class Robot, then we copy x into y with:

y = new Robot(x); 
If the class Robot doesn't have a copy constructor, we can copy x into y if the class Robot specifically provides some other copy method. One technique is to provide a method called clone that returns an Object:
Object clone(Robot r) { ... } 
If only a bit-wise copy of r is needed, the clone method inherited from the class Object can be invoked inside clone:
try {
   return super.clone();
  } catch(CloneNotSupported e) {}; 
The copy returned by clone can then be cast as a Robot:
y =
(Robot) x.clone(); 
This will raise the CloneNotSupported exception if Robot does not implement the Cloneable interface:
class Robot
implements Cloneable { ... } 
Unfortunately, the class Object does not implement Cloneable, hence we cannot automatically clone an object without going through the hoops described above.

In short, if you want to copy an instance of a class that doesn't provide a copy constructor or a specific method for copying, you need to create a subclass that either provides a copy method implemented by you, or implements Cloneable and provides a clone method that invokes the bitwise super.clone() (or overrides it with your algorithm).

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