Cloning and StringBuffering

[Dennis Noren]

I can make StringBuffer a verb, can't I?

I am implementing a cloneable interface, and it mostly seems to work correctly, except for the following.

I have a class Shape, with data elements
Color color;
StringBuffer description;

The clones seem to work correctly, except that when I try to run my setDesc method to change the description on the clone, it changes the description on the original also. My setColor method does not show the same behavior.

public void setColor(Color col) { color = col;}
public void setDesc(String desc) {
description.delete(0,description.length());
description.append(desc);
}

Here is my clone override method (there is also a rectangle subclass):

      public Object clone (){
try {
         Shape newShape = (Shape)super.clone();
         if (this.rect != null)
            newShape.rect = (Rectangle)this.rect.clone();
return newShape;
}
catch (CloneNotSupportedException e) {
System.out.println("Cloning failed");
return this;
}
      }

and here is my equals override method:

// override equals method from Object class
public boolean equals (Object obj) {
if ((obj != null) && (obj.getClass().equals(this.getClass()))) {
if (((Shape)obj).description == this.description  &&
((Shape)obj).color == this.color)
return true;// content matches completely
else
return false;// content does not match completely
}else{
return false;// from different classes, or object is null
}
}

I create and clone as follows:

// clone and then change description data of clone
Shape shape4 = (Shape) shape3.clone();
shape4.setDesc("short desc");

And then I print shape3 and shape4, and they both have the new text in the description data member. Any idea why this is happening?

[Ernest Friedman-Hill]

The default implementation of clone() (the one you get by calling super.clone()) does a "shallow copy" -- the reference variables in the object are copied, but not the objects they point to. This means that after you call clone(), both the old and new Shape objects share the same StringBuffer object to hold their description, which obviously gives you the problem you're observing.

Solution: do the same thing with description that you do with rect!