Extending Classes - The Java Programming Language - page 155

first.push(9);
IntegerStack second = first.clone();
With the default clone method, the data in memory will look something
like this:
Now consider what happens when future code invokes first.pop() , fol-
lowed by first.push(17) . The top element in the stack first will change
from 9 to 17 , which is expected. The programmer will probably be sur-
prised, however, to see that the top element of second will also change
to 17 because there is only one array that is shared by the two stacks.
The solution is to override clone to make a copy of the array:
public IntegerStack clone() {
try {
IntegerStack nObj = (IntegerStack) super.clone();
nObj.buffer = buffer.clone();
return nObj;
} catch (CloneNotSupportedException e) {
// Cannot happen -- we support
// clone, and so do arrays
throw new InternalError(e.toString());
}
}