Wrapper class for HashMap

Question

Why people write Wrapper class for HashMap? Why not directly use the HashMap?

Answer 1

I thought that the term wrapper class is generally
used to signify one class having an instance of
another class inside it. And not one class
deriving from another.
But thats just me.I think that is just you. :)
I consider "wrapping" as having a subset of the functionality of another class. This could be containing or extending, in my opinion.
Consider:
// only partially functional vector for JButtons...
// class containing Vector
public class JButtonVector
  Vector vec = new Vector();
  public void add(JButton button)
    vec.add(button);
  public JButton get(int index)
    if(index >= vec.size() || index < 0)
      throw new IllegalArgumentException(...);
    return (JButton)vec.get(index);
// class extending Vector
class JButtonVector2 extends Vector
  public void add(JButton but)
    super.add(but);
  public JButton get(int index)
    return (JButton)(super.get(index));
}Problem with the second class is that it also allows adding of regular Objects. That might not be desirable (in this case, it is definitely not).