What exactly is a wrapper class?

Question

In so far as Integer, Byte, Double et al. are concerned I understand that those wrapper classes are a way to handle primitive types as objects. But I've come across references to wrapper classes in documentation and other technical articles and I don't think I really understand what they are really used for. Why do you need a wrapper class for an object?
Could someone shed me some light on this issue?
Thanks,
John

Answer 1

An instance of a wrapper class holds a pointer to and instance of some other class or interface and provides access to it, typically supplying methods equivalent to all the methods of the underlying type.
The basic wrapper class a functional replacement for the underlying object.
When you're passed an object from some code outside your control you often don't know what exact class it is, or have access to it's class definition.
A wrapper object allows you to modify the functionality of such an object. You can extend the wrapper class, for example, adding extra methods. Or you can modify the methods, for example to provide logging of calls made.
As a concrete example, I have a wrapper class LoggedConnection which wraps a java.sql.Connection object. It also implements java.sql.Connection. If you ask a LoggedConnection for a Statement or PreparedStatement it fetches the appropriate object and returns a wrapped version which logs the SQL and the execution times etc..
My "get me a connection" routine can return either a raw Connection or a LoggedConnection according to whether a SQL log file is provided.
The main program doesn't need to know if it's using a genuine Connection object or the wrapped version.
Part of the definition would look like:
public class LoggedConnection implements Connection {
  private Connection wrappee;
  public LoggedConnection(Connection con) {
    wrapee = con;
  public createStatement() throws SQLExeception {
    return new LoggedStatement(wrappee.createStatement())
}(Of course this was tedious to write by hand because Connection has so many methods, all of which the wrapper must implement. Later I wrote a java program to generate Wrapper classes).