Polymorphism vs interfaces

Question

Hi,
I'm extending an interface that overrides a doStuff() method. When I call the method, the most specific method is not used. I've provided a code example. Do you know how I can achieve polymorphic behaviour using this kind of interface heirarchy?
public interface Stuff  {
  public void doStuff(Object o);
public interface FireGrainedStuff extends Stuff {
  public void doStuff(Integer i);
public class InterfaceTesting implements FireGrainedStuff {
  public static void main(String[] args) {
    InterfaceTesting tester = new InterfaceTesting();
    tester.runTest();
  public void runTest() {
    InterfaceTesting test1 = new InterfaceTesting();
    testIt(test1,new Object());
    testIt(test1,new Integer(0));
    Stuff test2 = new InterfaceTesting();
    testIt(test2,new Object());
    testIt(test2,new Integer(0));
    FireGrainedStuff test3 = new InterfaceTesting();
    testIt(test3,new Object());
    testIt(test3,new Integer(0));
  public void testIt(Stuff stuff, Object arg) {
    stuff.doStuff(arg);
  public void doStuff(Object o) {
    System.out.println("doStuff(Object o)");
  public void doStuff(Integer i) {
    System.out.println("doStuff(Integer i)");
// OUTPUT:
doStuff(Object o)
doStuff(Object o)
doStuff(Object o)
doStuff(Object o)
doStuff(Object o)
doStuff(Object o)

Answer 1

public void testIt(Stuff stuff, Object arg) {
stuff.doStuff(arg);
public void doStuff(Object o) {
System.out.println("doStuff(Object o)");
public void doStuff(Integer i) {
System.out.println("doStuff(Integer i)");
The problem is with your overloaded methods 'doStuff' in your InterfaceTesting class. Overloaded methods are not polymorphic. Which overloaded method gets called is determined by it's compile time types. Since the method 'testIt' is taking an argument of type Object, it's going to call the 'doStuff(Object o)' method .