Downcasting and Run-Time Type Identification (RTTI) : Polymorphism « Class Definition « Java Tutorial

1. Java Tutorial
2. Class Definition
3. Polymorphism

class Useful {
  public void f() {
  }

  public void g() {
  }
}

class MoreUseful extends Useful {
  public void f() {
  }

  public void g() {
  }

  public void u() {
  }

  public void v() {
  }

  public void w() {
  }
}

public class MainClass {
  public static void main(String[] args) {
    Useful[] x = { new Useful(), new MoreUseful() };
    x[0].f();
    x[1].g();

    // x[1].u();
    ((MoreUseful) x[1]).u(); // Downcast/RTTI
    ((MoreUseful) x[0]).u(); // Exception thrown
  }
}