Java/Class/class cast

Class Casting Demo

/*
 * Copyright (c) Ian F. Darwin, http://www.darwinsys.ru/, 1996-2002.
 * All rights reserved. Software written by Ian F. Darwin and others.
 * $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS""
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 * 
 * Java, the Duke mascot, and all variants of Sun"s Java "steaming coffee
 * cup" logo are trademarks of Sun Microsystems. Sun"s, and James Gosling"s,
 * pioneering role in inventing and promulgating (and standardizing) the Java 
 * language and environment is gratefully acknowledged.
 * 
 * The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
 * inventing predecessor languages C and C++ is also gratefully acknowledged.
 */

/** Class Casting Demo. */
public class ClassCast {
  public static void main(String[] argv) {
    CCA anA = new CCA();
    CCB aB = new CCB(1);
    CCC aC = new CCC(2,3);
    System.out.println("A = " + anA);
    System.out.println("B = " + aB);
    System.out.println("C = " + aC);
    CCA aCast = aC;
    System.out.println("aCast = " + aCast);
    CCC anotherC = (CCC) aCast;
    System.out.println("anotherC = " + anotherC);
  }
}
class CCA {
}
class CCB extends CCA {
  int one;
  CCB(int i) {
    one = i;
  }
  public String toString() {
    return "In a B object: " + one;
  }
}
class CCC extends CCB {
  int two;
  CCC(int i, int j) {
    super(i);  // does "one = i" for us.
    two = j;
  }
  public String toString() {
    return "In a C object: " + one + "," + two;
  }
}

Downcasting and Run-Time Type Identification (RTTI)

// : c07:RTTI.java
// Downcasting & Run-Time Type Identification (RTTI).
// {ThrowsException}
// From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002
// www.BruceEckel.ru. See copyright notice in CopyRight.txt.
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 RTTI {
  public static void main(String[] args) {
    Useful[] x = { new Useful(), new MoreUseful() };
    x[0].f();
    x[1].g();
    // Compile time: method not found in Useful:
    //! x[1].u();
    ((MoreUseful) x[1]).u(); // Downcast/RTTI
    ((MoreUseful) x[0]).u(); // Exception thrown
  }
} ///:~