Java/Class/Access Control

Composition with public objects

 
// : c06:Car.java
// Composition with public objects.
// From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002
// www.BruceEckel.ru. See copyright notice in CopyRight.txt.
class Engine {
  public void start() {
  }
  public void rev() {
  }
  public void stop() {
  }
}
class Wheel {
  public void inflate(int psi) {
  }
}
class Window {
  public void rollup() {
  }
  public void rolldown() {
  }
}
class Door {
  public Window window = new Window();
  public void open() {
  }
  public void close() {
  }
}
public class Car {
  public Engine engine = new Engine();
  public Wheel[] wheel = new Wheel[4];
  public Door left = new Door(), right = new Door(); // 2-door
  public Car() {
    for (int i = 0; i < 4; i++)
      wheel[i] = new Wheel();
  }
  public static void main(String[] args) {
    Car car = new Car();
    car.left.window.rollup();
    car.wheel[0].inflate(72);
  }
} ///:~

Create a Singleton Object

 
class MySingleton {
  // the static singleton object
  private static MySingleton theObject;
  private MySingleton() {
  }
  public static MySingleton createMySingleton() {
    if (theObject == null)
      theObject = new MySingleton();
    return theObject;
  }
}
public class Main {
  public static void main(String[] args) {
    MySingleton ms1 = MySingleton.createMySingleton();
    MySingleton ms2 = MySingleton.createMySingleton();
    System.out.println(ms1 == ms2);
  }
}

The protected keyword

 
// : c06:Orc.java
// The protected keyword.
// From "Thinking in Java, 3rd ed." (c) Bruce Eckel 2002
// www.BruceEckel.ru. See copyright notice in CopyRight.txt.

class Villain {
  private String name;
  protected void set(String nm) {
    name = nm;
  }
  public Villain(String name) {
    this.name = name;
  }
  public String toString() {
    return "I"m a Villain and my name is " + name;
  }
}
public class Orc extends Villain {
  private int orcNumber;
  public Orc(String name, int orcNumber) {
    super(name);
    this.orcNumber = orcNumber;
  }
  public void change(String name, int orcNumber) {
    set(name); // Available because it"s protected
    this.orcNumber = orcNumber;
  }
  public String toString() {
    return "Orc " + orcNumber + ": " + super.toString();
  }
  public static void main(String[] args) {
    Orc orc = new Orc("Limburger", 12);
    System.out.println(orc);
    orc.change("Bob", 19);
    System.out.println(orc);
  }
} ///:~

Visibility

 
/*
 * 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.
 */

public class Visibility {
  final int i = 42;
  void method1() {
    // int i = 13;        // would conflict with line below
    for (int i=0; i<10; i++)  // does not corrupt object"s i
      // for (int i=0; i<3; i++) // not allowed, use j
        System.out.println("i = " + i);  // prints 0, 1, ...
    System.out.println("i = " + i);    // prints 42
  }
}