Inheritance syntax

Bruce Eckel's Thinking in Java Contents | Prev | Next

//: Detergent.java
// Inheritance syntax & properties
 
class Cleanser {
  private String s = new String("Cleanser");
  public void append(String a) { s += a; }
  public void dilute() { append(" dilute()"); }
  public void apply() { append(" apply()"); }
  public void scrub() { append(" scrub()"); }
  public void print() { System.out.println(s); }
  public static void main(String[] args) {
    Cleanser x = new Cleanser();
    x.dilute(); x.apply(); x.scrub();
    x.print();
  }
}
 
public class Detergent extends Cleanser {
  // Change a method:
  public void scrub() {
    append(" Detergent.scrub()");
    super.scrub(); // Call base-class version
  }
  // Add methods to the interface:
  public void foam() { append(" foam()"); }
  // Test the new class:
  public static void main(String[] args) {
    Detergent x = new Detergent();
    x.dilute();
    x.apply();
    x.scrub();
    x.foam();
    x.print();
    System.out.println("Testing base class:");
    Cleanser.main(args);
  }
} ///:~ 

This demonstrates a number of features. First, in the Cleanser append( ) method, Strings are concatenated to s using the += operator, which is one of the operators (along with ‘ +’) that the Java designers “overloaded” to work with Strings.

Initializing the base class

//: Cartoon.java
// Constructor calls during inheritance
 
class Art {
  Art() {
    System.out.println("Art constructor");
  }
}
 
class Drawing extends Art {
  Drawing() {
    System.out.println("Drawing constructor");
  }
}
 
public class Cartoon extends Drawing {
  Cartoon() {
    System.out.println("Cartoon constructor");
  }
  public static void main(String[] args) {
    Cartoon x = new Cartoon();
  }
} ///:~ 

The output for this program shows the automatic calls:

Art constructor
Drawing constructor
Cartoon constructor

You can see that the construction happens from the base “outward,” so the base class is initialized before the derived-class constructors can access it.

Even if you don’t create a constructor for Cartoon( ), the compiler will synthesize a default constructor for you that calls the base class constructor.

Constructors with arguments
//: Chess.java
// Inheritance, constructors and arguments
 
class Game {
  Game(int i) {
    System.out.println("Game constructor");
  }
}
 
class BoardGame extends Game {
  BoardGame(int i) {
    super(i);
    System.out.println("BoardGame constructor");
  }
}
 
public class Chess extends BoardGame {
  Chess() {
    super(11);
    System.out.println("Chess constructor");
  }
  public static void main(String[] args) {
    Chess x = new Chess();
  }
} ///:~ 

If you don’t call the base-class constructor in BoardGame( ), the compiler will complain that it can’t find a constructor of the form Game( ). In addition, the call to the base-class constructor must be the first thing you do in the derived-class constructor. (The compiler will remind you if you get it wrong.)

Catching base constructor exceptions



Comments

  • Great for beginners!

    Posted by Sam on 10/01/2012 02:31pm

    The examples are really good for the beginners.

    Reply
Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • With JRebel, developers get to see their code changes immediately, fine-tune their code with incremental changes, debug, explore and deploy their code with ease (both locally and remotely), and ultimately spend more time coding instead of waiting for the dreaded application redeploy to finish. Every time a developer tests a code change it takes minutes to build and deploy the application. JRebel keeps the app server running at all times, so testing is instantaneous and interactive.

  • It's time high-level executives and IT compliance officers recognize and acknowledge the danger of malicious insiders, an increased attack surface and the potential for breaches caused by employee error or negligence. See why there is extra emphasis on insider threats.

Most Popular Programming Stories

More for Developers

Latest Developer Headlines

RSS Feeds