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Thinking in Java
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protected

Now that you’ve been introduced to inheritance, the keyword protected finally has meaning. In an ideal world, the private keyword would be enough. In real projects, there are times when you want to make something hidden from the world at large and yet allow access for members of derived classes. The protected keyword is a nod to pragmatism. It says “This is private as far as the class user is concerned, but available to anyone who inherits from this class or anyone else in the same package.” (In Java, protected also provides package access.)

The best approach is to leave the fields private; you should always preserve your right to change the underlying implementation. You can then allow controlled access to inheritors of your class through protected methods:

//: c06:Orc.java
// The protected keyword.
import com.bruceeckel.simpletest.*;
import java.util.*;

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 static Test monitor = new Test();
  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);
    monitor.expect(new String[] {
      "Orc 12: I'm a Villain and my name is Limburger",
      "Orc 19: I'm a Villain and my name is Bob"
    });
  }
} ///:~


You can see that change( ) has access to set( ) because it’s protected. Also note the way that Orc’s toString( ) method is defined in terms of the base-class version of toString( ).
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire