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Combining composition

Thursday Mar 1st 2001

It is very common to use composition and inheritance together. The following example shows the creation of a more complex class, using both inheritance and composition, along with the necessary constructor initialization:

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and inheritance

It is very common to use composition and inheritance together. The following example shows the creation of a more complex class, using both inheritance and composition, along with the necessary constructor initialization:

//: PlaceSetting.java
// Combining composition & inheritance
 
class Plate {
  Plate(int i) {
    System.out.println("Plate constructor");
  }
}
 
class DinnerPlate extends Plate {
  DinnerPlate(int i) {
    super(i);
    System.out.println(
      "DinnerPlate constructor");
  }
}
 
class Utensil {
  Utensil(int i) {
    System.out.println("Utensil constructor");
  }
}
 
class Spoon extends Utensil {
  Spoon(int i) {
    super(i);
    System.out.println("Spoon constructor");
  }
}
 
class Fork extends Utensil {
  Fork(int i) {
    super(i);
    System.out.println("Fork constructor");
  }
}
 
class Knife extends Utensil {
  Knife(int i) {
    super(i);
    System.out.println("Knife constructor");
  }
}
 
// A cultural way of doing something:
class Custom {
  Custom(int i) {
    System.out.println("Custom constructor");
  }
}
 
public class PlaceSetting extends Custom {
  Spoon sp;
  Fork frk;
  Knife kn;
  DinnerPlate pl;
  PlaceSetting(int i) {
    super(i + 1);
    sp = new Spoon(i + 2);
    frk = new Fork(i + 3);
    kn = new Knife(i + 4);
    pl = new DinnerPlate(i + 5);
    System.out.println(
      "PlaceSetting constructor");
  }
  public static void main(String[] args) {
    PlaceSetting x = new PlaceSetting(9);
  }
} ///:~ 

While the compiler forces you to initialize the base classes, and requires that you do it right at the beginning of the constructor, it doesn’t watch over you to make sure that you initialize the member objects, so you must remember to pay attention to that.

Guaranteeing proper cleanup

Java doesn’t have the C++ concept of a destructor, a method that is automatically called when an object is destroyed. The reason is probably that in Java the practice is simply to forget about objects rather than to destroy them, allowing the garbage collector to reclaim the memory as necessary.

Often this is fine, but there are times when your class might perform some activities during its lifetime that require cleanup. As mentioned in Chapter 4, you can’t know when the garbage collector will be called, or if it will be called. So if you want something cleaned up for a class, you must write a special method to do it explicitly, and make sure that the client programmer knows that they must call this method. On top of this, as described in Chapter 9 (exception handling), you must guard against an exception by putting such cleanup in a finally clause.

Consider an example of a computer-aided design system that draws pictures on the screen:

//: CADSystem.java
// Ensuring proper cleanup
import java.util.*;
 
class Shape {
  Shape(int i) {
    System.out.println("Shape constructor");
  }
  void cleanup() {
    System.out.println("Shape cleanup");
  }
}
 
class Circle extends Shape {
  Circle(int i) {
    super(i);
    System.out.println("Drawing a Circle");
  }
  void cleanup() {
    System.out.println("Erasing a Circle");
    super.cleanup();
  }
}
 
class Triangle extends Shape {
  Triangle(int i) {
    super(i);
    System.out.println("Drawing a Triangle");
  }
  void cleanup() {
    System.out.println("Erasing a Triangle");
    super.cleanup();
  }
}
 
class Line extends Shape {
  private int start, end;
  Line(int start, int end) {
    super(start);
    this.start = start;
    this.end = end;
    System.out.println("Drawing a Line: " +
           start + ", " + end);
  }
  void cleanup() {
    System.out.println("Erasing a Line: " +
           start + ", " + end);
    super.cleanup();
  }
}
 
public class CADSystem extends Shape {
  private Circle c;
  private Triangle t;
  private Line[] lines = new Line[10];
  CADSystem(int i) {
    super(i + 1);
    for(int j = 0; j < 10; j++)
      lines[j] = new Line(j, j*j);
    c = new Circle(1);
    t = new Triangle(1);
    System.out.println("Combined constructor");
  }
  void cleanup() {
    System.out.println("CADSystem.cleanup()");
    t.cleanup();
    c.cleanup();
    for(int i = 0; i < lines.length; i++)
      lines[i].cleanup();
    super.cleanup();
  }
  public static void main(String[] args) {
    CADSystem x = new CADSystem(47);
    try {
      // Code and exception handling...
    } finally {
      x.cleanup();
    }
  }
} ///:~ 

Everything in this system is some kind of Shape (which is itself a kind of Object since it’s implicitly inherited from the root class). Each class redefines Shape’s cleanup( ) method in addition to calling the base-class version of that method using super. The specific Shape classes Circle, Triangle and Line all have constructors that “draw,” although any method called during the lifetime of the object could be responsible for doing something that needs cleanup. Each class has its own cleanup( ) method to restore non-memory things back to the way they were before the object existed.

In main( ), you can see two keywords that are new, and won’t officially be introduced until Chapter 9: try and finally. The try keyword indicates that the block that follows (delimited by curly braces) is a guarded region , which means that it is given special treatment. One of these special treatments is that the code in the finally clause following this guarded region is always executed, no matter how the try block exits. (With exception handling, it’s possible to leave a try block in a number of non-ordinary ways.) Here, the finally clause is saying “always call cleanup( ) for x, no matter what happens.” These keywords will be explained thoroughly in Chapter 9.

Note that in your cleanup method you must also pay attention to the calling order for the base-class and member-object cleanup methods in case one subobject depends on another. In general, you should follow the same form that is imposed by a C++ compiler on its destructors: First perform all of the work specific to your class (which might require that base-class elements still be viable) then call the base-class cleanup method, as demonstrated here.

There can be many cases in which the cleanup issue is not a problem; you just let the garbage collector do the work. But when you must do it explicitly, diligence and attention is required.

Order of garbage collection
There’s not much you can rely on when it comes to garbage collection. The garbage collector might never be called. If it is, it can reclaim objects in any order it wants. In addition, implementations of the garbage collector in Java 1.0 often don’t call the finalize( ) methods. It’s best to not rely on garbage collection for anything but memory reclamation. If you want cleanup to take place, make your own cleanup methods and don’t rely on finalize( ). (As mentioned earlier, Java 1.1 can be forced to call all the finalizers.)

Name hiding

Only C++ programmers might be surprised by name hiding, since it works differently in that language. If a Java base class has a method name that’s overloaded several times, redefining that method name in the derived class will not hide any of the base-class versions. Thus overloading works regardless of whether the method was defined at this level or in a base class:

//: Hide.java
// Overloading a base-class method name
// in a derived class does not hide the
// base-class versions
 
class Homer {
  char doh(char c) {
    System.out.println("doh(char)");
    return 'd';
  }
  float doh(float f) {
    System.out.println("doh(float)");
    return 1.0f;
  }
}
 
class Milhouse {}
 
class Bart extends Homer {
  void doh(Milhouse m) {}
}
 
class Hide {
  public static void main(String[] args) {
    Bart b = new Bart();
    b.doh(1); // doh(float) used
    b.doh('x');
    b.doh(1.0f);
    b.doh(new Milhouse());
  }
} ///:~ 

As you’ll see in the next chapter, it’s far more common to override methods of the same name using exactly the same signature and return type as in the base class. It can be confusing otherwise (which is why C++ disallows it, to prevent you from making what is probably a mistake).

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