Combining composition

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

and
inheritance

//: 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

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.

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.


Order
of garbage collection

Name
hiding

//: 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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