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Study guide: Object-oriented language basics, Part 5

Brush up on Java terms, learn tips and cautions, and review homework assignments

By Jeff Friesen, JavaWorld.com, 08/03/01

Glossary of terms

deep cloning

A process where the clone() method duplicates a composite object's contained objects.

finalization

The act of the garbage collector calling an object's finalize() method to perform cleanup tasks prior to the object's garbage collection.

garbage collector

JVM code dedicated to reclaiming the memory occupied by objects no longer in use.

notification method

A method that wakes up one or more of several waiting threads.

shallow cloning

A process where the clone() method duplicates the contents of instance fields, but does not duplicate the objects referenced by those fields.

waiting method

A method that causes a thread to wait either indefinitely until notified by another thread or for a specific time period if not notified before the time expires.

Tips and cautions

These tips and cautions will help you write better programs and save you from agonizing over why the compiler produces error messages.

Tips

• You can obtain a Class object for any type by appending .class to the type name.
• When you need to compare a class's objects, override that class's equals() method. Do not rely on the default equals() implementation because that implementation uses == to compare object references—not object contents.
• Consider overriding a class's toString() method to return a String object that contains the current values of the class's fields. Those values are useful for debugging purposes.

Cautions

• It is illegal to call Object's getClass() method on a primitive type.
• When assigning a clone() method call's returned reference to a reference variable, you must explicitly cast the reference to the reference variable's type. Otherwise, the compiler reports an error.
• Because you can't clone String objects, attempts to call String's inherited clone() method result in compiler errors.
• Do not use == to compare two objects' contents. The == operator only compares two object references—not the objects' contents.

Reader questions

Find out what questions your fellow readers are asking and my answers to those questions.

Jeff,

I have difficulty understanding the effect or implications of overriding the clone method inside the AnotherClass in Listing 6, CloneDemo2.java. Unfortunately, you didn't explain the reason as to why overriding the clone method produces the result it does. Could you clarify this please?

Heejoon Park

// CloneDemo1.java
class CloneDemo1 implements Cloneable
{
   int instanceField = 3;
   public static void main (String [] args) throws
CloneNotSupportedException
   {
      CloneDemo1 cd1 = new CloneDemo1 ();
      CloneDemo1 x = (CloneDemo1) cd1.clone ();
      System.out.println (x.instanceField);
      AnotherClass ac = new AnotherClass ();
//      AnotherClass y = (AnotherClass) ac.clone ();
   }
}
class AnotherClass
{
   int anotherInstanceField = 2;
}

CloneDemo1.java:17: clone() has protected access in java.lang.Object
           AnotherClass y = (AnotherClass) ac.clone ();

// CloneDemo1.java
class CloneDemo1 implements Cloneable
{
   int instanceField = 3;
   public static void main (String [] args) throws
CloneNotSupportedException
   {
      CloneDemo1 cd1 = new CloneDemo1 ();
      CloneDemo1 x = (CloneDemo1) cd1.clone ();
      System.out.println (x.instanceField);
      AnotherClass ac = new AnotherClass ();
//      AnotherClass y = (AnotherClass) ac.clone ();
   }
}
class AnotherClass
{
   int anotherInstanceField = 2;
   void foo ()
   {
      CloneDemo1 cd1 = new CloneDemo1 ();
      CloneDemo1 cd2 = (CloneDemo1) cd1.clone ();
   }
}

 AnotherClass implements Cloneable
{
   int anotherInstanceField = 2;
   protected Object clone () throws CloneNotSupportedException
   {
      return super.clone ();
   }
}

// Shapes.java
class Point
{
   private double x, y;
   Point (double x, double y)
   {
      this.x = x;
      this.y = y;
   }
   double getX ()
   {
      return x;
   }
   double getY ()
   {
      return y;
   }
}
class Square
{
   private double width;
   Square (double width)
   {
      this.width = width;
   }
   double getWidth ()
   {
      return width;
   }
}
class Rectangle extends Square
{
   private double length;
   Rectangle (double width, double length)
   {
      // Pass the width parameter value to the Square layer, so that 
      // layer can initalize.
      super (width);
      this.length = length;
   }
   double getLength ()
   {
      return length;
   }
}
class Shapes
{
   public static void main (String [] args)
   {
      Square s = new Square (100);
      System.out.println ("s.width = " + s.getWidth ());
      Rectangle r = new Rectangle (50, 25);
      System.out.println ("r.width = " + r.getWidth ());
      System.out.println ("r.length = " + r.getLength ());
   }
}