Summary: In this lab, you will experiment with implementations of methods for collections.

Preparation

Exercises

Exercise 1

  /** Adds all of the elements in the specified collection to this 
   *  collection. */
  public boolean addAll(Collection<? extends AnyType> c);

  /** Removes all this collection's elements that are also contained
   *  in the specified collection. */
  public boolean removeAll(Collection<?> c);

  /** Returns true if this collection contains all of the elements in
   *  the specified collection.  */
  public boolean containsAll(Collection<?> c);

1. Add the above methods to the Collection interface in package weiss.util.
2. Implement the above methods in AbstractCollection in the same package.
3. Recompile the book code using Build from the JDE menu.
4. Test the implementation of your methods by writing a small driver program that creates two instances of collections from the book code (e.g., use weiss.util.LinkedList and its add method) and uses one of the above methods on them.

   Note: Remember that your program will have to know where to find the weiss packages. In an earlier lab, you modified your .emacs file so that JDE knew this. If you try to compile your driver program from the command line, you will have to manually specify it with the -classpath argument to javac.

Exercise 2

• Both objects implement the List interface AND
• Contain the same items (as determined by equals) in precisely the same order,

• Both objects implement the Set interface AND
• Contain the same items (as determined by equals) in any order.

1. Why is it impossible for a Collection to implement both the Set and the List interface?
2. Add an implementation of equals to AbstractCollection that adheres to the specification above.
3. Test your implementation by writing a small driver program.

For those with extra time

• Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
• If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

-- Object (Java 2 Platform SE 5.0), hashCode