Well, as a Java newbie, I'm in a constant state of confusion.  In the
1.4.2 doc for the HashMap put() method, it says:

<quote>
public Object put(Object key,
                 Object value)

Associates the specified value with the specified key in this map. If
the map previously contained a mapping for this key, the old value is
replaced.
</quote>

So could I trouble you to explain (or point to an explanation) of why
your example and the doc SEEM to contradict each other?

http://java.sun.com/j2se/1.4.2/docs/api/java/util/HashMap.html#put(java.lang.Obj
ect,%20java.lang.Object)

I await humiliation.

Excellent example, Patricia. In the spirit of your
example, here is another one:

/*
* This program illustrates that if equals() is appropriately
* defined, then even a mediocre hashCode() will not prevent a
* hash collection from working correctly (although a mediocre
* hashCode() will defeat the performance purpose of a hash
* collection.
*/

import java.util.HashSet;
import java.util.Set;

public class SillyHashCodeTester {
  private String name;
  private int id;

  public SillyHashCodeTester(String name, int id) {
    this.name = name;
    this.id = id;
  }

  public String toString() {
    return name + ", " + id;
  }

  public boolean equals(Object o) {
    return (
      o instanceof SillyHashCodeTester
      && this.name.equals( ((SillyHashCodeTester)o).name )
      && this.id == ((SillyHashCodeTester)o).id
    );
  }

  //A: Define a silly hashCode():
  public int hashCode() {
    return 42;
  }

  public static void main(String[] args) {
    Set<SillyHashCodeTester> hashSet;
    hashSet = new HashSet<SillyHashCodeTester>();

    System.out.println( "Set size: " + hashSet.size() );
    System.out.println("Add an object to the set.");
    hashSet.add( new SillyHashCodeTester("abc", 100) );
    System.out.println( "Set size: " + hashSet.size() + "\n" );

    System.out.println("Add a non-duplicate object to the set.");
    hashSet.add( new SillyHashCodeTester("abc", 200) );
    System.out.println( "Set size: " + hashSet.size() + "\n" );

    System.out.println("Try to add a duplicate object to the set.");
    hashSet.add( new SillyHashCodeTester("abc", 100) );
    System.out.println( "Set size: " + hashSet.size() + "\n" );

    System.out.println("Try to add a duplicate object to the set.");
    hashSet.add( new SillyHashCodeTester("abc", 200) );
    System.out.println( "Set size: " + hashSet.size() + "\n" );

    System.out.println("Add a non-duplicate object to the set.");
    hashSet.add( new SillyHashCodeTester("xyz", 100) );
    System.out.println( "Set size: " + hashSet.size() + "\n" );
  }
}

/*
This program should print:

Set size: 0
Add an object to the set.
Set size: 1

Add a non-duplicate object to the set.
Set size: 2

Try to add a duplicate object to the set.
Set size: 2

Try to add a duplicate object to the set.
Set size: 2

Add a non-duplicate object to the set.
Set size: 3
*/

George

I beg to differ with you, huddler.
You are NOT stupid--the stupid
persist in their ignorance.

George