Implementing hashcode and equals

Question

Is there anything more than performance reasons to always implement hashCode when you implement equals? If I understand correctly, when you put items in a hashtable it chooses the bucket by the hashcode and checks there is no equal item with the equals method. So it should still work if you never implement the hashcode method. But is there any other reason to implement hashCode than performance? Or have I just understood this completely wrong?

Answer 1

Everything you say is true. And your example shows that point well. However, what if I just implement a hashCode that always returns 0? How will this work then? And does it have any differences than performance?
Here is an example
import junit.framework.*;
import java.util.*;
public class EqualsTest extends TestCase
  public void testHashCode()
    Map hashTable = new Hashtable();
    hashTable.put( new TestItem( 0 ), "0" );
    hashTable.put( new TestItem( 1 ), "1" );
    assertEquals( 2, hashTable.size() );
    Set hashSet = new HashSet();
    hashSet.add( new TestItem( 0 ));
    hashSet.add( new TestItem( 1 ) );
    assertEquals( 2, hashSet.size() );
  private class TestItem
    private final int i;
    public TestItem( int i )
      this.i = i;
    public boolean equals( Object o )
      final TestItem testItem = ( TestItem ) o;
      return i == testItem.i;
    public int hashCode()
      return 0;
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