Re: HashTable

George wrote:


Here's a different approach to explaining what I think is going on. I've
written two trivial classes, NoEquals and HasEquals. Each has two String
fields. The difference is that NoEquals inherits equals and hashCode
from Object. HasEquals has a trivial, dummy hashCode and an equals that
treats HasEquals instances with equal fields as being equal.

I also wrote a test method that takes a pair of references and does some
tests of the sorts of things that I gather, from your messages, are
going on in your program, constructing a Set of instances of the class
and using it as a Map key. It is called three times, with a pair of
NoEquals objects, with an equal pair of HasEquals objects, and with an
unequal pair of HasEquals objects.

The test demonstrates how the results of the Set contains and Map
containsKey methods depend on the underlying equals implementation in my
classes. If this program does not explain what is going on in your
code, try to modify it to demonstrate the problem and post the result.

Output:

NoEquals objects
left == right is false
left.equals(right) is false
s1.contains(right) is false
m.containsKey(s1) is false

equal HasEquals objects
left == right is false
left.equals(right) is true
s1.contains(right) is true
m.containsKey(s1) is true

unequal HasEquals objects
left == right is false
left.equals(right) is false
s1.contains(right) is false
m.containsKey(s1) is false

Source code - copy to an EqualsDemo.java file:

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

public class EqualsDemo {

   static void test(Object left, Object right) {
     System.out.printf("left == right is %b%n", left == right);
     System.out.printf("left.equals(right) is %b%n", left
         .equals(right));
     Set<Object> s1 = new HashSet<Object>();
     s1.add(left);
     System.out.printf("s1.contains(right) is %b%n", s1
         .contains(right));
     Map<Object, String> m = new HashMap<Object, String>();
     m.put(s1, "Dummy string");
     Set<Object> s2 = new HashSet<Object>();
     s2.add(right);
     System.out.printf("m.containsKey(s1) is %b%n", m
         .containsKey(s2));
   }

   public static void main(String[] args) {
     System.out.println("NoEquals objects");
     test(new NoEquals("a", "b"), new NoEquals("a", "b"));
     System.out.println();
     System.out.println("equal HasEquals objects");
     test(new HasEquals("a", "b"), new HasEquals("a", "b"));
     System.out.println();
     System.out.println("unequal HasEquals objects");
     test(new HasEquals("a", "b"), new HasEquals("a", "B"));
   }
}

class NoEquals {
   String field1;
   String field2;
   NoEquals(String field1, String field2) {
     this.field1 = field1;
     this.field2 = field2;
   }
}

class HasEquals {
   String field1;
   String field2;
   HasEquals(String field1, String field2) {
     this.field1 = field1;
     this.field2 = field2;
   }
   public boolean equals(Object obj) {
     if (obj == null || !(obj instanceof HasEquals)) {
       return false;
     } else {
       HasEquals other = (HasEquals) obj;
       return field1.equals(other.field1)
           && field2.equals(other.field2);
     }
   }

   /*
    * Dummy hashCode for testing equals, do not use in production
    * code.
    */
   public int hashCode() {
     return 0;
   }
}