Question

Here’s Pair.java

import java.lang.*;  import java.util.*;   public class Pair<TYPEA, TYPEB> implements Comparable< Pair<TYPEA, TYPEB> > {   protected final TYPEA Key_;   protected final TYPEB Value_;    public Pair(TYPEA key, TYPEB value) {     Key_   = key;     Value_ = value;   }   public TYPEA getKey() {     return Key_;   }   public TYPEB getValue() {     return Value_;   }   public String toString() {     System.out.println('in toString()');     StringBuffer buff = new StringBuffer();       buff.append('Key: ');       buff.append(Key_);       buff.append('\tValue: ');       buff.append(Value_);     return(buff.toString() );   }   public int compareTo( Pair<TYPEA, TYPEB> p1 ) {      System.out.println('in compareTo()');     if ( null != p1 ) {        if ( p1.equals(this) ) {          return 0;        } else if ( p1.hashCode() > this.hashCode() ) {              return 1;       } else if ( p1.hashCode() < this.hashCode() ) {          return -1;         }     }     return(-1);   }   public boolean equals( Pair<TYPEA, TYPEB> p1 ) {      System.out.println('in equals()');     if ( null != p1 ) {        if ( p1.Key_.equals( this.Key_ ) && p1.Value_.equals( this.Value_ ) ) {          return(true);       }     }     return(false);   }   public int hashCode() {      int hashCode = Key_.hashCode() + (31 * Value_.hashCode());     System.out.println('in hashCode() [' + Integer.toString(hashCode) + ']');     return(hashCode);   } } 

Here’s the testcase:

import java.lang.*;  import java.util.*;  import junit.framework.*;  public class PairTest extends TestCase {     public void testPair() {      String key   = new String('key');      String value = new String('asdf');       Pair<String, String> pair = new Pair<String, String>( key, value );       assertTrue( pair.getKey().equals( key ) );     assertTrue( pair.getValue().equals( value ) );     assertTrue( pair.equals( new Pair<String, String>(key, value)) );   }    public void testPairCollection() {       HashMap< Pair<String, String>, String> hm1 = new HashMap<Pair<String,String>, String>();       Pair<String, String> p1 = new Pair<String, String>('Test1', 'Value1');         hm1.put(p1, 'ONE');       Pair<String, String> p2 = new Pair<String, String>('Test1', 'Value2');         hm1.put(p2, 'TWO');       Pair<String, String> p3 = new Pair<String, String>('Test2', 'Value1');         hm1.put(p3, 'THREE');         Pair<String, String> p4 = new Pair<String, String>('Test2', 'Value2');         hm1.put(p4, 'FOUR');      Pair<String, String> p5 = new Pair<String, String>('Test3', 'Value1');         hm1.put(p5, 'FIVE');      Pair<String, String> p6 = new Pair<String, String>('Test3', 'Value2');         hm1.put(p6, 'SIX');       Pair<String, String> p7 = new Pair<String, String>('Test3', 'Value3');         hm1.put(p7, 'SEVEN');          assertTrue( hm1.size() == 7 );       Pair<String, String> pSrch = new Pair<String, String>('Test3', 'Value3');      assertTrue( p7.equals(pSrch) );     assertTrue( pSrch.equals(p7) );     assertTrue( p7.hashCode() == pSrch.hashCode() );      assertTrue( 0 == p7.compareTo( pSrch ) );     assertTrue( 0 == pSrch.compareTo(p7) );      System.out.println('starting containsKey search');     assertTrue( hm1.containsKey( p7 ) );     System.out.println('starting containsKey search2');     assertTrue( hm1.containsKey( pSrch ) );     System.out.println('finishing containsKey search');      String result = hm1.get( pSrch );     assertTrue( null != result );     assertTrue( 0 == result.compareTo('SEVEN'));    }  } 

Here’s my problem, the last hm1.containsKey call should (I naively expect) return the value stored where Pair<‘Three’, ‘Three’> is true – I should get a String with a value of ‘SEVEN’. Here is the output:

Running in equals() in hashCode() [1976956095] in hashCode() [1976956126] in hashCode() [1976956096] in hashCode() [1976956127] in hashCode() [1976956097] in hashCode() [1976956128] in hashCode() [1976956159] in equals() in equals() in hashCode() [1976956159] in hashCode() [1976956159] in compareTo() in equals() in compareTo() in equals() starting containsKey search in hashCode() [1976956159] starting containsKey search2 in hashCode() [1976956159]     <--- Bug here?  Never reaches            String result = hm1.get( pSrch ); 

So is both p7.hashCode() and pSrch.hashCode() are equal and p7.equals(pSrch) and pSrch.equals(p7), and hm1.containsValue(p7) == true, I would expect hm1.containsValue(pSrch) would also return true, but it does not. What am I missing?

Answer 1

You need to override the equals method from the java.lang.Object class.

Instead, you’ve overloaded the method with an additional version that takes a Pair. Totally different method that never gets called. Replace your equals with something like this:

@Override public boolean equals(Object o) {    System.out.println('in equals()');   if (o instanceof Pair) {      Pair<?, ?> p1 = (Pair<?, ?>) o;     if ( p1.Key_.equals( this.Key_ ) && p1.Value_.equals( this.Value_ ) ) {        return(true);     }   }   return(false); } 

To avoid this kind of mistake, use the @Override annotation on methods you intend to act as overrides. You’ll get a compile time error when they don’t.