Can anyone see what is wrong here?

Question

Here we go.....I am working on an assessment at Uni. I have written the following code. It compiles with no errors.....it starts to run (ie it asks for the 2 file names to impliment the equi-join) but it doesnt seem to pass a return value. The program produces no output!!! HELP!!!
private static Map equiJoin(Map relation1, Map relation2)
          Map resultRelation = new TreeMap();
          Iterator keys1Iterator = relation1.keySet().iterator();
          Iterator values1Iterator = relation1.values().iterator();
          Iterator keys2Iterator = relation2.keySet().iterator();
          Iterator values2Iterator = relation2.values().iterator();
          while (keys1Iterator.hasNext())     //loop through relation1
               String primaryKey1 = (String)keys1Iterator.next();
               String foreignKey1 = (String)values1Iterator.next();
               while (keys2Iterator.hasNext())//loop through realtion2
               String primaryKey2 = (String)keys2Iterator.next();
               String foreignKey2 = (String)values2Iterator.next();
               if (foreignKey1 == primaryKey2)
               resultRelation.put(primaryKey1, primaryKey2 + "\t" + foreignKey2);
          return resultRelation;
     }

Answer 1

You have to construct the second iterators inside the first while loop, otherwise you will always check against the value of the last key in the first iterator.
private static Map equiJoin(Map relation1, Map relation2){
     Map resultRelation = new TreeMap();
     Iterator keys1Iterator = relation1.keySet().iterator();
     Iterator values1Iterator = relation1.values().iterator();          
     while (keys1Iterator.hasNext()){
          String primaryKey1 = (String)keys1Iterator.next();
          String foreignKey1 = (String)values1Iterator.next();
          Iterator keys2Iterator = relation2.keySet().iterator();
          Iterator values2Iterator = relation2.values().iterator();
          while (keys2Iterator.hasNext()){          
               String primaryKey2 = (String)keys2Iterator.next();
               String foreignKey2 = (String)values2Iterator.next();
               if (foreignKey1 == primaryKey2)
                    resultRelation.put(primaryKey1, primaryKey2 + "\t" + foreignKey2);
     return resultRelation;
}