Exception is being thrown

Question

I tried out this small code for using hashset but it gives exceptions. I have no idea why is it so?
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
class Students implements Comparable
     private String name;
     private float gpa = 0.0F;
     Students(String n, float g)
          name = n;
          gpa = g;
     Students(){}
     public String getName() {
          return name;
     public float getGpa() {
          return gpa;
     @Override
     public int compareTo(Object o) {
          // TODO Auto-generated method stub
          if(this.getGpa() > ((Students)o).getGpa())
               return 1;
          else if(this.getGpa() < ((Students)o).getGpa())
               return -1;
          else
               return 0;
     public boolean equals(Object o){
          if(this.getGpa() == ((Students)o).getGpa())
               return true;
          else
               return false;
     public int hashCode(){
          return (int)(10*gpa);
public class HashFunction {
     public static void main(String args[]){
          Students s1= new Students("Fred", 3.0F);
          Students s2 = new Students("Sam", 3.1F);
          Students s3 = new Students("Steve", 3.5F);
          Set s = new HashSet();
          s.add(s1);
          s.add(s2);
          s.add(s3);
          Iterator i = s.iterator();
          while(i.hasNext())
               System.out.println(((Students)i.next()).getName() + " " + ((Students)i.next()).getGpa());
}The exception trace that is generated is as follows:
Exception in thread "main" java.util.NoSuchElementException
     at java.util.HashMap$HashIterator.nextEntry(HashMap.java:796)
     at java.util.HashMap$KeyIterator.next(HashMap.java:828)
     at HashFunction.main(HashFunction.java:68)

Answer 1

roaan wrote:
I had one more question. I am reading from the file like this
          String word;
          word = file.nextWord();
          try{
               while(word != null)
                    word = word.toLowerCase();
                    wf.put(word);
                    word = file.nextWord();
          catch(Exception e){
               e.printStackTrace();
     }Now when the end of the file is reached word contains null and throws a nullpointer exception. I am catching that exception but is there a way out that when my word is null i should simply exit out of the loop (i check that word is not equal to null in my while loop but still it throws an excpetion).Nothing there can throw NPE from word being null. The only possibilities of NPE in the code you posted are
1. if file is null
2. if wf is null
3. if NPE is being thrown inside nextWord(), which has nothing to do with word being null.
Once you make sure that the above are not happening, that code will be fine and you won't need to catch NPE. And in fact, catching NPE like that is bad practice anyway.