generics

[maggie karve]

Ques from Examlab...

 package Coll;

import java.util.*;

public class PQT{
     public static void main(String args[]){
          Map<Ad,Ad> mp=new TreeMap<Ad,Ad>(new CP());
          mp.put(new Ad("L"),new Ad("U"));
          mp.put(new Ad("T"),new Ad("O"));
          System.out.println(mp);
     }
}

class Ad{
     String gen;
     Ad(String gen){
          this.gen=gen;
     }
     public int hashCode(){
          return gen.toUpperCase().hashCode();
     }
     public boolean equals(Object ob){
          return ((Ad)ob).gen.equalsIgnoreCase(this.gen);
     }
     public String toString(){
          return "<"+gen+">";
     }
}

class CP implements Comparator{
     public int compareTo(Object a, Object b){
          return 1;
     }

	@Override
	public int compare(Object arg0, Object arg1) {
		// TODO Auto-generated method stub
		return 0;
	}
}   

output is
{<L>=<O>}???

[Deepak Bala]

output is
{<L>=<O>}???

Do you have a question to go along with the code ? What output were you expecting ?

[maggie karve]

why is it not printing all the values???

[Deepak Bala]

Analyze this code and discern the output. That will answer your question

  Map<Ad,Ad> mp=new TreeMap<Ad,Ad>(new CP());
mp.put(new Ad("L"),new Ad("U"));
Entry<Ad, Ad> next = mp.entrySet().iterator().next();
System.out.println(next.getKey() + " " + next.getValue());
mp.put(new Ad("T"),new Ad("O"));
next = mp.entrySet().iterator().next();
System.out.println(next.getKey() + " " + next.getValue());
mp.put(new Ad("X"),new Ad("Y"));
next = mp.entrySet().iterator().next();
System.out.println(next.getKey() + " " + next.getValue());
System.out.println(mp);  

[Harpreet Singh janda]

Your overridden method is always returning 0,

  
@Override  
    public int compare(Object arg0, Object arg1) {  
        // TODO Auto-generated method stub  
        return 0;  
    }  
  

which means whatever will you pass for TreeMap both the values will be equal. So each time when you pass the new element to TreeMap, it will compare the key and will found them same (Because of compare method implementation) and it will replace the existing value with new one.

If you try it with

    mp.put(new Ad("T"),new Ad("O")); 

as the first entry of map then the output will be

{<T>=<Y>}

[Harpreet Singh janda]

If you will change your code like

   class Ad{
     String gen;
     Ad(String gen){
          this.gen=gen;
     }
     public int hashCode(){
          return gen.toUpperCase().hashCode();
     }
     public boolean equals(Object ob){
          return ((Ad)ob).gen.equalsIgnoreCase(this.gen);
     }
     public String toString(){
          return "<"+gen+">";
     }
     
    // Changed
    public int compareTo(Ad ad2)	{
    	 return gen.compareTo(ad2.gen);
     }
}

class CP implements Comparator<Ad>{
       // Changed
        public int compare(Ad arg0, Ad arg1) {
			return ((Ad)arg0).compareTo((Ad)arg1);
	}
} 

It will give the result you were expecting

[Simran Dass]

This is a question on Collections.
Kindly put it under the topic Collections and not Generics next time.