TreeMap vs HashMap

[Raymond Fung]

Can anyone please tell me why in this program; I can retrieve "A" value from TreeMap but not HashMap.

It returns null value with HashMap.

Is there anyway that I can fix it?

Thanks

import java.util.*;
 
public class Test2 {
 
    static void says(Map<APerson, String> map){
 
        APerson a = new APerson("A");
        System.out.println("A says - " + map.get(a));
    }
 
 
    public static void main(String[] args) {
 
        SortedMap<APerson, String> sortedSayings = new TreeMap<APerson, String>();
        sortedSayings.put(new APerson("A"), "I am A");
        sortedSayings.put(new APerson("B"), "I am B");
        says(sortedSayings);

        Map<APerson, String> sayings = new HashMap<APerson, String>();
        sayings.put(new APerson("A"), "I am A");
        sayings.put(new APerson("B"), "I am B");
        says(sayings);
    }
 
    static class APerson implements Comparable<APerson>{
        String name;
 
        public APerson(String name) {
            this.name = name;
        }
 
        public boolean equals(APerson o) {
          if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
 
            final APerson p = (APerson) o;
            return name.equals(p.name);
        }
                     
        // Needed if this object is to be used as a key in a TreeMap
        public int compareTo(APerson o) {
            return this.name.compareTo(o.name);
        }
    }  
}

---------------------------Result---------------------------------
A says - I am A
A says - null

[I added [code] tags - Jim]
[ September 06, 2005: Message edited by: Jim Yingst ]

[Ernest Friedman-Hill]

Hi,

Welcome to JavaRanch!

HashMap uses the hashCode() method, and expects it to return values "consistent with equals()". What this means is that if you override equals() for a class, you always need to override hashCode() as well, so that two objects that are equal() will have the same hashCode() return value.

In your case, an excellent implementation would just delegate to "name" -- i.e.,

public int hashCode() {
  return name.hashCode();
}

Since your equals() method is also based solely on the "name" member, this will do the exactly right thing.

[Jim Yingst]

public boolean equals(APerson o) {

This is a subtle but serious problem. You have not overridden the equals(Object) method here - you have merely overloaded it.

[Ernest Friedman-Hill]

Originally posted by Jim Yingst:

public boolean equals(APerson o) {

This is a subtle but serious problem.

Good eye! Thanks, Jim.

[Raymond Fung]

Thanks guys!!!

It works now