I am trying to sort a TreeMap on key. Key is some custom DataStructure having int, List, String, etc.
The member on which I am expecting a sort has some duplicates. Let's say that member is Rank. More than 1 object can have same rank.
Simplified version example:
NOTE: in the CompareTo method below 0 is not returned intentionally to NOT ignore duplicates.(Please correct me if this is not the right way to avoid duplicates)
import java.util.TreeMap;
public class TreeTest {
public static void main(String[] args) {
TreeMap<Custom,String> t = new TreeMap<Custom,String>();
Custom c1 = new Custom();
c1.setName("a");
c1.setRank(0);
Custom c2 = new Custom();
c2.setName("b");
c2.setRank(1);
Custom c3 = new Custom();
c3.setName("c");
c3.setRank(0);
t.put(c1, "first");
t.put(c2, "Second");
t.put(c3, "Third");
System.out.println(t.keySet());
for(Custom c:t.keySet()){
System.out.println(t.get(c));
}
}
}
And Custom Object
package com.example.ui;
public class Custom implements Comparable<Custom>{
int rank;
String name;
public int getRank() {
return rank;
}
public void setRank(int rank) {
this.rank = rank;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((name == null) ? 0 : name.hashCode());
result = prime * result + rank;
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Custom other = (Custom) obj;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (rank != other.rank)
return false;
return true;
}
// 0 is not returned intentionally to NOT ignore duplicates.
public int compareTo(Custom o) {
if(o.rank>this.rank)
return 1;
if(o.rank==this.rank)
return -1;
return -1;
}
}
Output::
[com.example.ui.Custom@fa0, com.example.ui.Custom@fbe, com.example.ui.Custom@f80] null null null
Expected:
First, Second, Third based on Rank 0,1,0 respectively.
I looked at couple of examples on Google. Most of them were basic usage on TreeMap sort using keys or values with primitive datatypes, but none with duplicates when sorting member
is a part of custom key DataStructure.
Please help?
Best Answer
The problem is that your implementation of
compareTo
is not consistent with equals, which is required byTreeMap
. From the API docs:One possible consistent implementation would be to first compare by rank and then by name if the rank values are equal. For two instances of Custom with equal ranks and identical names you should not expect to be able to store them both as keys within the same Map - This violates the contract of Map.