What is comparable type casting & Map.Entry?

Question

I am new in Java. I am implementing Comparator interface & i am having a compareTo function below:-

public int compare(Object o1, Object o2) {
    Map.Entry e1 = (Map.Entry) o1;
    Map.Entry e2 = (Map.Entry) o2;
    return ((Comparable) e1.getValue()).compareTo(e2.getValue());
}

I am not be able to understand what does this line means

Map.Entry e1 = (Map.Entry) o1;

What is the use of two Map.Entry ??

Why Comparable in type casting ??

Someone please give me a reference so that i can figure it out. Thanks.

Edited:- Here is my whole class,i am having a HashMap I want to sort by values using Generic class,hence writing this class

package via;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

public class SortMap<K, V> {
    public Map<K, V> getSortedMap(Map<K, V> mapToSort) {
        List<Map.Entry> list = new ArrayList<Map.Entry>(mapToSort.entrySet());
        Collections.sort(list, new Comparator() {

            @Override
            public int compare(Object o1, Object o2) {
                Map.Entry e1 = (Map.Entry) o1;
                Map.Entry e2 = (Map.Entry) o2;
                return ((Comparable) e2.getValue()).compareTo(e1.getValue());
            }

        });
        Map sortedMap = new LinkedHashMap();
        for(Iterator i=list.iterator();i.hasNext();)
        {
            Map.Entry entry=(Map.Entry)i.next();
            sortedMap.put(entry.getKey(), entry.getValue());
        }
        System.out.println(list);
        return sortedMap;
    }

}

Can you post the complete class? And you should implement the parameterized `Comparator` rather than the raw type. — Rohit Jain, Mar 03 '14 at 10:05
It looks like you're implementing the *raw* `Comparator` interface - any reason you're not implementing it as `Comparator>` or whatever? — Jon Skeet, Mar 03 '14 at 10:05
As i mentioned earlier,i am new bee to java so don'w know what is happening here. I am having only little idea about it. — NameNotFoundException, Mar 03 '14 at 10:11
If you are new to Java, then try some simple comparator/comparable example. there are loads of them on Google. — TheLostMind, Mar 03 '14 at 10:13
Why don't you use a TreeMap instead of writing this class? Try this: http://www.tutorialspoint.com/java/java_treemap_class.htm — user2424380, Mar 03 '14 at 10:28
@vicky : sorry, didn't notice. And what about this (see the accepted answer here) : http://stackoverflow.com/questions/12947088/java-treemap-comparator — user2424380, Mar 03 '14 at 10:54

AmitG · Answer 1 · 2014-03-03T12:23:19.093

I am considering here, both K,V are String values. If you have some other values then first your classes for K,V should follow equals() and hashcode() rules. Comparable is not required if you follow the following program.

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class SortMap {
    public Map<String, String> getSortedMap(Map<String, String> mapToSort) {
        Set<Map.Entry<String,String>> mapEntry = mapToSort.entrySet();

        List<Map.Entry<String,String>> listMap= new ArrayList<Map.Entry<String,String>>();
        listMap.addAll(mapEntry);
        Collections.sort(listMap, new Comparator<Map.Entry<String,String>>() {

            @Override
            public int compare(Map.Entry<String,String> o1, Map.Entry<String,String> o2) {
                return  o1.getValue().compareTo(o2.getValue());
            }

        });
        Map<String,String> sortedMap = new LinkedHashMap<String,String>();
        for(Map.Entry<String,String> mapE:listMap)
        {
            sortedMap.put(mapE.getKey(), mapE.getValue());
        }
        System.out.println(sortedMap);
        return sortedMap;
    }

}

Map.Entry is nothing but Entry class written inside HashMap class which implements Entry interface which is written inside Map interface. so there are rules to use inner class references.

static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;
        final int hash;

        /**
         * Creates new entry.
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }
}

Same way i have approached earlier. It is not giving compareTo function to compare the objects values. — NameNotFoundException, Mar 03 '14 at 13:09
Because that reference is of type `Object` and it doesn't have `compareTo` method. — AmitG, Mar 03 '14 at 15:40

What is comparable type casting & Map.Entry?

1 Answers1