Map sorted by Key

why comparator ?

• comparable : adj

• comparator : n

Some class can't be changed , which like Map , Set and their subclass . They are given being unchangeable . Thus , we can't

public TreeMap implements comparable{
...
}

So , here comparator works .

// mapCompare is the class with comparator()
TreeMap<String,Integer> map = new TreeMap<String,Integer>( new mapCompare() ); 

// The configure  <String>  is same as the key of *TreeMap*
class mapCompare implements Comparator<String > {
public int compare( String o1, String o2){
   o1.compareTo(o2);
}

Code

import java.util.Comparator;
import java.util.Iterator;
import java.util.Map;
import java.util.TreeMap;

public class mapSortedByKey {
	public static void main(String[] args) {
		
		// New one map
		
		TreeMap<String, Integer> map = new TreeMap<String, Integer>(new mapCompare());
		map.put("E", 1);
		map.put("A", 2);
		map.put("B", 3);
		map.put("C", 4);
		map.put("D", 5);
		
		// print 
		/*for( Map.Entry<String,Integer> one: map.entrySet() ){
			System.out.println(one.getKey()+":"+one.getValue());
		}
		*/
		
		Iterator<Map.Entry<String,Integer> > iter = map.entrySet().iterator();
		while(iter.hasNext()){
			Map.Entry<String,Integer> one = iter.next();
			System.out.println(one.getKey() +": "+one.getValue());
		}
	}
}

class mapCompare implements Comparator<String> {
	public int compare(String o1, String o2) {
		return o2.compareTo(o1);
	}
}

E: 1
D: 5
C: 4
B: 3
A: 2