new Comparator<K>() isn't Comparator an interface

[Ines Mew]

Hi,
this is probably not a new question . This is someone's code that I am trying to grasp.
I do not get how is it possible to use

Comparator<K> valueComparator = new Comparator<K>()

.

1) Comparator is an interface and I should not be able to create an instance of interface?
2) why should it return 1

if (compare == 0) 
					return 1;

public static <K, V extends Comparable<V>> Map<K, V> sortByValues(final Map<K, V> map) {

		//START of anonymous class
		Comparator<K> valueComparator = new Comparator<K>() {
			public int compare(K k1, K k2) {
				
				//using compareTo() same as for Comparable interface 
				int compare = map.get(k1).compareTo(map.get(k2));//compare VALUES 
				if (compare == 0) 
					return 1;
				//Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
				//the implementor must ensure that compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.
				else 
					return compare;
			}
		};

[Dave Tolls]

From the comments int hat code:
//START of anonymous class

That bit in the curly braces is defining an anonymous class that implements the Comparator interface's method.

If you look at the files produced when it's compiled you'll see a <YourClassName>.class file, which is the compiled version of your class, but also a <YourClassName>$1.class, which is the anonymous class.

[Knute Snortum]

And welcome to the Ranch, Ines. Did Dave's explanation make sense to you? This is an idiom you will probably see a lot of in old code, and it is confusing, so it's a good thing to fully understand it.

[Campbell Ritchie]

Ines ilovebears wrote: . . . This is someone's code that I am trying to grasp. . . .

So at least you are not to blame for this:-

if (compare == 0) 
					return 1;

. . .

[Ines Mew]

Hi Campbell, Dave, Knute

Thanks for your answers..I am still not clear on the whole topic of anonymous classes and generics but I feel like I am getting some progress. On a scale from 1 to 10 I went
from 0 to 2.. or maybe 1

How about this line:

//using TreeMap(Comparator c) constructor. Constructs a new, empty tree map, ordered according to the given comparator.
		Map<K, V> sortedByValues = new TreeMap<K, V>(valueComparator);

1) new TreeMap(Comparator<T>) it creates an empty map ->this new map is ORDERED based on the rules I specified in
my Comparator -> then I take my original UNORDERED map and put it into this new map. Correct?

2) on line 56

sorted.put("AL",22);

I am trying to add another entry to my ORDERED map. I get NPE. Why?

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



public class MapValueComparator 
{
	public static <K, V extends Comparable<V>> Map<K, V> sortByValues(final Map<K, V> map) {

		//START of anonymous class
		Comparator<K> valueComparator = new Comparator<K>() {
			public int compare(K k1, K k2) {
				
				//using compareTo() same as for Comparable interface 
				int compare = map.get(k2).compareTo(map.get(k1));//compare VALUES 
				if (compare == 0) 
					return 1;
				//Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
				//the implementor must ensure that compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.
				else 
					return compare;
			}
		}; //END of anonymous class 

		//using TreeMap(Comparator c) constructor. Constructs a new, empty tree map, ordered according to the given comparator.
		Map<K, V> sortedByValues = new TreeMap<K, V>(valueComparator);
		sortedByValues.putAll(map);
		return sortedByValues;
	}
	
}

class TestMapValueComparator {

	public static void main(String[] args) {
		
		Map <String, Integer> map= new TreeMap<String, Integer>();
		map.put("CA", 6);
		map.put("NY", 3);
		map.put("NJ", 8);
		map.put("MN", 5);
		map.put("CA", 5);//cannot have dup key; will keep ONE key and replace value 6 with 5
		
		System.out.println("Original map:"+"\t");

		for (Map.Entry<String, Integer> entry:map.entrySet())
		{
			System.out.println(entry);
		}
		
		System.out.println("=============================");
		
		Map<String, Integer> sorted=MapValueComparator.sortByValues(map);

		sorted.put("AL",22);
		System.out.println("Sorted map:"+"\t");

		for (Map.Entry<String, Integer> entry:sorted.entrySet())
		{
			System.out.println(entry);
		}
	}
}