Programming

  minte9
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Collections / Comparable

Sort

ArrayList doesn't have a sort method. 
 
/**
 * ArrayList doesn't have a sort method.
 * 
 * There is a sort method in the Collections class ...
 * that takes a List as argument
 */

package com.minte9.collections.comparable;

import java.util.ArrayList;
import java.util.Collections;

public class Sort {
    public static void main(String[] args) {

        ArrayList<String> myList = new ArrayList<>();
        myList.add("AB");
        myList.add("BC");
        myList.add("AD");

        Collections.sort(myList);   // Look Here
        System.out.println(myList); // [AB, AD, BC]
    }
}

Comparable

When you are sorting objects, the compiler doesn't know what to sort. 
 
/**
 * When you are sorting objects, the compiler doesn't know what to sort
 * We need to implements Comparable
 * 
 * Comparator is external to the element type you're comparing ...
 * it is a separate class.
 * 
 * You can make as many of these as you like!
 */

package com.minte9.collections.comparable;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

public class Album {    
    public static void main(String[] args) {

        ArrayList<Song> myList = new ArrayList<Song>();
        myList.add(new Song("A", "1"));
        myList.add(new Song("B", "3"));
        myList.add(new Song("C", "2"));
        
        Collections.sort(myList); 
        System.out.println(myList); // [A:1, B:3, C:2]

        Collections.sort(myList, new ArtistComparator());
        System.out.println(myList); // [A:1, C:2, B:3]
    }
}

class Song implements Comparable<Song> {
        
    String title;
    String artist;
    
    @Override public int compareTo(Song s) {
        return title.compareTo(s.title); // Look Here
            // Uses overrided method toString()
    }
    
    public Song(String t, String a) {
        title = t;
        artist = a;
    }
    
    @Override public String toString() {
        return title + ":" + artist;
    }
}

class ArtistComparator implements Comparator<Song> {
    @Override public int compare(Song first, Song second) {

        return first.artist.compareTo(second.artist); // Look Here
            // second comparison using Comparator
    }
}




References