PROGRAMMING

m9/ JAVA
REMEMBERS




Last update:   24-10-2021

HashSets

With HashSet with have no duplicates, but we loose sorting.
 
/**
 * HashSet have no duplicates, but no sorting.
 */

package com.minte9.collections.sets;

import java.util.HashSet;

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

        HashSet<Song> mySet = new HashSet<Song>();
        mySet.add(new Song("A", "2"));
        mySet.add(new Song("C", "1"));
        mySet.add(new Song("B", "4"));
        mySet.add(new Song("B", "3"));
        
        System.out.println(mySet); // [A:2, B:3, C:1]
            // no duplicates
            // hash code
            /*
                65
                67
                66
                66
            */
    }
}

class Song implements Comparable<Song>{

    public String title;
    public String artist;
    
    @Override public boolean equals(Object o) {
        Song s = (Song) o;
        return title.equals(s.title); // Look Here
    }

    @Override public int hashCode() {
        return title.hashCode();  // Look Here
    }
    
    @Override public int compareTo(Song s) {
        return title.compareTo(s.title);
    }
    
    public Song(String t, String a) {
        title = t;
        artist = a;
    }
    
    @Override public String toString() {
        return title + ":" + artist;
    }
}
... 38 lines
 

Trees

TreeSet prevents duplicates and keeps the list sorted.
 
/**
 * TreeSet is similar to HashSet in that it prevents duplicates.
 * But it also keeps the list sorted (very small performance hit).
 */

package com.minte9.collections.sets;

import java.util.ArrayList;
import java.util.TreeSet;

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

        ArrayList<A> myList = new ArrayList<>();
        myList.add(new A("F", "1"));
        myList.add(new A("G", "2"));
        myList.add(new A("H", "4"));
        myList.add(new A("H", "3"));
        
        TreeSet<A> myTree = new TreeSet<A>();
        myTree.addAll(myList);
        System.out.println(myTree); // [F:1, G:2, H:4]
    }
}

class A implements Comparable<A> {

    public String title;
    public String artist;
    
    public A(String t, String a) {
        title = t;
        artist = a;
    }

    @Override public int compareTo(A a) {
        return title.compareTo(a.title);
    }
    
    @Override public String toString() {
        return title + ":" + artist;
    }
}
... 27 lines
 

Questions     github Collections
Maps

        A B C D E F
🔔
1/1