A tutorial on sets in Java; in this tutorial I show you how to use HashSet, TreeSet and LinkedHashSet, plus how to use the Set interface methods and how to do stuff like finding the intersection of two sets.

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Code For This Tutorial

App.java:

import java.util.HashSet;
import java.util.Set;
import java.util.TreeSet;

public class App {

	public static void main(String[] args) {

		// HashSet does not retain order.
		// Set<String> set1 = new HashSet<String>();

		// LinkedHashSet remembers the order you added items in
		// Set<String> set1 = new LinkedHashSet<String>();

		// TreeSet sorts in natural order
		Set<String> set1 = new TreeSet<String>();

		if (set1.isEmpty()) {
			System.out.println("Set is empty at start");
		}

		set1.add("dog");
		set1.add("cat");
		set1.add("mouse");
		set1.add("snake");
		set1.add("bear");

		if (set1.isEmpty()) {
			System.out.println("Set is empty after adding (no!)");
		}

		// Adding duplicate items does nothing.
		set1.add("mouse");

		System.out.println(set1);

		// ///////// Iteration ////////////////

		for (String element : set1) {
			System.out.println(element);
		}

		// ////////// Does set contains a given item? //////////
		if (set1.contains("aardvark")) {
			System.out.println("Contains aardvark");
		}

		if (set1.contains("cat")) {
			System.out.println("Contains cat");
		}

		/// set2 contains some common elements with set1, and some new

		Set<String> set2 = new TreeSet<String>();

		set2.add("dog");
		set2.add("cat");
		set2.add("giraffe");
		set2.add("monkey");
		set2.add("ant");
		
		////////////// Intersection ///////////////////
		
		Set<String> intersection = new HashSet<String>(set1);
		
		intersection.retainAll(set2);
		
		System.out.println(intersection);
		
		////////////// Difference /////////////////////////
		
		Set<String> difference = new HashSet<String>(set2);
		
		difference.removeAll(set1);
		System.out.println(difference);
	}

}

Set is empty at start
[bear, cat, dog, mouse, snake]
bear
cat
dog
mouse
snake
Contains cat
[cat, dog]
[giraffe, ant, monkey]

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