Java - Collection Framework Set
Introduction
Set interface models a set in mathematics, which is a collection of unique elements.
A set cannot contain duplicate elements.
Java allows at most one null element in a Set.
Set does not guarantee the ordering of the elements.
HashSet class is an implementation for the Set interface.
The following code creates a Set and adds elements to it.
If you attempt to add duplicate elements to a Set and they are ignored silently.
Two elements in a Set are considered equal if the equals() method returns true.
Demo
import java.util.HashSet; import java.util.Set; public class Main { public static void main(String[] args) { // Create a set Set<String> s1 = new HashSet<>(); // Add a few elements s1.add("XML"); s1.add("Json"); s1.add("Java"); s1.add("Java"); // Duplicate!!! No effect // Create another set by copying s1 Set<String> s2 = new HashSet<>(s1); // Add a few more elements s2.add("Python"); s2.add("Ruby"); s2.add(null); // one null is fine s2.add(null); // Duplicate!!! No effect // Print the sets System.out.println("s1: " + s1); System.out.println("s1.size(): " + s1.size()); System.out.println("s2: " + s2); System.out.println("s2.size(): " + s2.size()); }/*from ww w . j a v a2s . c o m*/ }
Result
