boolean addAll(Collection c);

package in.bench.resources.java.collections;

import java.util.HashSet;

public class AddOneHashSetToAnotherHashSet {

	public static void main(String[] args) {

		// 1: creating HashSet object of type String
		HashSet<String> hset1 = new HashSet<String>();

		// adding elements to HashSet object
		hset1.add("Sundar Pichai");
		hset1.add("Satya Nadella");
		hset1.add("Shiv Nadar");
		hset1.add("Shantanu Narayen");

		// Iterating using enhanced for-loop
		System.out.println("Iterating original HashSet-1\n");
		for(String str : hset1) {
			System.out.println(str);
		}

		// 2: creating HashSet object of type String
		HashSet<String> hset2 = new HashSet<String>();

		// adding elements to HashSet object
		hset2.add("Narayan Murthy");
		hset2.add("Nandan Nilekeni");
		hset2.add("Shibulhset SD");

		boolean boolAddAll = hset1.addAll(hset2);
		System.out.println("\n\nWhether invoking HashSet-1 "
				+ "changed : " + boolAddAll);

		// Iterating using enhanced for-loop
		System.out.println("\n\nIterating after addAll operation\n");
		for(String str : hset1) {
			System.out.println(str);
		}
	}
}

Iterating original HashSet-1

Shantanu Narayen
Shiv Nadar
Sundar Pichai
Satya Nadella

Whether invoking HashSet-1 changed : true

Iterating after addAll operation

Shantanu Narayen
Shiv Nadar
Sundar Pichai
Nandan Nilekeni
Shibulhset SD
Satya Nadella
Narayan Murthy