Java HashSet contains() method example

java.util.HashSet contains()

Description

On this document we will be showing a java example on how to use the contains(Object o) method of HashSet Class. Basically this method check the HashSet object if it contains the specified method argument Object. A boolean returned value will indicate if the set is contains the parameter Object or not.It will return true if it is, otherwise false. Make a note that a HashSet is unsorted thus the order of elements are random, better watch out on this concept.

The HashSet as majority of the Collection classes make use of Generics such that if we have declared our set like HashSet set = new HashSet() then the elements of inside our set is of type String. And we cannot insert different object type not unless its a subclass of the object declaration, otherwise a runtime error will be thrown.

The HashSet elements are unique, there would be no two identical elements that can exist.

Important notes for isEmpty() method:

specified by contains in interface Collection<E>
specified by contains in interface Set<E>
overrides contains in class AbstractCollection<E>

Method Syntax

public boolean contains(Object o)

Method Argument

Data Type	Parameter	Description
Object	o	element whose presence in this set is to be tested

Method Returns

The contains(Object o) method returns true if this set contains the specified element.

Compatibility

Requires Java 1.2 and up

Java HashSet contains() Example

Below is a java code demonstrates the use of contains() method of HashSet class. The example presented might be simple however it shows the behavior of the contains() method.

package com.javatutorialhq.java.examples;

import static java.lang.System.*;

import java.util.HashSet;
import java.util.Scanner;


/*
 * This example source code demonstrates the use of  
 * contains() method of HashSet class
 */

public class HashSetContainsExample {

	public static void main(String[] args)  {

		// get the HashMap object from the method init()
				HashSet studentSet = init();
				
				// ask user input
				out.println("Enter student name:");
				Scanner s = new Scanner(System.in);
				String name = s.nextLine();
				s.close();
						
				//check if the user input is in the student db
				
				if(studentSet.contains(name)){
					out.println(name +" found on student list");
				}
				
				else{
					out.println(name +" name not on the student list");
				}


		
	}

	private static HashSet init() {
		// declare the HashSet object
		HashSet studentSet = new HashSet<>();
		// put contents to our HashMap
		studentSet.add("Shyra Travis");
		studentSet.add("Sharon Wallace");
		studentSet.add("Leo Batista");
		
		return studentSet;
	}

}

There are two methods created on the above example, main() and init. The main method calls the init() which initializes and assign values to a HashSet object. The main() method assign to a new HashSet object the returned object by the init() method. As you would have noticed we have used the Scanner class to get user input and then we check it on our current set if it is in there. The discussion on the use of scanner is out of scope however you can go to our scanner tutorial for more information.

Sample Output

Below is the sample output when you run the above example.

References

Oracle Java Documentation