Check If Element Exists in Java HashSet Example (contains)

This example shows how to check if an element exists in HashSet in Java. This example also shows how to check if the HashSet contains the element using the contains method.

How to check if element exists in Java HashSet using the contains method?

The contains method of the HashSet class returns true if the specified object exists in the HashSet.

1	public boolean contains(Object o)

The contains method returns true if the HashSet contains the specified element, false otherwise.

import java.util.HashSet;

import java.util.Set;

public class JavaHashSetContainsExample {

public static void main(String[] args) {

Set<String> hSetOddNumbers = new HashSet<String>();

hSetOddNumbers.add("One");

hSetOddNumbers.add("Three");

hSetOddNumbers.add("Five");

hSetOddNumbers.add("Seven");

* To check if the element exists in the HashSet, use

* the contains method

//this will return true as HashSet contains element "Three"

System.out.println( hSetOddNumbers.contains("Three") );

//this will return false as HashSet does not contain element "Four"

System.out.println( hSetOddNumbers.contains("Four") );

}

Output

1 2	true false

How to check if an object of a custom class exists in the HashSet?

The HashSet contains method relies on the equals and hashCode methods to check if the set contains the specified element. For the HashSet of custom class objects, if the custom class does not override the equals and hashCode methods then the contains method fails to find the specified object.

import java.util.HashSet;

import java.util.Set;

class Account{

private int accountNumber;

public Account(int accountNumber){

this.accountNumber = accountNumber;

}

public String toString(){

return "Account: " + this.accountNumber;

}

public class JavaHashSetContainsExample {

public static void main(String[] args) {

Set<Account> hSetAccounts = new HashSet<Account>();

hSetAccounts.add(new Account(100));

hSetAccounts.add(new Account(101));

hSetAccounts.add(new Account(102));

System.out.println( hSetAccounts.contains(new Account(101)) );

}

Output

false

As we can see from the output, even if the account with id 101 exists in the HashSet object, the contains method returned false. When the custom class does not override the equals and hashCode methods, methods inherited from the Object class are used. The equals method of the Object class compares the object references not the actual object content and thus returns false.

In order for the contains method to work properly, we need to implement the equals and hashCode methods in our custom class as given below.