Java ArrayList contains() method
Syntax and Examples

Introduction

The contains() method in Java's ArrayList is a handy way to check if an element already exists within your list. Think of it like looking through a box of toys – you want to quickly see if the toy car you’re holding is already in the box.

Syntax

public boolean contains(Object o)

Parameters

Parameter	Description
o	The element you want to check for within the list. It can be any object, as long as it's properly compared for equality (using equals() method).

Return Value

The contains() method returns a boolean value:

• true: If the element is found within the list.
• false: If the element is not found in the list.

Examples

Example 1: Checking for a String

This example demonstrates how to use contains() with a list of strings.

import java.util.ArrayList;

public class ContainsExample {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");

        String searchName = "Bob";

        if (names.contains(searchName)) {
            System.out.println(searchName + " is in the list.");
        } else {
            System.out.println(searchName + " is not in the list.");
        }
    }

Bob is in the list.

Explanation: We create an ArrayList of strings called names. We then use contains() to see if “Bob” exists in the list. Since it does, the message “Bob is in the list.” is printed.

Example 2: Checking for an Integer

This example shows how to check for an integer within an ArrayList of Integers.

import java.util.ArrayList;

public class ContainsIntegerExample {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);

        int searchNumber = 25;

        if (numbers.contains(searchNumber)) {
            System.out.println(searchNumber + " is in the list.");
        } else {
            System.out.println(searchNumber + " is not in the list.");
        }
    }

25 is not in the list.

Explanation: We create an ArrayList of integers named numbers. We then try to find 25 using contains(). Since 25 isn't present, we print “25 is not in the list.”

Example 3: Using a Custom Object

This example demonstrates how to use contains() with objects of a custom class.

import java.util.ArrayList;

class Person {
    String name;

    public Person(String name) {
        this.name = name;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj instanceof Person) {
            return this.name.equals(((Person) obj).name);
        } else {
            return false;
        }
    }

    @Override
    public int hashCode(){
        return name.hashCode(); //important for proper equals implementation
    }
}

public class CustomObjectContainsExample {
    public static void main(String[] args) {
        ArrayList<Person> people = new ArrayList<>();
        people.add(new Person("Alice"));
        people.add(new Person("Bob"));

        Person searchPerson = new Person("Charlie");

        if (people.contains(searchPerson)) {
            System.out.println(searchPerson.name + " is in the list.");
        } else {
            System.out.println(searchPerson.name + " is not in the list.");
        }
    }

Charlie is not in the list.

Explanation: Here, we're using a custom Person class. Importantly, we’ve overridden the equals() method to compare Person objects based on their names. Without overriding equals, ArrayList would use object reference equality which is almost certainly not what you want when comparing objects with the same content.

Important Considerations

The contains() method relies on the equals() method of the objects being compared. Make sure that your custom classes have properly implemented equals() and hashCode() methods for accurate comparisons. If you don't, you might get unexpected results.