How do you check if an ArrayList of objects contains a value in Java?

The problem

The reason why your approach did not work is because the List#contains (documentation) method uses the result of Person#equals to decide if an element from the list is the same as the argument. If you have not implemented this method than it searches the method in the inheritance tree of Person. As every object inherits from the class Object you will fall back to Object#equals (documentation) if no class on the way to Object has implemented equals. However this Object#equals method compairs objects by their identity (place in the memory). That means that if you create an object that has the same properties (like name and so on) it just will be a copy but not the same as the one in the list, per identity.

Table of Contents Show

The problem
1. ArrayList contains() syntax
“how to check if an arraylist of objects contains a value in java” Code Answer
Java answers related to “how to check if an arraylist of objects contains a value in java”
Java queries related to “how to check if an arraylist of objects contains a value in java”
1. Overview
Arraylist.contains() in Java
ArrayList contains() Method

Note that there are many questions here at StackOverflow for this topic. If you're not familiar with it you should check it out. Here is an example explaining it in more detail: What issues should be considered when overriding equals and hashCode in Java?

There are multiple ways of solving this problem. Let's first consider a very straightforward and easy way.

1. ArrayList contains() syntax

The contains() method is pretty simple. It simply checks the index of element in the list. If the index is greater than '0' than element is present in the list.

public boolean contains(Object o) { return indexOf(o) >= 0; }

“how to check if an arraylist of objects contains a value in java” Code Answer

java list contains object with property

java by The Red Five on Feb 19 2020 Donate Comment

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1. Overview

Finding an element in a list is a very common task we come across as developers.

In this quick tutorial, we'll cover different ways we can do this with Java.

Arraylist.contains() in Java

ArrayList contains() method in Java is used for checking if the specified element exists in the given list or not.

Syntax:

public boolean contains(Object) object-element to be searched for

Parameters:
object- element whose presence in this list is to be tested

Returns:
It returns true if the specified element is found in the list else it returns false.

Code#1: Demonstrate the working of the method contains() in integer

// Java code to demonstrate the working of 
// contains() method in ArrayList 

// for ArrayList functions 

import java.util.ArrayList;

class GFG {

public static void main(String[] args)

{

// creating an Empty Integer ArrayList

ArrayList<Integer> arr = new ArrayList<Integer>(4);

// using add() to initialize values

// [1, 2, 3, 4]

arr.add(1);

arr.add(2);

arr.add(3);

arr.add(4);

// use contains() to check if the element

// 2 exits or not

boolean ans = arr.contains(2);

if (ans)

System.out.println("The list contains 2");

else

System.out.println("The list does not contains 2");

// use contains() to check if the element

// 5 exits or not

ans = arr.contains(5);

if (ans)

System.out.println("The list contains 5");

else

System.out.println("The list does not contains 5");

}
}

Output: The list contains 2 The list does not contains 5

Code#2: Demonstrate the working of the method contains() in string

// Java code to demonstrate the working of 
// contains() method in ArrayList of string 

// for ArrayList functions 

import java.util.ArrayList;

class GFG {

public static void main(String[] args)

{

// creating an Empty String ArrayList

ArrayList<String> arr = new ArrayList<String>(4);

// using add() to initialize values

// ["geeks", "for", "geeks"]

arr.add("geeks");

arr.add("for");

arr.add("geeks");

// use contains() to check if the element

// "geeks" exits or not

boolean ans = arr.contains("geeks");

if (ans)

System.out.println("The list contains geeks");

else

System.out.println("The list does not contains geeks");

// use contains() to check if the element

// "coding" exits or not

ans = arr.contains("coding");

if (ans)

System.out.println("The list contains coding");

else

System.out.println("The list does not contains coding");

}
}

Output: The list contains geeks The list does not contains coding

Practical Application:
In search operations, we can check if a given element exists in a list or not.

Reference:
Oracle Docs

Article Tags :

Java

Java - util package

Java-ArrayList

Java-Collections

java-list

Practice Tags :

Java

Java-Collections

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ArrayList contains() Method

Below is the syntax of the contains() method, defined in ArrayList class:

public boolean contains(Object o)

This method takes one object as its parameter. It checks if this object is in the ArrayList or not.It returns one boolean value. If the ArrayList contains at least one element, then it returns true. Else it returns false. It tries to find out at least one element vsuch that (o == null ? v == null : o.equals(v)). i.e. if ois null,it returns trueonly if one element isnull in the ArrayList. If ois not null, it returns trueonly if at least one element equal to v.