ArrayList element

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.

Further reading:

Checking If a List Is Sorted in Java

Learn several algorithms for checking whether a list is sorted in Java.

Read more

Java List Initialization in One Line

In this quick tutorial, we'll investigate how can we initialize a List using one-liners.

Read more

2. Setup

First let's start by defining a Customer POJO:

public class Customer { private int id; private String name; // getters/setters, custom hashcode/equals }

Then an ArrayList of customers:

List customers = new ArrayList[]; customers.add[new Customer[1, "Jack"]]; customers.add[new Customer[2, "James"]]; customers.add[new Customer[3, "Kelly"]];

Note that we've overridden hashCodeand equalsin our Customer class.

Based on our current implementation of equals, two Customer objects with the same id will be considered equal.

We'll use this list of customersalong the way.

3. Using Java API

Java itself provides several ways of finding an item in a list:

• Thecontains method
• TheindexOfmethod
• An ad-hoc for loop
• TheStreamAPI

3.1. contains[]

Listexposes a method calledcontains:

boolean contains[Object element]

As the name suggests, this method returns true if the list contains the specified element,and returns false otherwise.

So when we need to check if a specific item exists in our list, we can:

Customer james = new Customer[2, "James"]; if [customers.contains[james]] { // ... }

3.2. indexOf[]

indexOfis another useful method for finding elements:

int indexOf[Object element]

This method returns the index of the first occurrence of the specified element in the given list, or -1 if the list doesn't contain the element.

So logically, if this method returns anything other than -1, we know that the list contains the element:

if[customers.indexOf[james] != -1] { // ... }

The main advantage of using this method is that it can tell us the position of the specified element in the given list.

3.3. Basic Looping

Now what if we want to do a field-based search for an element? For example, say we're announcing a lottery and we need to declare a Customer with a specific name as the winner.

For such field-based searches, we can turn to iteration.

A traditional way of iterating through a list is to use one of Java's looping constructs. In each iteration, we compare the current item in the list with the element we're looking for to see if it's a match:

public Customer findUsingEnhancedForLoop[ String name, List customers] { for [Customer customer : customers] { if [customer.getName[].equals[name]] { return customer; } } return null; }

Here the name refers to the name we are searching for in the given list of customers. This method returns the first Customer object in the list with a matching name, or null if no such Customer exists.

3.4. Looping With an Iterator

Iteratoris another way that we can traverse a list of items.

We can simply take our previous example and tweak it a bit:

public Customer findUsingIterator[ String name, List customers] { Iterator iterator = customers.iterator[]; while [iterator.hasNext[]] { Customer customer = iterator.next[]; if [customer.getName[].equals[name]] { return customer; } } return null; }

Consequently, the behavior is the same as before.

3.5. Java 8 Stream API

As of Java 8, we can alsouse the Stream APIto find an element in a List.

To find an element matching specific criteria in a given list, we:

• invoke stream[] on the list
• call thefilter[]method with a proper Predicate
• call thefindAny[]construct, which returns the first element that matches thefilter predicate wrapped in an Optionalif such an element exists
  

Customer james = customers.stream[] .filter[customer -> "James".equals[customer.getName[]]] .findAny[] .orElse[null];

For convenience, we default to null in case an Optional is empty, but this might not always be the best choice for every scenario.

4. Third-Party Libraries

Now, while the Stream API is more than sufficient, what should we do if we're stuck on an earlier version of Java?

Fortunately, there are many third-party libraries like Google Guava and Apache Commons which we can use.

4.1. Google Guava

Google Guava provides functionality that is similar to what we can do with streams:

Customer james = Iterables.tryFind[customers, new Predicate[] { public boolean apply[Customer customer] { return "James".equals[customer.getName[]]; } }].orNull[];

Just like with Stream API, we can optionally choose to return a default value instead of null:

Customer james = Iterables.tryFind[customers, new Predicate[] { public boolean apply[Customer customer] { return "James".equals[customer.getName[]]; } }].or[customers.get[0]];

The above code will pick the first element in the list if no match is found.

Also, don't forget that Guava throws a NullPointerException if either the list or the predicate is null.

4.2. Apache Commons

We can find an element in almost the exact same way using Apache Commons:

Customer james = IterableUtils.find[customers, new Predicate[] { public boolean evaluate[Customer customer] { return "James".equals[customer.getName[]]; } }];

There are a couple of important differences though:

1. Apache Commons just returns nullif we pass a null list.
2. Itdoesn't provide default value functionality like Guava's tryFind.

5. Conclusion

In this article, we learned different ways of finding an element in a List, starting with quick existence checks and finishing with field-based searches.

We also looked at the third-party librariesGoogle Guava and Apache Commons as alternatives to the Java 8 Streams API.

Thanks for stopping by, and remember to check out all the sources for these examples over onGitHub.