ArrayList

Syntax

ArrayList<Type> list = new ArrayList<>();

Examples

import java.util.ArrayList;

ArrayList<String> fruits = new ArrayList<>();
fruits.add("apple");
fruits.add("banana");
fruits.add("cherry");

System.out.println(fruits);         // [apple, banana, cherry]
System.out.println(fruits.size());  // 3

import java.util.ArrayList;

ArrayList<String> names = new ArrayList<>();
names.add("Fola");
names.add("Zain");
names.add("Jamal");

System.out.println(names.get(1));   // Zain

names.set(1, "Priya");               // replace element at index 1
System.out.println(names);           // [Fola, Priya, Jamal]

names.remove("Jamal");                // remove by value
names.remove(0);                       // remove by index
System.out.println(names);            // [Priya]

import java.util.ArrayList;

ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(10);
numbers.add(20);
numbers.add(30);

for (int num : numbers) {
    System.out.println(num);
}

// Checking membership and finding an index
System.out.println(numbers.contains(20));  // true
System.out.println(numbers.indexOf(30));    // 2

import java.util.ArrayList;
import java.util.Collections;

ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(5);
numbers.add(1);
numbers.add(3);

Collections.sort(numbers);
System.out.println(numbers);  // [1, 3, 5]

numbers.sort(Collections.reverseOrder());
System.out.println(numbers);  // [5, 3, 1]

Best practices

• Use ArrayList when you need fast index-based access and mostly add elements at the end - avoid frequent inserts/removals in the middle, which are slow
• Declare the variable using the List interface (List<String> list = new ArrayList<>();) so the implementation can be swapped later without changing calling code
• Use an enhanced for-each loop for reading, but a regular Iterator (or indexed loop counting down) when removing elements while iterating, to avoid a ConcurrentModificationException
• Specify an initial capacity (new ArrayList<>(100)) when you know roughly how many elements to expect, to reduce internal resizing

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation