Java ArrayList

last modified February 23, 2024

In this article we show how to work with ArrayList collection in Java. Located in the java.util package, ArrayList is an important collection of the Java collections framework.

Java collections framework is a unified architecture for representing and manipulating collections, enabling collections to be manipulated independently of implementation details. A collection is an object that represents a group of objects.

ArrayList definition

ArrayList is an ordered sequence of elements. It is dynamic and resizable. It provides random access to its elements. Random access means that we can grab any element at constant time. An ArrayList automatically expands as data is added. Unlike simple arrays, an ArrayList can hold data of multiple data types. It permits all elements, including null.

Elements in the ArrayList are accessed via an integer index. Indexes are zero-based. Indexing of elements and insertion and deletion at the end of the ArrayList takes constant time.

An ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. As elements are added to an ArrayList, its capacity grows automatically. Choosing a proper capacity can save some time.

The add method

Single elements can be added to an ArrayList with the add method.

import java.util.ArrayList;
import java.util.List;

void main() {

    List<String> langs = new ArrayList<>();

    langs.add("Java");
    langs.add("Python");
    langs.add(1, "C#");
    langs.add(0, "Ruby");

    for (String lang : langs) {

        System.out.printf("%s ", lang);
    }

    System.out.println();
}

The example adds elements to an array list one by one.

List<String> langs = new ArrayList<>();

An ArrayList is created. The data type specified inside the diamond brackets (< >) restricts the elements to this data type; in our case, we have a list of strings.

langs.add("Java");

An element is appended at the end of the list with the add method.

langs.add(1, "C#");

This time the overloaded add method inserts the element at the specified position; The "C#" string will be located at the second position of the list; remember, the ArrayList is an ordered sequence of elements.

for (String lang : langs) {

    System.out.printf("%s ", lang);
}

With the for loop, we go through the ArrayList list and print its elements.

$ java Main.java
Ruby Java C# Python

Note that the elements keep the order they were inserted.

The List.of method

Since Java 9, we have a couple of factory methods for creating lists having a handful of elements. The created list is immutable.

import java.util.List;

void main() {

    var words = List.of("wood", "forest", "falcon", "eagle");
    System.out.println(words);

    var values = List.of(1, 2, 3);
    System.out.println(values);
}

In the example, we create two lists that have four and three elements.

The get and size methods

The get returns the element at the specified position in this list and the size returns the size of the list.

import java.util.ArrayList;
import java.util.List;

void main() {

    List<String> colours = new ArrayList<>();

    colours.add("blue");
    colours.add("orange");
    colours.add("red");
    colours.add("green");

    String col = colours.get(1);
    System.out.println(col);

    int size = colours.size();

    System.out.printf("The size of the ArrayList is: %d%n", size);
}

The example uses the get and size methods of the ArrayList

String col = colours.get(1);

The get method returns the second element, which is "orange".

int size = colours.size();

The size method determines the size of our colours list; we have four elements.

$ java Main.java
orange
The size of the ArrayList is: 4

The copy method

A copy of a list can be generated with List.copy method.

import java.util.List;

void main() {

    var words = List.of("forest", "wood", "eagle", "sky", "cloud");
    System.out.println(words);

    var words2 = List.copyOf(words);
    System.out.println(words2);
}

The example creates a copy of a list with List.copy.

Raw ArrayList

An ArrayList can contain various data types. These are called raw lists.

Raw lists often require casts and they are not type safe.

import java.util.ArrayList;
import java.util.List;

class Base {}

enum Level {

    EASY,
    MEDIUM,
    HARD
}

void main() {

    Level level = Level.EASY;

    List da = new ArrayList();

    da.add("Java");
    da.add(3.5);
    da.add(55);
    da.add(new Base());
    da.add(level);

    for (Object el : da) {

        System.out.println(el);
    }
}

The example adds five different data types into an array list — a string, double, integer, object, and enumeration.

List da = new ArrayList();

When we add multiple data types to a list, we omit the angle brackets.

$ java Main.java
Java
3.5
55
com.zetcode.Base@659e0bfd
EASY

The addAll method

The following example uses the addAll method to add multiple elements to a list in one step.

import java.util.ArrayList;
import java.util.List;

void main() {

    List<String> colours1 = new ArrayList<>();

    colours1.add("blue");
    colours1.add("red");
    colours1.add("green");

    List<String> colours2 = new ArrayList<>();

    colours2.add("yellow");
    colours2.add("pink");
    colours2.add("brown");

    List<String> colours3 = new ArrayList<>();
    colours3.add("white");
    colours3.add("orange");

    colours3.addAll(colours1);
    colours3.addAll(2, colours2);

    for (String col : colours3) {

        System.out.println(col);
    }
}

Two lists are created. Later, the elements of the lists are added to the third list with the addAll method.

colours3.addAll(colours1);

The addAll method adds all of the elements to the end of the list.

colours3.addAll(2, colours2);

This overloaded method adds all of the elements starting at the specified position.

$ java Main.java
white
orange
yellow
pink
brown
blue
red
green

Modifying elements

The next example uses methods to modify the ArrayList.

import java.util.ArrayList;
import java.util.List;

void main() {

    List<String> items = new ArrayList<>();
    fillList(items);

    items.set(3, "watch");
    items.add("bowl");
    items.remove(0);
    items.remove("pen");

    for (Object el : items) {

        System.out.println(el);
    }

    items.clear();

    if (items.isEmpty()) {

        System.out.println("The list is empty");
    } else {
        System.out.println("The list is not empty");
    }
}

void fillList(List<String> data) {

    data.add("coin");
    data.add("pen");
    data.add("pencil");
    data.add("clock");
    data.add("book");
    data.add("spectacles");
    data.add("glass");
}

An ArrayList is created and modified with the set, add, remove, and clear methods.

items.set(3, "watch");

The set method replaces the fourth element with the "watch" item.

items.add("bowl");

The add method adds a new element at the end of the list.

items.remove(0);

The remove method removes the first element, having index 0.

items.remove("pen");

The overloaded remove method remove the first occurrence of the "pen" item.

items.clear();

The clear method removes all elements from the list.

if (items.isEmpty()) {

The isEmpty method determines if the list is empty.

$ java Main.java
pencil
watch
book
spectacles
glass
bowl
The list is empty

The removeIf method

The removeIf method removes all of the elements of a collection that satisfy the given predicate.

import java.util.ArrayList;
import java.util.List;

void main() {

    List<Integer> values = new ArrayList<>();
    values.add(5);
    values.add(-3);
    values.add(2);
    values.add(8);
    values.add(-2);
    values.add(6);

    values.removeIf(val -> val < 0);

    System.out.println(values);
}

In our example, we have an ArrayList of integers. We use the removeIf method to delete all negative values.

values.removeIf(val -> val < 0);

All negative numbers are removed from the array list.

$ java Main.java
[5, 2, 8, 6]

The removeAll method

The removeAll method removes from this list all of its elements that are contained in the specified collection. Note that all elements are removed with clear.

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

void main() {

    List<String> letters = new ArrayList<>();
    letters.add("a");
    letters.add("b");
    letters.add("c");
    letters.add("a");
    letters.add("d");

    System.out.println(letters);

    letters.removeAll(Collections.singleton("a"));
    System.out.println(letters);
}

In the example, we remove all "a" letters from the list.

The replaceAll method

The replaceAll method replaces each element of a list with the result of applying the operator to that element.

import java.util.ArrayList;
import java.util.List;
import java.util.function.UnaryOperator;

void main() {

    List<String> items = new ArrayList<>();
    items.add("coin");
    items.add("pen");
    items.add("cup");
    items.add("notebook");
    items.add("class");

    UnaryOperator<String> uo = (x) -> x.toUpperCase();

    items.replaceAll(uo);

    System.out.println(items);
}

The example applies an operator on each of the list elements; the elements' letters are transformed to uppercase.

UnaryOperator<String> uo = (x) -> x.toUpperCase();

A UnaryOperator that transforms letters to uppercase is created.

items.replaceAll(uo);

The operator is applied on the list elements with the replaceAll method.

$ java Main.java
[COIN, PEN, CUP, NOTEBOOK, CLASS]

The second example uses the replaceAll method to capitalize string items.

import java.util.ArrayList;
import java.util.List;
import java.util.function.UnaryOperator;

class MyOperator<T> implements UnaryOperator<String> {

    @Override
    public String apply(String s) {

        if (s == null || s.length() == 0) {
            return s;
        }

        return s.substring(0, 1).toUpperCase() + s.substring(1);
    }
}

void main() {

    List<String> items = new ArrayList<>();

    items.add("coin");
    items.add("pen");
    items.add("cup");
    items.add("notebook");
    items.add("glass");

    items.replaceAll(new MyOperator<>());

    System.out.println(items);
}

We have a list of string items. These items are capitalized with the help of the replaceAll method.

class MyOperator<T> implements UnaryOperator<String> {

A custom UnaryOperator is created.

@Override
public String apply(String s) {

    if (s == null || s.length() == 0) {
        return s;
    }

    return s.substring(0, 1).toUpperCase() + s.substring(1);
}

Inside the UnaryOperator's apply method, we return the string with its first letter in uppercase.

items.replaceAll(new MyOperator<>());

The operator is applied on the list items.

$ java Main.java
[Coin, Pen, Cup, Notebook, Glass]

The contains method

The contains method returns true if a list contains the specified element.

import java.util.ArrayList;
import java.util.List;

void main() {

    List<String> items = new ArrayList<>();

    items.add("coin");
    items.add("pen");
    items.add("cup");
    items.add("notebook");
    items.add("class");

    String item = "pen";

    if (items.contains(item)) {

        System.out.printf("There is a %s in the list%n", item);
    }
}

The example checks if the specified item is in the list.

if (items.contains(item)) {

    System.out.printf("There is a %s in the list%n", item);
}

The message is printed if the item is in the list.

$ java Main.java
There is a pen in the list

Getting index of elements

Each of the elements in an ArrayList has its own index number. The indexOf returns the index of the first occurrence of the specified element, or -1 if the list does not contain the element. The lasindexOf returns the index of the last occurrence of the specified element, or -1 if the list does not contain the element.

import java.util.ArrayList;
import java.util.List;

void main() {

    List<String> colours = new ArrayList<>();

    colours.add(0, "blue");
    colours.add(1, "orange");
    colours.add(2, "red");
    colours.add(3, "green");
    colours.add(4, "orange");

    int idx1 = colours.indexOf("orange");
    System.out.println(idx1);

    int idx2 = colours.lastIndexOf("orange");
    System.out.println(idx2);
}

The example prints the first and last index of the "orange" element.

$ java Main.java
1
4

List of lists

We can add other lists into a list.

import java.util.ArrayList;
import java.util.List;

void main() {

    List<Integer> l1 = new ArrayList<>();
    l1.add(1);
    l1.add(2);
    l1.add(3);

    List<Integer> l2 = new ArrayList<>();
    l2.add(4);
    l2.add(5);
    l2.add(6);

    List<Integer> l3 = new ArrayList<>();
    l3.add(7);
    l3.add(8);
    l3.add(9);

    List<List<Integer>> nums = new ArrayList<>();
    nums.add(l1);
    nums.add(l2);
    nums.add(l3);

    System.out.println(nums);

    for (List<Integer> list : nums) {

        for (Integer n : list) {

            System.out.printf("%d ", n);
        }

        System.out.println();
    }
}

The example creates three lists of integers. Later, the lists are added into another fourth list.

List<Integer> l1 = new ArrayList<>();
l1.add(1);
l1.add(2);
l1.add(3);

A list of integers is created.

List<List> nums = new ArrayList<>();
nums.add(l1);
nums.add(l2);
nums.add(l3);

A list of lists is created.

for (List<Integer> list : nums) {

    for (Integer n : list) {

        System.out.printf("%d ", n);
    }

    System.out.println();
}

We use two for loops to go through all the elements.

$ java Main.java
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
1 2 3
4 5 6
7 8 9

The subList method

The subList method returns a view of the portion of a list between the specified fromIndex, inclusive, and toIndex, exclusive. The changes in a sublist are reflected in the original list.

import java.util.ArrayList;
import java.util.List;

void main() {

    List<String> items = new ArrayList<>();

    items.add("coin");
    items.add("pen");
    items.add("cup");
    items.add("notebook");
    items.add("glass");
    items.add("chair");
    items.add("ball");
    items.add("bowl");

    List<String> items2 = items.subList(2, 5);

    System.out.println(items2);

    items2.set(0, "bottle");

    System.out.println(items2);
    System.out.println(items);
}

The example creates a sublist from a list of items.

List<String> items2 = items.subList(2, 5);

A sublist is created with the subList method; it contains items with indexes 2, 3, and 4.

items2.set(0, "bottle");

We replace the first item of the sublist; the modification is reflected in the original list, too.

$ java Main.java
[cup, notebook, glass]
[bottle, notebook, glass]
[coin, pen, bottle, notebook, glass, chair, ball, bowl]

Traversing elements

In the following example, we show five ways to traverse an ArrayList.

import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;

void main() {

    List<Integer> nums = new ArrayList<>();
    nums.add(2);
    nums.add(6);
    nums.add(7);
    nums.add(3);
    nums.add(1);
    nums.add(8);

    for (int i = 0; i < nums.size(); i++) {

        System.out.printf("%d ", nums.get(i));
    }

    System.out.println();

    for (int num : nums) {

        System.out.printf("%d ", num);
    }

    System.out.println();

    int j = 0;
    while (j < nums.size()) {

        System.out.printf("%d ", nums.get(j));
        j++;
    }

    System.out.println();

    ListIterator<Integer> it = nums.listIterator();

    while(it.hasNext()) {

        System.out.printf("%d ", it.next());
    }

    System.out.println();

    nums.forEach(e -> System.out.printf("%d ", e));
    System.out.println();
}

In the example, we traverse an array list of integers with for loops, while loop, iterator, and forEach construct.

List<Integer> nums = new ArrayList<>();
nums.add(2);
nums.add(6);
nums.add(7);
nums.add(3);
nums.add(1);
nums.add(8);

We have created an ArrayList of integers.

for (int i = 0; i < nums.size(); i++) {

    System.out.printf("%d ", nums.get(i));
}

Here, we use the classic for loop to iterate over the list.

for (int num : nums) {

    System.out.printf("%d ", num);
}

The second way uses the enhanced-for loop, which was introduced int Java 5.

int j = 0;
while (j < nums.size()) {

    System.out.printf("%d ", nums.get(j));
    j++;
}

The third way uses the while loop.

ListIterator<Integer> it = nums.listIterator();

while(it.hasNext()) {

    System.out.printf("%d ", it.next());
}

Here, a ListIterator is used to traverse the list.

nums.forEach(e -> System.out.printf("%d ", e));

In the last way, we use the forEach method, which was introduced in Java 8.

$ java Main.java
2 6 7 3 1 8
2 6 7 3 1 8
2 6 7 3 1 8
2 6 7 3 1 8
2 6 7 3 1 8

Sorting elements

There are different ways to sort an ArrayList.

Sorting with the sort method

The ArrayList's sort method sorts a list according to the order induced by the specified comparator.

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;


void main() {

    List<Person> persons = createList();
    persons.sort(Comparator.comparing(Person::age).reversed());

    System.out.println(persons);

}

List<Person> createList() {

    List<Person> persons = new ArrayList<>();

    persons.add(new Person(17, "Jane"));
    persons.add(new Person(32, "Peter"));
    persons.add(new Person(47, "Patrick"));
    persons.add(new Person(22, "Mary"));
    persons.add(new Person(39, "Robert"));
    persons.add(new Person(54, "Greg"));

    return persons;
}

record Person(int age, String name) {}

We have an ArrayList of custom Person classes. We sort the persons according to their age in a reversed order.

persons.sort(Comparator.comparing(Person::age).reversed());

This line sorts the persons by their age, from the oldest to the youngest.

$ java Main.java
[Age: 54 Name: Greg, Age: 47 Name: Patrick, Age: 39 Name: Robert, Age: 32 Name: Peter,
    Age: 22 Name: Mary, Age: 17 Name: Jane]

Sorting elements using stream

In the second example, we use Java stream to sort the ArrayList. The Stream API allows a more powerful way to do sorting.

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

void main() {

    List<Country> countries = createList();

    List<Country> sorted_countries = countries.stream()
            .sorted((e1, e2) -> Integer.compare(e1.population,
                    e2.population))
            .collect(Collectors.toList());

    System.out.println(sorted_countries);
}

List<Country> createList() {

    List<Country> countries = new ArrayList<>();

    countries.add(new Country("Slovakia", 5424000));
    countries.add(new Country("Hungary", 9845000));
    countries.add(new Country("Poland", 38485000));
    countries.add(new Country("Germany", 81084000));
    countries.add(new Country("Latvia", 1978000));

    return countries;
}

record Country(String name, int population) {
}

In this example, we have a list of countries. Each country has a name and population. The countries are sorted by population.

List<Country> sorted_countries = countries.stream()
        .sorted((e1, e2) -> Integer.compare(e1.population,
                e2.population))
        .collect(Collectors.toList());

With the stream method, we create a stream from a list. The sorted method sorts elements according to the provided comparator. With Integer.compare we compare the populations of countries. With collect, we transform the stream into a list of countries.

$ java Main.java
[Country{name=Latvia, population=1978000}, Country{name=Slovakia, population=5424000},
Country{name=Hungary, population=9845000}, Country{name=Poland, population=38485000},
Country{name=Germany, population=81084000}]

The countries are sorted by their population in ascending mode.

Working with ArrayList and simple Java array

The following example uses an ArrayList with a simple Java array.

import java.util.Arrays;
import java.util.List;

void main() {

    String[] a = new String[] { "Mercury", "Venus", "Earth",
            "Mars", "Jupiter", "Saturn", "Uranus", "Neptune" };

    List<String> planets = List.of(a);
    System.out.println(planets);

    String[] planets2 = planets.toArray(new String[0]);
    System.out.println(Arrays.toString(planets2));
}

An ArrayList is converted to an array and vice versa.

String[] a = new String[] { "Mercury", "Venus", "Earth",
        "Mars", "Jupiter", "Saturn", "Uranus", "Neptune" };

We have an array of strings.

List<String> planets = List.of(a);

We generate an immutable list from the array with List.of;

String[] planets2 = planets.toArray(new String[0]);

The ArrayList's toArray is used to convert a list to an array.

Stream to list

Java streams can be converted to lists using collectors.

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

void main() {

    var words = Stream.of("forest", "eagle", "river", "cloud", "sky");

    List<String> words2 = words.collect(Collectors.toList());
    System.out.println(words2.getClass());
}

We have a stream of strings. We convert the stream to a list with Collectors.toList.

$ java Main.java
class java.util.ArrayList

In this article we have worked with the Java ArrayList container.

Source

Java ArrayList - language reference

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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