Java ArrayList

last modified January 10, 2023

Java ArrayList tutorial shows 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.

Java ArrayList

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.

Java ArrayList adding single items

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

package com.zetcode;

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

public class ListAddItem {

    public static void main(String[] args) {

        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.

Ruby Java C# Python

Note that the elements keep the order they were inserted.

Java List.of

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

package com.zetcode;

import java.util.List;

public class ListOf {

    public static void main(String[] args) {

        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.

Java ArrayList get() and size()

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

package com.zetcode;

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

public class GetAndSizeEx {

    public static void main(String[] args) {

        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.

orange
The size of the ArrayList is: 4

Java ArrayList copy

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

package com.zetcode;

import java.util.List;

public class ListCopy {

    public static void main(String[] args) {

        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.

package com.zetcode;

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

class Base {}

enum Level {

    EASY,
    MEDIUM,
    HARD
}

public class DataTypesEx {

    public static void main(String[] args) {

        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
3.5
55
com.zetcode.Base@659e0bfd
EASY

Java ArrayList add multiple elements

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

package com.zetcode;

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

public class AddingMultipleItemsEx {

    public static void main(String[] args) {

        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.

white
orange
yellow
pink
brown
blue
red
green

Java ArrayList modifying elements

The next example uses methods to modify the ArrayList.

package com.zetcode;

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

public class ModifyingListEx {

    public static void main(String[] args) {

        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");
        }
    }

    public static void fillList(List<String> list) {

        list.add("coin");
        list.add("pen");
        list.add("pencil");
        list.add("clock");
        list.add("book");
        list.add("spectacles");
        list.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.

pencil
watch
book
spectacles
glass
bowl
The list is empty

Java ArrayList removeIf

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

package com.zetcode;

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

public class RemoveIfEx {

    public static void main(String[] args) {

        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.

[5, 2, 8, 6]

Java ArrayList removeAll

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.

package com.zetcode;

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

public class RemoveAll {

    public static void main(String[] args) {

        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.

Java ArrayList replaceAll

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

package com.zetcode;

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

public class ReplaceAllEx {

    public static void main(String[] args) {

        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.

[COIN, PEN, CUP, NOTEBOOK, CLASS]

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

package com.zetcode;

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

class MyOperator<T> implements UnaryOperator<String> {

    @Override
    public String apply(String var) {

        if (var == null || var.length() == 0) {
            return var;
        }
        return var.substring(0, 1).toUpperCase() + var.substring(1);
    }
}

public class ReplaceAllEx2 {

    public static void main(String[] args) {

        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 var) {

    if (var == null || var.length() == 0) {
        return var;
    }
    return var.substring(0, 1).toUpperCase() + var.substring(1);
}

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

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

The operator is applied on the list items.

[Coin, Pen, Cup, Notebook, Glass]

Java ArrayList contains()

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

package com.zetcode;

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

public class ContainsEx {

    public static void main(String[] args) {

        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.

There is a pen in the list

Getting index of elements in ArrayList

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.

package com.zetcode;

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

public class GetIndexEx {

    public static void main(String[] args) {

        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.

1
4

Java list of lists

We can add other lists into a list.

package com.zetcode;

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

public class ListOfLists {

    public static void main(String[] args) {

        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.

[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
1 2 3
4 5 6
7 8 9

Java ArrayList subList

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.

package com.zetcode;

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

public class SubListEx {

    public static void main(String[] args) {

        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.

[cup, notebook, glass]
[bottle, notebook, glass]
[coin, pen, bottle, notebook, glass, chair, ball, bowl]

Java ArrayList traversing

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

package com.zetcode;

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

public class TraversingArrayListEx {

    public static void main(String[] args) {

        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.

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

The example prints the elements of a list to the console, utilizing various techniques.

Java ArrayList sorting

There are different wasy to sort an ArrayList.

Sorting ArrayList with its sort method

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

package com.zetcode;

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

class Person {

    private int age;
    private String name;

    public Person(int age, String name) {

        this.age = age;
        this.name = name;
    }

    public int getAge() {

        return age;
    }

    @Override
    public String toString() {
        return "Age: " + age + " Name: " + name;
    }
}

public class ArrayListSortingEx {

    public static void main(String[] args) {

        List<Person> persons = createList();

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

        System.out.println(persons);

    }

    private static 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;
    }
}

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::getAge).reversed());

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

[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 ArrayList with Java 8 stream

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

package com.zetcode;

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

class Country {

    private String name;
    private int population;

    public Country(String name, int population) {
        this.name = name;
        this.population = population;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getPopulation() {
        return population;
    }

    public void setPopulation(int population) {
        this.population = population;
    }

    @Override
    public String toString() {
        return "Country{" + "name=" + name
                + ", population=" + population + '}';
    }
}

public class ArrayListSortingEx2 {

    public static void main(String[] args) {

        List<Country> countries = createList();

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

        System.out.println(sorted_countries);
    }

    private static 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;
    }
}

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.getPopulation(),
                e2.getPopulation())).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.

[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.

package com.zetcode;

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

public class ListToArray {

    public static void main(String[] args) {

        List<String> planets = Arrays.asList("Mercury", "Venus", "Earth",
                "Mars", "Jupiter", "Saturn", "Uranus", "Neptune");

        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.

List<String> planets = Arrays.asList("Mercury", "Venus", "Earth",
        "Mars", "Jupiter", "Saturn", "Uranus", "Neptune");

With the Arrays.asList method, we create a fixed-size list backed by the specified array.

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.

package com.zetcode;

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

public class ToList {

    public static void main(String[] args) {

        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.

class java.util.ArrayList

List all Java tutorials.

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