The Java ArrayList can be initialized in number of ways depending on the requirement. In this tutorial, we will learn to initialize ArrayList based on some frequently seen usecases.
Table of Contents 1. Initialize ArrayList in single line 2. Create ArrayList and add objects 3. Initialize arraylist of lists1. Initialize ArrayList in one line
1.1. Arrays.asList[] – Initialize arraylist from array
To initialize an arraylist in single line statement, get all elements in form of array using Arrays.asList method and pass the array argument to ArrayList constructor.
ArrayList names = new ArrayList[ Arrays.asList["alex", "brian", "charles"] ]; System.out.println[names];Program output.
[alex, brian, charles]1.2. List.of[] – Immutable list – Java 9
We can use List.of[] static factory methods to create immutable lists. Only drawback is that add operation is not supported in these lists.
List names = List.of["alex", "brian"]; System.out.println[names];Program output.
[alex, brian]Read More : Java 9 Immutable Collections
2. Create ArrayList and add objects – ArrayList constructor
Using ArrayList constructor is traditional approach. We create a blank arraylist using constructor and add elements to list using add[] method. We can add elements either one by one, or we can pass another collection to add all elements in one step.
ArrayList names = new ArrayList[]; //1. Add elements one by one names.add["alex"]; names.add["brian"]; names.add["charles"]; System.out.println[names]; HashMap details = new HashMap[]; details.put["keanu", 23]; details.put["max", 24]; details.put["john", 53]; //2. Add elements from other collection names.addAll[details.keySet[]]; System.out.println[names];Program output.
[alex, brian, charles] [alex, brian, charles, max, john, keanu]3. Initialize arraylist of lists
At times, we may need to initialize arraylist of lists.
List marks = new ArrayList[]; marks.add[ Arrays.asList[10, 20, 30] ]; marks.add[ Arrays.asList[40, 50, 60] ]; marks.add[ Arrays.asList[70, 80, 90] ]; for [List mark : marks] { System.out.println[mark]; }Program output.
[10, 20, 30] [40, 50, 60] [70, 80, 90]Please note that Arrays.asList[] does not return java.util.ArrayList instance. It returns java.util.Arrays$ArrayList instance instead.
So if you must have an ArrayList only, then create ArrayList for Arrays.asList[] instance in below manner.
marks.add[new ArrayList[ Arrays.asList[10, 20, 30] ]];That’s all about to create an arraylist in Java. Drop me your questions in comments.
Happy Learning !!
Reference:
ArrayList Java Docs
A Guide to Java ArrayList
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ArrayList in Java is used to store dynamically sized collection of elements. Contrary to Arrays that are fixed in size, an ArrayList grows its size automatically when new elements are added to it.
ArrayList is part of Java’s collection framework and implements Java’s List interface.
Following are few key points to note about ArrayList in Java -
An ArrayList is a re-sizable array, also called a dynamic array. It grows its size to accommodate new elements and shrinks the size when the elements are removed.
ArrayList internally uses an array to store the elements. Just like arrays, It allows you to retrieve the elements by their index.
Java ArrayList allows duplicate and null values.
Java ArrayList is an ordered collection. It maintains the insertion order of the elements.
You cannot create an ArrayList of primitive types like int, char etc. You need to use boxed types like Integer, Character, Boolean etc.
Java ArrayList is not synchronized. If multiple threads try to modify an ArrayList at the same time, then the final outcome will be non-deterministic. You must explicitly synchronize access to an ArrayList if multiple threads are gonna modify it.
Creating an ArrayList and adding new elements to it
This example shows:
- How to create an ArrayList using the ArrayList[] constructor.
- Add new elements to an ArrayList using the add[] method.
import java.util.ArrayList; import java.util.List; public class CreateArrayListExample { public static void main[String[] args] { List animals = new ArrayList[]; animals.add["Lion"]; animals.add["Tiger"]; animals.add["Cat"]; animals.add["Dog"]; System.out.println[animals]; animals.add[2, "Elephant"]; System.out.println[animals]; } }
[Lion, Tiger, Cat, Dog] [Lion, Tiger, Elephant, Cat, Dog]
Creating an ArrayList from another collection
This example shows:
How to create an ArrayList from another collection using the ArrayList[Collection c] constructor.
How to add all the elements from an existing collection to the new ArrayList using the addAll[] method.
import java.util.ArrayList; import java.util.List; public class CreateArrayListFromCollectionExample { public static void main[String[] args] { List firstFivePrimeNumbers = new ArrayList[]; firstFivePrimeNumbers.add[2]; firstFivePrimeNumbers.add[3]; firstFivePrimeNumbers.add[5]; firstFivePrimeNumbers.add[7]; firstFivePrimeNumbers.add[11]; List firstTenPrimeNumbers = new ArrayList[firstFivePrimeNumbers]; List nextFivePrimeNumbers = new ArrayList[]; nextFivePrimeNumbers.add[13]; nextFivePrimeNumbers.add[17]; nextFivePrimeNumbers.add[19]; nextFivePrimeNumbers.add[23]; nextFivePrimeNumbers.add[29]; firstTenPrimeNumbers.addAll[nextFivePrimeNumbers]; System.out.println[firstTenPrimeNumbers]; } }
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
Accessing elements from an ArrayList
This example shows:
- How to check if an ArrayList is empty using the isEmpty[] method.
- How to find the size of an ArrayList using the size[] method.
- How to access the element at a particular index in an ArrayList using the get[] method.
- How to modify the element at a particular index in an ArrayList using the set[] method.
import java.util.ArrayList; import java.util.List; public class AccessElementsFromArrayListExample { public static void main[String[] args] { List topCompanies = new ArrayList[]; System.out.println["Is the topCompanies list empty? : " + topCompanies.isEmpty[]]; topCompanies.add["Google"]; topCompanies.add["Apple"]; topCompanies.add["Microsoft"]; topCompanies.add["Amazon"]; topCompanies.add["Facebook"]; System.out.println["Here are the top " + topCompanies.size[] + " companies in the world"]; System.out.println[topCompanies]; String bestCompany = topCompanies.get[0]; String secondBestCompany = topCompanies.get[1]; String lastCompany = topCompanies.get[topCompanies.size[] - 1]; System.out.println["Best Company: " + bestCompany]; System.out.println["Second Best Company: " + secondBestCompany]; System.out.println["Last Company in the list: " + lastCompany]; topCompanies.set[4, "Walmart"]; System.out.println["Modified top companies list: " + topCompanies]; } }
Is the topCompanies list empty? : true Here are the top 5 companies in the world [Google, Apple, Microsoft, Amazon, Facebook] Best Company: Google Second Best Company: Apple Last Company in the list: Facebook Modified top companies list: [Google, Apple, Microsoft, Amazon, Walmart]
Removing elements from an ArrayList
This example shows:
How to remove the element at a given index in an ArrayList | remove[int index]
How to remove an element from an ArrayList | remove[Object o]
How to remove all the elements from an ArrayList that exist in a given collection | removeAll[]
How to remove all the elements matching a given predicate | removeIf[]
How to clear an ArrayList | clear[]
import java.util.ArrayList; import java.util.List; import java.util.function.Predicate; public class RemoveElementsFromArrayListExample { public static void main[String[] args] { List programmingLanguages = new ArrayList[]; programmingLanguages.add["C"]; programmingLanguages.add["C++"]; programmingLanguages.add["Java"]; programmingLanguages.add["Kotlin"]; programmingLanguages.add["Python"]; programmingLanguages.add["Perl"]; programmingLanguages.add["Ruby"]; System.out.println["Initial List: " + programmingLanguages]; programmingLanguages.remove[5]; System.out.println["After remove[5]: " + programmingLanguages]; boolean isRemoved = programmingLanguages.remove["Kotlin"]; System.out.println["After remove[\"Kotlin\"]: " + programmingLanguages]; List scriptingLanguages = new ArrayList[]; scriptingLanguages.add["Python"]; scriptingLanguages.add["Ruby"]; scriptingLanguages.add["Perl"]; programmingLanguages.removeAll[scriptingLanguages]; System.out.println["After removeAll[scriptingLanguages]: " + programmingLanguages]; programmingLanguages.removeIf[new Predicate[] { @Override public boolean test[String s] { return s.startsWith["C"]; } }]; System.out.println["After Removing all elements that start with \"C\": " + programmingLanguages]; programmingLanguages.clear[]; System.out.println["After clear[]: " + programmingLanguages]; } }
Initial List: [C, C++, Java, Kotlin, Python, Perl, Ruby] After remove[5]: [C, C++, Java, Kotlin, Python, Ruby] After remove["Kotlin"]: [C, C++, Java, Python, Ruby] After removeAll[scriptingLanguages]: [C, C++, Java] After Removing all elements that start with "C": [Java] After clear[]: []
Iterating over an ArrayList
The following example shows how to iterate over an ArrayList using
import java.util.ArrayList; import java.util.Iterator; import java.util.List; import java.util.ListIterator; public class IterateOverArrayListExample { public static void main[String[] args] { List tvShows = new ArrayList[]; tvShows.add["Breaking Bad"]; tvShows.add["Game Of Thrones"]; tvShows.add["Friends"]; tvShows.add["Prison break"]; System.out.println["=== Iterate using Java 8 forEach and lambda ==="]; tvShows.forEach[tvShow -> { System.out.println[tvShow]; }]; System.out.println["\n=== Iterate using an iterator[] ==="]; Iterator tvShowIterator = tvShows.iterator[]; while [tvShowIterator.hasNext[]] { String tvShow = tvShowIterator.next[]; System.out.println[tvShow]; } System.out.println["\n=== Iterate using an iterator[] and Java 8 forEachRemaining[] method ==="]; tvShowIterator = tvShows.iterator[]; tvShowIterator.forEachRemaining[tvShow -> { System.out.println[tvShow]; }]; System.out.println["\n=== Iterate using a listIterator[] to traverse in both directions ==="]; ListIterator tvShowListIterator = tvShows.listIterator[tvShows.size[]]; while [tvShowListIterator.hasPrevious[]] { String tvShow = tvShowListIterator.previous[]; System.out.println[tvShow]; } System.out.println["\n=== Iterate using simple for-each loop ==="]; for[String tvShow: tvShows] { System.out.println[tvShow]; } System.out.println["\n=== Iterate using for loop with index ==="]; for[int i = 0; i { System.out.println["Name : " + user.getName[] + ", Age : " + user.getAge[]]; }]; } }
Name : Rajeev, Age : 25 Name : John, Age : 34 Name : Steve, Age : 29
Sorting an ArrayList
Sorting an ArrayList is a very common task that you will encounter in your programs. In this section, I’ll show you how to -
1. Sort an ArrayList using Collections.sort[] method
import java.util.ArrayList; import java.util.Collections; import java.util.List; public class ArrayListCollectionsSortExample { public static void main[String[] args] { List numbers = new ArrayList[]; numbers.add[13]; numbers.add[7]; numbers.add[18]; numbers.add[5]; numbers.add[2]; System.out.println["Before : " + numbers]; Collections.sort[numbers]; System.out.println["After : " + numbers]; } }
Before : [13, 7, 18, 5, 2] After : [2, 5, 7, 13, 18]
2. Sort an ArrayList using ArrayList.sort[] method
import java.util.ArrayList; import java.util.Comparator; import java.util.List; public class ArrayListSortExample { public static void main[String[] args] { List names = new ArrayList[]; names.add["Lisa"]; names.add["Jennifer"]; names.add["Mark"]; names.add["David"]; System.out.println["Names : " + names]; names.sort[new Comparator[] { @Override public int compare[String name1, String name2] { return name1.compareTo[name2]; } }]; names.sort[[name1, name2] -> name1.compareTo[name2]]; names.sort[Comparator.naturalOrder[]]; System.out.println["Sorted Names : " + names]; } }
# Output Names : [Lisa, Jennifer, Mark, David] Sorted Names : [David, Jennifer, Lisa, Mark]
3. Sort an ArrayList of Objects using custom Comparator
import java.util.ArrayList; import java.util.Collections; import java.util.Comparator; import java.util.List; class Person { private String name; private Integer age; public Person[String name, Integer age] { this.name = name; this.age = age; } public String getName[] { return name; } public void setName[String name] { this.name = name; } public Integer getAge[] { return age; } public void setAge[Integer age] { this.age = age; } @Override public String toString[] { return "{" + "name='" + name + '\'' + ", age=" + age + '}'; } } public class ArrayListObjectSortExample { public static void main[String[] args] { List people = new ArrayList[]; people.add[new Person["Sachin", 47]]; people.add[new Person["Chris", 34]]; people.add[new Person["Rajeev", 25]]; people.add[new Person["David", 31]]; System.out.println["Person List : " + people]; people.sort[[person1, person2] -> { return person1.getAge[] - person2.getAge[]; }]; people.sort[Comparator.comparingInt[Person::getAge]]; System.out.println["Sorted Person List by Age : " + people]; Collections.sort[people, Comparator.comparing[Person::getName]]; System.out.println["Sorted Person List by Name : " + people]; } }
# Output Person List : [{name='Sachin', age=47}, {name='Chris', age=34}, {name='Rajeev', age=25}, {name='David', age=31}] Sorted Person List by Age : [{name='Rajeev', age=25}, {name='David', age=31}, {name='Chris', age=34}, {name='Sachin', age=47}] Sorted Person List by Name : [{name='Chris', age=34}, {name='David', age=31}, {name='Rajeev', age=25}, {name='Sachin', age=47}]
Synchronizing Access to an ArrayList
The ArrayList class is not synchronized. If multiple threads try to modify an ArrayList at the same time then the final result becomes not-deterministic because one thread might override the changes done by another thread.
The following example shows what happens when multiple threads try to modify an ArrayList at the same time.
import java.util.ArrayList; import java.util.List; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.TimeUnit; public class UnsafeArrayListExample { public static void main[String[] args] throws InterruptedException { List unsafeArrayList = new ArrayList[]; unsafeArrayList.add[1]; unsafeArrayList.add[2]; unsafeArrayList.add[3]; ExecutorService executorService = Executors.newFixedThreadPool[10]; Runnable task = [] -> { incrementArrayList[unsafeArrayList]; }; for[int i = 0; i