Initialize a List of Lists in Java

This post will discuss different ways to initialize a List of Lists in Java.

1. Using `List.add()` method

You can declare a List of Lists in Java, using the following syntax. It uses the new operator to instantiate the list by allocating memory and returning a reference to that memory.

import java.util.ArrayList;

import java.util.List;

class Main

{

public static void main(String[] args)

{

List<List<Integer>> lists = new ArrayList<>();

System.out.println(lists);

}

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Output:

[]

To add elements to it, call the add() method. Since each element of a List of Lists is a List itself, pass the new List instance to the add() method. Be extra careful not to pass the same mutable instance of a list to the add() method again.

import java.util.ArrayList;

import java.util.List;

class Main

{

public static void main(String[] args)

{

List<List<Integer>> lists = new ArrayList<>();

int M = 3;

for (int i = 0; i < M; i++) {

lists.add(new ArrayList<>());

}

lists.forEach(System.out::println);

}

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Output:

[]
[]
[]

You can easily add lists of a specific size, initialized by nulls or with some desired value.

import java.util.ArrayList;

import java.util.Collections;

import java.util.List;

class Main

{

public static void main(String[] args)

{

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

int M = 3;

int N = 3;

for (int i = 0; i < M; i++) {

lists.add(new ArrayList<>(Collections.nCopies(N, null)));

}

lists.forEach(System.out::println);

}

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Output:

[null, null, null]
[null, null, null]
[null, null, null]

2. Using `Arrays.asList()` method

You can initialize a List of Lists in a single line in the following manner using the Arrays.asList() method:

import java.util.Arrays;

import java.util.List;

class Main

{

public static void main(String[] args)

{

List<List<Integer>> lists = Arrays.asList(Arrays.asList(1, 2, 3),

Arrays.asList(4, 5, 6),

Arrays.asList(6, 7, 8));

lists.forEach(System.out::println);

}

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Output:

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

With Java 9 onwards, you can use the static factory method List.of().

import java.util.List;

class Main

{

public static void main(String[] args)

{

List<List<Integer>> lists = List.of(

List.of(1, 2, 3), List.of(4, 5, 6), List.of(6, 7, 8)

);

lists.forEach(System.out::println);

}

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Output:

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

Note that both Arrays.asList() and List.of() method creates an unmodifiable instance of the list. If you need a mutable instance, wrap each list instance using the ArrayList constructor.

import java.util.ArrayList;

import java.util.List;

class Main

{

public static void main(String[] args)

{

List<List<Integer>> lists = new ArrayList<>(List.of(

new ArrayList<>(List.of(1, 2, 3)),

new ArrayList<>(List.of(4, 5, 6)),

new ArrayList<>(List.of(6, 7, 8))

));

lists.forEach(System.out::println);

}

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Output:

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

3. Using `IntStream`

Finally, with Java 8 you can use IntStream to initialize a List of Lists as demonstrated below:

import java.util.Collections;

import java.util.List;

import java.util.stream.Collectors;

import java.util.stream.IntStream;

class Main

{

public static void main(String[] args)

{

List<List<Integer>> lists = IntStream.rangeClosed(1, 3).boxed()

.map(i -> Collections.nCopies(4, i))

.collect(Collectors.toList());

lists.forEach(System.out::println);

}

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Output:

[1, 1, 1, 1]
[2, 2, 2, 2]
[3, 3, 3, 3]

That’s all about initializing a List of Lists in Java.

Initialize a List of Lists in Java

1. Using List.add() method

2. Using Arrays.asList() method

3. Using IntStream

Share this:

1. Using `List.add()` method

2. Using `Arrays.asList()` method

3. Using `IntStream`