Convert a List of Lists to a 2D array in Java

This post will discuss how to convert a List of Lists to a two-dimensional primitive or Object array in Java.

There is no straightforward way to convert a List of Lists to a two-dimensional array. However, introduction of Stream API in Java 8 made this task a little simpler:

1. Stream API

The standard solution to convert a List of Lists to a two-dimensional primitive array in Java is using Stream API. The idea is to get a stream of the list of lists and use map() to replace each of the nested lists with the corresponding single-dimensional array. Then, finally call toArray() with a generator to produce the two-dimensional primitive array.

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

class Main

{

public static void main(String[] args)

{

// Input list

List<List<Integer>> list = Arrays.asList(

Arrays.asList(1, 3, 2),

Arrays.asList(1, 2, 2),

Arrays.asList(1, 2)

);

int[][] arr = list.stream()

.map(l -> l.stream().mapToInt(Integer::intValue).toArray())

.toArray(int[][]::new);

System.out.println(Arrays.deepToString(arr));

}

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Output:

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

A similar idea can be applied to the list of lists of Wrapper class to get the two-dimensional array, as shown below:

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

class Main

{

public static void main(String[] args)

{

// Input list

List<List<String>> list = Arrays.asList(

Arrays.asList("A", "B", "C"),

Arrays.asList("B", "D"),

Arrays.asList("E", "F")

);

String[][] arr = list.stream()

.map(l -> l.stream().toArray(String[]::new))

.toArray(String[][]::new);

System.out.println(Arrays.deepToString(arr));

}

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Output:

[[A, B, C], [B, D], [E, F]]

Here’s an alternative version of the above code that uses List’s toArray() method:

import java.util.Arrays;

import java.util.List;

class Main

{

public static void main(String[] args)

{

// Input list

List<List<String>> list = Arrays.asList(

Arrays.asList("A", "B", "C"),

Arrays.asList("B", "D"),

Arrays.asList("E", "F")

);

String[][] arr = list.stream()

.map(l -> l.toArray(new String[l.size()]))

.toArray(String[][]::new);

System.out.println(Arrays.deepToString(arr));

}

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Output:

[[A, B, C], [B, D], [E, F]]

2. Using Loops

Before Java 8, you can create a two-dimensional array of the same size as that of the given list and use a for-loop to fill each array row with the results of the calling toArray() method upon each of the nested lists.

import java.util.Arrays;

import java.util.List;

class Main

{

public static void main(String[] args)

{

// Input list

List<List<String>> list = Arrays.asList(

Arrays.asList("A", "B", "C"),

Arrays.asList("B", "D"),

Arrays.asList("E", "F")

);

String[][] arr = new String[list.size()][];

int i = 0;

for (List<String> l: list) {

arr[i++] = l.toArray(new String[l.size()]);

}

System.out.println(Arrays.deepToString(arr));

}

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Output:

[[A, B, C], [B, D], [E, F]]

That’s all about converting a List of Lists to a two-dimensional array in Java.

Convert a List of Lists to a 2D array in Java

1. Stream API

2. Using Loops

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