How to make lists of lists in Java?

Hi, In this tutorial, I will show you how to make lists of lists in Java.

A list of lists in Java is just a list where each entry is a list in and of itself. More intricate data structures like matrices, graphs, and other multi-dimensional arrays can be represented with this. The java.util.List interface and the java.util.ArrayList class in Java offers a robust and adaptable foundation for managing lists.

Step-by-Step Guide

1. Import Necessary Classes

First, you need to import the necessary classes from the java.util package:

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

2. Create a List of Lists

You must instantiate a List of List objects to generate a list of lists. ArrayList is the most popular implementation of the List interface.

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

3. Add Lists to the List of Lists

You can add individual lists to your listOfLists:

List<Integer> innerList1 = new ArrayList<>();
innerList1.add(1);
innerList1.add(2);
innerList1.add(3);

List<Integer> innerList2 = new ArrayList<>();
innerList2.add(4);
innerList2.add(5);
innerList2.add(6);

listOfLists.add(innerList1);
listOfLists.add(innerList2);

4. Access Elements in the List of Lists

To access elements in the list of lists, you need to use two get methods, one for the outer list and one for the inner list.

int firstElement = listOfLists.get(0).get(0); // Accessing the first element of the first inner list
int secondElement = listOfLists.get(1).get(1); // Accessing the second element of the second inner list

5. Iterate Over the List of Lists

You can use nested loops to iterate over the list of lists:

for (List<Integer> innerList : listOfLists) {
    for (Integer element : innerList) {
        System.out.print(element + " ");
    }
    System.out.println();
}

6. Modify Elements in the List of Lists

You can also modify elements in the list of lists by using the set method:

listOfLists.get(0).set(0, 10); // Changing the first element of the first inner list to 10

7. Remove Elements from the List of Lists

To remove elements, you can use the remove method:

listOfLists.get(1).remove(1); // Removing the second element of the second inner list

Example Program

This is a full example use that shows how to create, edit, and access a list of lists in Java:

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

public class ListOfListsExample 
{
    public static void main(String[] args) 
	{
        List<List<Integer>> listOfLists = new ArrayList<>();

        // Adding lists to the list of lists
        List<Integer> innerList1 = new ArrayList<>();
        innerList1.add(1);
        innerList1.add(2);
        innerList1.add(3);

        List<Integer> innerList2 = new ArrayList<>();
        innerList2.add(4);
        innerList2.add(5);
        innerList2.add(6);

        listOfLists.add(innerList1);
        listOfLists.add(innerList2);

        // Accessing elements
        int firstElement = listOfLists.get(0).get(0);
        int secondElement = listOfLists.get(1).get(1);

        System.out.println("First element of the first list: " + firstElement);
        System.out.println("Second element of the second list: " + secondElement);

        // Modifying elements
        listOfLists.get(0).set(0, 10);

        // Removing elements
        listOfLists.get(1).remove(1);

        // Iterating over the list of lists
        for (List<Integer> innerList : listOfLists) {
            for (Integer element : innerList) {
                System.out.print(element + " ");
            }
            System.out.println();
        }
    }
}

Output:

First element of the first list: 1
Second element of the second list: 5
10 2 3
4 6

Conclusion

Lists of lists in Java provide a flexible and powerful way to handle multi-dimensional data structures. By using the List interface and ArrayList class, you can easily create, modify, and traverse lists of lists. This guide should give you a good starting point for working with these structures in your Java programs.

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