Access List of lists java

Table of Contents

Nội dung chính Show

One of the Usecase
Question on list of lists
You may also like:

One of the Usecase

Question on list of lists

In this posts, we will see how to create a list of lists in java.
You can easily create a list of lists using below syntax

List<ArrayList<String>> listOfLists = new ArrayList<ArrayList<String>>();
or
ArrayList<ArrayList<String>> listOfLists = new ArrayList<ArrayList<String>>();

One of the Usecase

This is generally useful when you are trying to read a CSV file and then you need to handle list of lists to get it in memory, perform some processing and write back to another CSV file.

Lets take a simple example for list of lists.

package org.arpit.java2blog;

import java.util.ArrayList;

import java.util.List;

public class JavaListOfListsMain {

public static void main(String[] args) {

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

ArrayList<String> list1 = new ArrayList<String>();

list1.add("Delhi");

list1.add("Mumbai");

listOfLists.add(list1);

ArrayList<String> anotherList = new ArrayList<String>();

anotherList.add("Beijing");

anotherList.add("Shanghai");

listOfLists.add(anotherList);

listOfLists.forEach((list)->

{

list.forEach((city)->System.out.println(city));

}

);

}

When you run above program, you will get below output.

Delhi
Mumbai
Beijing
Shanghai

Question on list of lists

Can you instantiate List as below

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

No, you can't.

Lets understand the reason for it.
Lets say you have list as below:

ArrayList<ArrayList<String>> list1 = new ArrayList<ArrayList<String>>();

Now suppose you could assign that to

List<List<String>> list2 = list1.

Now, you should be able to do this:

list2.add(new LinkedList<String>());

But that means you have just added a LinkedList to a list whose elements are supposed to be ArrayList only.Thats why it is not allowed to do it.
Thats all about how to create list of lists in java.

import_contacts