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Working With a List of Lists in Java

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1. Overview

List is a pretty commonly used data structure in Java. Sometimes, we may need a nested List structure for some requirements, such as List>.

In this tutorial, we’ll take a closer look at this “List of Lists” data structure and explore some everyday operations.

2. List Array vs. List of Lists

We can look at the “List of Lists” data structure as a two-dimensional matrix. So, if we want to group a number of List objects, we have two options:

Next, let’s have a look at when to choose which one.

Array is fast for “get” and “set” operations, which run in O(1) time. However, since the array’s length is fixed, it’s costly to resize an array for inserting or deleting elements.

On the other hand, List is more flexible on insertion and deletion operations, which run in O(1) time. Generally speaking, List is slower than Array on “get/set” operations. But some List implementations, such as ArrayList, are internally based on arrays. So, usually, the difference between the performance of Array and ArrayList on “get/set” operations is not noticeable.

Therefore, we would pick the List> data structure in most cases to gain better flexibility.

Of course, if we’re working on a performance-critical application, and we don’t change the size of the first dimension – for instance, we never add or remove inner Lists – we can consider using the List[] structure.

We’ll mainly discuss List> in this tutorial.

3. Common Operations on List of Lists

Now, let’s explore some everyday operations on List>.

For simplicity, we’ll manipulate the List> object and verify the result in unit test methods.

Further, to see the change straightforwardly, let’s also create a method to print the content of the List of Lists:

private void printListOfLists(List> listOfLists) < System.out.println("\n List of Lists "); System.out.println("-------------------------------------"); listOfLists.forEach(innerList ->< String line = String.join(", ", innerList); System.out.println(line); >); > 

Next, let’s first initialize a list of lists.

3.1. Initializing a List of Lists

We’ll import data from a CSV file into a List> object. Let’s first look at the CSV file’s content:

Linux, Microsoft Windows, Mac OS, Delete Me Kotlin, Delete Me, Java, Python Delete Me, Mercurial, Git, Subversion

Let’s say we name the file as example.csv and put it under the resources/listoflists directory.

Next, let’s create a method to read the file and store the data in a List> object:

private List> getListOfListsFromCsv() throws URISyntaxException, IOException < Listlines = Files.readAllLines(Paths.get(getClass().getResource("/listoflists/example.csv") .toURI())); List> listOfLists = new ArrayList<>(); lines.forEach(line -> < ListinnerList = new ArrayList<>(Arrays.asList(line.split(", "))); listOfLists.add(innerList); >); return listOfLists; > 

In the getListOfListsFromCsv method, we first read all lines from the CSV file into a List object. Then, we walk through the lines List and convert each line (String) into List .

List> listOfLists = getListOfListsFromCsv(); assertThat(listOfLists).hasSize(3); assertThat(listOfLists.stream() .map(List::size) .collect(Collectors.toSet())).hasSize(1) .containsExactly(4); printListOfLists(listOfLists); 

 List of Lists ------------------------------------- Linux, Microsoft Windows, Mac OS, Delete Me Kotlin, Delete Me, Java, Python Delete Me, Mercurial, Git, Subversion

List> listOfLists = getListOfListsFromCsv(); List newList = new ArrayList<>(Arrays.asList("Slack", "Zoom", "Microsoft Teams", "Telegram")); listOfLists.add(2, newList); assertThat(listOfLists).hasSize(4); assertThat(listOfLists.get(2)).containsExactly("Slack", "Zoom", "Microsoft Teams", "Telegram"); printListOfLists(listOfLists); 

 List of Lists ------------------------------------- Linux, Microsoft Windows, Mac OS, Delete Me Kotlin, Delete Me, Java, Python Slack, Zoom, Microsoft Teams, Telegram Delete Me, Mercurial, Git, Subversion 

List innerList = listOfLists.get(x); // innerList.add(), remove() . 

List> listOfLists = getListOfListsFromCsv(); listOfLists.forEach(innerList -> innerList.remove("Delete Me")); assertThat(listOfLists.stream() .map(List::size) .collect(Collectors.toSet())).hasSize(1) .containsExactly(3); printListOfLists(listOfLists); 

 List of Lists ------------------------------------- Linux, Microsoft Windows, Mac OS Kotlin, Java, Python Mercurial, Git, Subversion 

package com.crunchify.tutorial; import java.io.FileInputStream; import java.util.jar.JarEntry; import java.util.jar.JarInputStream; import org.json.JSONArray; import org.json.JSONObject; /** * @author Crunchify.com * */ public class CrunchifyFindClassesFromJar < @SuppressWarnings("resource") public static JSONObject getCrunchifyClassNamesFromJar(String crunchifyJarName) < JSONArray listofClasses = new JSONArray(); JSONObject crunchifyObject = new JSONObject(); try < JarInputStream crunchifyJarFile = new JarInputStream(new FileInputStream(crunchifyJarName)); JarEntry crunchifyJar; while (true) < crunchifyJar = crunchifyJarFile.getNextJarEntry(); if (crunchifyJar == null) < break; >if ((crunchifyJar.getName().endsWith(".class"))) < String className = crunchifyJar.getName().replaceAll("/", "\\."); String myClass = className.substring(0, className.lastIndexOf('.')); listofClasses.put(myClass); >> crunchifyObject.put("Jar File Name", crunchifyJarName); crunchifyObject.put("List of Class", listofClasses); > catch (Exception e) < System.out.println("Oops.. Encounter an issue while parsing jar" + e.toString()); >return crunchifyObject; > public static void main(String[] args) < JSONObject myList = getCrunchifyClassNamesFromJar("/Users//Documents/javax.servlet.jsp.jar"); System.out.println(myList); > >

< "List of Class": [ "javax.el.ArrayELResolver", "javax.el.BeanELResolver$BeanProperties", "javax.el.BeanELResolver$BeanProperty", "javax.el.BeanELResolver", "javax.el.CompositeELResolver$CompositeIterator", "javax.el.CompositeELResolver", "javax.el.ELContext", "javax.el.ELContextEvent", . . so many more. ], "Jar File Name": "/Users//Documents/javax.servlet.jsp.jar" >