Learn how to use lists in Java, how to use List<T> and ArrayList<T>, how to process lists of elements.
Watch the video lesson and access the hands-on exercises here: https://softuni.org/code-lessons/java-foundations-certification-lists
8. List<E> – Overview
List<E> holds a list of elements of any type
9
List<String> names = new ArrayList<>();
// Create a list of strings
names.add("Peter");
names.add("Maria");
names.add("George");
names.remove("Maria");
for (String name : names)
System.out.println(name);
// Peter, George
9. List<E> – Overview (2)
10
List<Integer> nums = new ArrayList<>(
Arrays.asList(10, 20, 30, 40, 50, 60));
nums.remove(2);
nums.remove(Integer.valueOf(40));
nums.add(100);
nums.add(0, -100);
for (int i = 0; i < nums.size(); i++)
System.out.print(nums.get(i) + " ");
-100 10 20 50 60 100
Inserts an element to index
Items count
Remove by index
Remove by value (slow)
10. List<E> holds a list of elements (like array, but extendable)
Provides operations to add / insert / remove / find elements:
size() – number of elements in the List<E>
add(element) – adds an element to the List<E>
add(index, element) – inserts an element to given position
remove(element) – removes an element (returns true / false)
remove(index) – removes element at index
contains(element) – determines whether an element is in the list
set(index, item) – replaces the element at the given index
List<E> – Data Structure
11
11. add – Appends an Element
12
10
5
2
0
Count: 1
2
3
List<Integer>
10
5
2
15. First, read from the console the array length:
Next, create a list of given size n and read its elements:
Reading Lists From the Console
Scanner sc = new Scanner(System.in);
int n = Integer.parseInt(sc.nextLine());
List<Integer> list = new ArrayList<>();
for (int i = 0; i < n; i++) {
int number = Integer.parseInt(sc.nextLine());
list.add(number);
}
16. Lists can be read from a single line of space separated values:
Reading List Values from a Single Line
17
2 8 30 25 40 72 -2 44 56
String values = sc.nextLine();
List<String> items = Arrays.stream(values.split(" "))
.collect(Collectors.toList());
List<Integer> nums = new ArrayList<>();
for (int i = 0; i < items.size(); i++)
nums.add(Integer.parseInt(items.get(i)));
Convert a collection
into List
List<Integer> items = Arrays.stream(values.split(" "))
.map(Integer::parseInt).collect(Collectors.toList());
17. Printing a list using a for-loop:
Printing a list using a String.join():
Printing Lists on the Console
18
List<String> list = new ArrayList<>(Arrays.asList(
"one", "two", "three", "four", "five", "six"));
for (int index = 0; index < list.size(); index++)
System.out.printf
("arr[%d] = %s%n", index, list.get(index));
List<String> list = new ArrayList<>(Arrays.asList(
"one", "two", "three", "four", "five", "six"));
System.out.println(String.join("; ", list));
Gets an element
at given index
18. Write a program to sum all adjacent equal numbers in a list of
decimal numbers, starting from left to right
Examples:
Problem: Sum Adjacent Equal Numbers
19
3 3 6 1 12 1
8 2 2 4 8 16 16 8 16
5 4 2 1 1 4 5 8 4
19. Scanner sc = new Scanner(System.in);
List<Double> numbers = Arrays.stream(sc.nextLine().split(" "))
.map(Double::parseDouble).collect(Collectors.toList());
for (int i = 0; i < numbers.size() - 1; i++)
if (numbers.get(i).equals(numbers.get(i + 1))) {
numbers.set(i, numbers.get(i) + numbers.get(i + 1));
numbers.remove(i + 1);
i = -1;
}
// Continues on the next slide
Solution: Sum Adjacent Equal Numbers (1)
0
21. Write a program that sums all numbers in a list in the
following order:
first + last, first + 1 + last - 1, first + 2 + last - 2, … first + n, last – n
Examples:
Problem: Gauss' Trick
22
1 2 3 4 5 6 6 3
1 2 3 4 5 5
22. Solution: Gauss' Trick
3
Scanner sc = new Scanner(System.in);
List<Integer> numbers = Arrays.stream(sc.nextLine().split(" "))
.map(Integer::parseInt).collect(Collectors.toList());
int size = numbers.size();
for (int i = 0; i < size / 2; i++) {
numbers.set(i, numbers.get(i) + numbers.get(numbers.size() - 1));
numbers.remove(numbers.size() - 1);
}
System.out.println(numbers.toString().replaceAll("[[],]", ""));
23. You receive two lists with numbers. Print a result list which
contains the numbers from both lists
If the length of the two lists is not equal, just add the
remaining elements at the end of the list
list1[0], list2[0], list1[1], list2[1], …
Problem: Merging Lists
24
1 2 3 4 5
6 7 8
1 6 2 7 3 8 4 5
24. // TODO: Read the input
List<Integer> resultNums = new ArrayList<>();
for (int i = 0; i < Math.min(nums1.size(), nums2.size()); i++) {
// TODO: Add numbers in resultNums
}
if (nums1.size() > nums2.size())
resNums.addAll(getRemainingElements(nums1, nums2));
else if (nums2.size() > nums1.size())
resNums.addAll(getRemainingElements(nums2, nums1));
System.out.println(resNums.toString().replaceAll("[[],]", ""));
Solution: Merging Lists (1)
25
25. public static List<Integer> getRemainingElements
(List<Integer> longerList, List<Integer> shorterList) {
List<Integer> nums = new ArrayList<>();
for (int i = shorterList.size(); i < longerList.size(); i++)
nums.add(longerList.get(i));
return nums;
}
Solution: Merging Lists (2)
26
28. Sorting a list == reorder its elements incrementally: Sort()
List items should be comparable, e.g. numbers, strings, dates, …
Sorting Lists
List<String> names = new ArrayList<>(Arrays.asList(
"Peter", "Michael", "George", "Victor", "John"));
Collections.sort(names);
System.out.println(String.join(", ", names));
// George, John, Michael, Peter, Victor
Collections.sort(names);
Collections.reverse(names);
System.out.println(String.join(", ", names));
// Victor, Peter, Michael, John, George
Sort in natural
(ascending) order
Reverse the sorted result
29. Read a number n and n lines of products. Print a numbered list
of all the products ordered by name
Examples:
Problem: List of Products
30
4
Potatoes
Tomatoes
Onions
Apples
1.Apples
2.Onions
3.Potatoes
4.Tomatoes
A
Z
30. int n = Integer.parseInt(sc.nextLine());
List<String> products = new ArrayList<>();
for (int i = 0; i < n; i++) {
String currentProduct = sc.nextLine();
products.add(currentProduct);
}
Collections.sort(products);
for (int i = 0; i < products.size(); i++)
System.out.printf("%d.%s%n", i + 1, products.get(i));
Solution: List of Products
31. Read a list of integers, remove all negative numbers from it
Print the remaining elements in reversed order
In case of no elements left in the list, print "empty"
Problem: Remove Negatives and Reverse
32
10 -5 7 9 -33 50 50 9 7 10
7 -2 -10 1 1 7
-1 -2 -3 empty
32. List<Integer> nums = Arrays.stream(sc.nextLine().split(" "))
.map(Integer::parseInt).collect(Collectors.toList());
for (int i = 0; i < nums.size(); i++)
if (nums.get(i) < 0)
nums.remove(i--);
Collections.reverse(nums);
if (nums.size() == 0)
System.out.println("empty");
else
System.out.println(nums.toString().replaceAll("[[],]", ""));
Solution: Remove Negatives and Reverse
34. …
…
…
Summary
35
Lists hold a sequence of elements
(variable-length)
Can add / remove / insert elements
at runtime
Creating (allocating) a list:
new ArrayList<E>()
Accessing list elements by index
Printing list elements: String.join(…)
35. …
…
…
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Notas del editor
Hello, I am Svetlin Nakov from SoftUni (the Software University). Together with my colleague George Georgiev, we shall teach this free Java Foundations course, which covers important concepts from Java programming, such as arrays, lists, methods, strings, classes, objects and exceptions, and prepares you for the "Java Foundations" official exam from Oracle.In this lesson your instructor George will explain and demonstrate how to use lists in Java, how to allocate a list using the ArrayList generic class, how to access list elements by index, how to add, modify, insert, and delete elements from list and how to read, traverse and print a list.Lists in Java are like arrays: they hold an indexed sequence of elements of the same type. But unlike arrays, lists can resize. In this tutorial on Java arrays, along with the live coding examples, your instructor George will give you some hands-on exercises to gain practical experience.
Are you ready? Let's start!
Before the start, I would like to introduce your course instructors: Svetlin Nakov and George Georgiev, who are experienced Java developers, senior software engineers and inspirational tech trainers.
They have spent thousands of hours teaching programming and software technologies and are top trainers from SoftUni.
I am sure you will like how they teach programming.
Most of this course will be taught by George Georgiev, who is a senior software engineer with many years of experience with Java, JavaScript and C++.
George enjoys teaching programming very much and is one of the top trainers at the Software University, having delivered over 300 technical training sessions on the topics of data structures and algorithms, Java essentials, Java fundamentals, C++ programming, C# development and many others.
I have no doubt you will benefit greatly from his lessons, as he always does his best to explain the most challenging concepts in a simple and fun way.
Before we dive into the course, I want to show you the SoftUni judge system, where you can get instant feedback for your exercise solutions.
SoftUni Judge is an automated system for code evaluation. You just send your code for a certain coding problem and the system will tell you whether your solution is correct or not and what exactly is missing or wrong.I am sure you will love the judge system, once you start using it!
// Solution to problem "01. Student Information".
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
String name = sc.nextLine();
int age = Integer.parseInt(sc.nextLine());
double grade = Double.parseDouble(sc.nextLine());
System.out.printf("Name: %s, Age: %d, Grade: %.2f",
name, age, grade);
}
}
In this section your instructor George will explain the concept of "Lists in Java" and how to use the ArrayList class.
Lists in Java are like arrays, but they can change their size. You can add, modify, insert and delete elements from a list. Lists hold indexed sequence of elements, which can be freely modified.
Let's learn how to use lists in Java and solve a few hands-on exercises to gain experience in processing sequences of elements.
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