Learn how to use arrays in Java, how to enter array, how to traverse an array, how to print array and more array operations.
Watch the video lesson and access the hands-on exercises here: https://softuni.org/code-lessons/java-foundations-certification-arrays
8. In programming, an array is a sequence of elements
Arrays have fixed size (array.length)
cannot be resized
Elements are of the same type (e.g. integers)
Elements are numbered from 0 to length-1
What are Arrays?
9
Array of 5
elements
Element’s index
Element of an array
… … … … …
0 1 2 3 4
9. Allocating an array of 10 integers:
Assigning values to the array elements:
Accessing array elements by index:
Working with Arrays
10
int[] numbers = new int[10];
for (int i = 0; i < numbers.length; i++)
numbers[i] = 1;
numbers[5] = numbers[2] + numbers[7];
numbers[10] = 1; // ArrayIndexOutOfBoundsException
All elements are
initially == 0
The length holds
the number of
array elements
The [] operator
accesses
elements by index
10. The days of a week can be stored in an array of strings:
Days of Week – Example
11
String[] days = {
"Monday",
"Tuesday",
"Wednesday",
"Thursday",
"Friday",
"Saturday",
"Sunday"
};
Operator Value
days[0] Monday
days[1] Tuesday
days[2] Wednesday
days[3] Thursday
days[4] Friday
days[5] Saturday
days[6] Sunday
11. Enter a day number [1…7] and print
the day name (in English) or "Invalid day!"
Problem: Day of Week
12
String[] days = { "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday", "Sunday" };
int day = Integer.parseInt(sc.nextLine());
if (day >= 1 && day <= 7)
System.out.println(days[day - 1]);
else
System.out.println("Invalid day!");
The first day in our array
is on index 0, not 1.
13. First, read the array length from the console :
Next, create an array of given size n and read its elements:
Reading Arrays From the Console
14
int n = Integer.parseInt(sc.nextLine());
int[] arr = new int[n];
for (int i = 0; i < n; i++) {
arr[i] = Integer.parseInt(sc.nextLine());
}
14. Arrays can be read from a single line of separated values
Reading Array Values from a Single Line
15
String values = sc.nextLine();
String[] items = values.split(" ");
int[] arr = new int[items.length];
for (int i = 0; i < items.length; i++)
arr[i] = Integer.parseInt(items[i]);
2 8 30 25 40 72 -2 44 56
15. Read an array of integers using functional programming:
Shorter: Reading Array from a Single Line
16
int[] arr = Arrays
.stream(sc.nextLine().split(" "))
.mapToInt(e -> Integer.parseInt(e)).toArray();
String inputLine = sc.nextLine();
String[] items = inputLine.split(" ");
int[] arr = Arrays.stream(items)
.mapToInt(e -> Integer.parseInt(e)).toArray();
You can chain
methods
import
java.util.Arrays;
16. To print all array elements, a for-loop can be used
Separate elements with white space or a new line
Printing Arrays on the Console
17
String[] arr = {"one", "two"};
// == new String [] {"one", "two"};
// Process all array elements
for (int i = 0; i < arr.length; i++) {
System.out.printf("arr[%d] = %s%n", i, arr[i]);
}
17. Read an array of integers (n lines of integers), reverse it and
print its elements on a single line, space-separated:
Problem: Reverse an Array of Integers
18
3
10
20
30
30 20 10
4
-1
20
99
5
5 99 20 -1
18. Solution: Reverse an Array of Integers
19
// Read the array (n lines of integers)
int n = Integer.parseInt(sc.nextLine());
int[] arr = new int[n];
for (int i = 0; i < n; i++)
arr[i] = Integer.parseInt(sc.nextLine());
// Print the elements from the last to the first
for (int i = n - 1; i >= 0; i--)
System.out.print(arr[i] + " ");
System.out.println();
19. Use for-loop:
Use String.join(separator, array):
Printing Arrays with for / String.join(…)
20
String[] strings = { "one", "two" };
System.out.println(String.join(" ", strings)); // one two
int[] arr = { 1, 2, 3 };
System.out.println(String.join(" ", arr)); // Compile error
String[] arr = {"one", "two"};
for (int i = 0; i < arr.length; i++)
System.out.println(arr[i]);
Works only
with strings
20. Read an array of strings (space separated values), reverse it and
print its elements:
Reversing array elements:
Problem: Reverse Array of Strings
21
a b c d e e d c b a -1 hi ho w w ho hi -1
a b c d e
exchange
21. Solution: Reverse Array of Strings
22
String[] elements = sc.nextLine().split(" ");
for (int i = 0; i < elements.length / 2; i++) {
String oldElement = elements[i];
elements[i] = elements[elements.length - 1 - i];
elements[elements.length - 1 - i] = oldElement;
}
System.out.println(String.join(" ", elements));
23. Iterates through all elements in a collection
Cannot access the current index
Read-only
For-each Loop
for (var item : collection) {
// Process the value here
}
24. int[] numbers = { 1, 2, 3, 4, 5 };
for (int number : numbers) {
System.out.println(number + " ");
}
Print an Array with Foreach
25
1 2 3 4 5
25. Read an array of integers
Sum all even and odd numbers
Find the difference
Examples:
Problem: Even and Odd Subtraction
26
1 2 3 4 5 6 3
3 5 7 9 11 -35
2 4 6 8 10 30
2 2 2 2 2 2 12
26. int[] arr = Arrays.stream(sc.nextLine().split(" "))
.mapToInt(e -> Integer.parseInt(e)).toArray();
int evenSum = 0;
int oddSum = 0;
for (int num : arr) {
if (num % 2 == 0) evenSum += num;
else oddSum += num;
}
// TODO: Find the difference and print it
Solution: Even and Odd Subtraction
27
28. …
…
…
Summary
29
Arrays hold a sequence of elements
Elements are numbered
from 0 to length – 1
Creating (allocating) an array
Accessing array elements by index
Printing array elements
29. …
…
…
Next Steps
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Notas do 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 work with arrays: reading arrays from the console, processing arrays, using the for-each loop, printing arrays and simple array algorithms. You will learn also how to declare and allocate an array of certain length, two ways to read an array from the console, how to traverse and print the elements from array, how to access an element by index and to modify an element at certain index.
Along with the live coding examples, your instructor George will give you some hands-on exercises to gain practical experience with the mentioned coding concepts.
Let's start learning arrays!
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 programming arrays are indexed sequences of elements, which naturally map to the real-world sequences and sets of objects.
In this section, you will learn the concept of "arrays" and how to use arrays in Java: how to declare and allocate an array of certain length, how to read an array from the console, how to traverse and print the elements from array, how to access an element by index and to modify an element at certain index.
Working with arrays and lists is an essential skill for the software development profession, so you should spend enough time to learn it in depth. In this course we have prepared many hands-on exercises to practice working with arrays. Don't skip them!
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