Workshop1

Introduction to Java

This workshop will cover Java, a popular object-oriented programming language, from the basics to intermediate object-oriented concepts. The course is fast paced and recommended for those who have a little bit of experience already. If you are a complete beginner we recommend the Intro to Python workshop.

If you want the exact code from the workshop click here

Installing Java and Eclipse

Eclipse is the Integrated Development Environment (IDE) where we will be writing our Java code.

Windows and Mac

1. Download Java from here
2. Select Windows x86 if your computer is 32-bit or Windows x64 if your computer is 64-bit or macOS for Mac
3. Double-click the downloaded file and follow the prompts to install Java
4. Download Eclipse from here
5. Under Get Eclipse click the orange button, it should download the appropriate version
6. Double-click the downloaded file and select Eclipse IDE for Java Developers
7. Press Install and follow the prompts.
8. You can now open Eclipse and run the example Hello, world! program

Linux (Ubuntu)

1. Install Java by running the following

sudo apt-get install openjdk-8-jdk

2. Install Eclipse by following this tutorial

Hello World

1. From the Welcome screen press Create a Hello World application
2. Follow the steps on the right
3. Once done you should see this in your editor

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello world");
    }
}

The first line of code denotes the name of your program. It should be the same as the file your program is in. We will learn more about public class later in the workshop.

public class HelloWorld { // Should be in a file called HelloWorld.java

The second line of code denotes the starting point for your code. It is always as follows. We learn more about it later.

public static void main(String[] args) {

Both of these lines have to have a matching curly brace to denote the end of each section. Like this

public static void main(String[] args) {
}// This is the matching curly brace

The third line of code is what prints "Hello world" to the screen, without the quotes.

System.out.println("Hello world");

You can print anything with this line. Whatever you want on the screen will go within the quotes

System.out.println("Anything!"); // Anything!

Every line of code needs a ';' otherwise your code won't work!

You may have noticed that some lines we put // followed by some text. This is a comment and lets you describe what is happening. There are also mult-line comments.

// Regular comment
/* 
  Multi-line
  comment
*/

Primitive data types

In Java, there are 8 primitive data types.

Data Type	Description	Example
boolean	true or false	true
byte	holds small numbers, 8 bits	64
char	holds letters	'a'
short	hold slightly larger numbers	2345
int	holds large numbers	1231232
long	holds even large numbers	2322L
float	holds decimal numbers	23.2f
double	holds decimal numbers	23.2

They are used when defining variables so we know what is inside them.

int x = 12;
int y = x + 14;
System.out.println(y); // Prints 26 to the screen

Bonus: Reading user input

This topic is a little advanced since it requires knowledge from later in the course but is usually necessary to make useful programs.

Scanner in = new Scanner(System.in)
int x = in.nextInt();
System.out.println(x);

Here we are creating what we call a Scanner object. Scanner is another data type but it was defined by someone else. We put System.in in the paretheses to denote we are reading from the user.

int x = in.nextInt();

.nextInt() is a function that reads a number from the user in format of int. The user's number gets put into the x variable. We then print to the screen so we know it was read.

There are functions for all the primite data types as well as String, which holds text.

double y = in.nextDouble(); // Reads the next double
String s = in.nextLine(); // Reads the whole line in the format of String