To
install Java 8 on your Pi use the following command:
sudo apt-get install oracle-java8-jdk
To
check that everything installed OK, type:
java -version
The
output shows that Java 1.8.0 has been installed. 1.8 means Java 8. The initial
releases of Java were all “point” releases: 1.0, 1.1, 1.2, 1.3, and 1.4. But in
2004 when Java 1.5 was released, it was marketed as Java 5; however, 1.5
still remained the internal version number. So 1.5 is Java 5, 1.6 is Java 6,
1.7 is Java 7, and 1.8 is Java 8.
Now
to write a simple Java program. Create a file called “Hello.java” using nano:
nano Hello.java
The
capital “H” is important, so be sure you type “Hello.java” and not
“hello.java”. Copy and paste the following code into the file:
public class Hello {
public static void main(String[] args) {
System.out.println("Hello Make Tech
Easier");
}
}
In
Java, the public class name must be the same as the file name. This makes it
easier to find classes when you have big programs. If you need to find the
class “TheNetworkListener,” then it will be in the file
“TheNetworkListener.java,” and so on.
To
compile the program type:
javac Hello.java
If
the compilation was successful then there will be no output. But if the program
has errors, then they will be displayed.
To
run the program type:
java Hello
The
output will be the string “Hello Make Tech Easier”
Congratulations,
you have now written and run your first Java program on a Raspberry Pi.
You
can also write GUI programs with Java. There are integrated development
environments (IDE) like NetBeans, which allow you to design Java desktop
applications graphically. But it is also possible to write GUI programs using
just a text editor.
Create
a file called “HelloDialog.java” with nano:
nano HelloDialog.java
Copy
and paste the following code:
import javax.swing.JOptionPane;
public class HelloDialog {
public static void main(String[] args) {
}
}
Make
sure you have the Raspberry Pi desktop running (using “startx” if necessary).
Compile the program using “javac HelloDialog.java” and run it using “java
HelloDialog“.
The
program uses Java’s GUI widget toolkit called Swing. All it does is show a
message dialog box. However, more complex programs can be written using Swing.
Create a file called “SimpleSwing.java” and edit it with nano. Copy and paste
the following code into the file.
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
public class SimpleSwing extends JFrame
{
public SimpleSwing() {
setTitle("Simple
example");
setSize(300, 200);
setLocationRelativeTo(null);
setDefaultCloseOperation(EXIT_ON_CLOSE);
}
public static void main(String[] args) {
@Override
public void run() {
SimpleSwing ex = new SimpleSwing();
ex.setVisible(true);
}
});
}
}
The
program is quite simple; however, it does show that you can create a
window with a title. The window can be resized and closed, just as you would
expect. This is the building block for more complex applications.
There are lots of tutorials online about the Java programming
language. Here is a small selection:
If you have any problems with the examples given above or
with getting Java to work on the Raspberry Pi, then please feel free to leave a
comment below and we will see if we can help.
