Installing Java and Running Programs

Let’s get Java running on your computer. You will install the JDK, compile a program from the terminal, try JShell, and set up an editor.

Step 1: Install the JDK#

Download a free JDK build. Eclipse Temurin 21 (LTS) from adoptium.net is a great choice. Oracle JDK 21 also works.

• Windows: download the .msi installer and tick “Set JAVA_HOME variable” and “Add to PATH”.
• Mac: download the .pkg, or run brew install --cask temurin@21.
• Linux (Ubuntu): sudo apt install openjdk-21-jdk.

Step 2: Check the installation#

Open a new terminal (Command Prompt / PowerShell on Windows) and run:

java -version
javac -version

You should see something like openjdk version "21.0.4". If you see “command not found”, the JDK’s bin folder is not on your PATH. Reinstall with the PATH option ticked, or add it manually.

Step 3: Compile and run from the terminal#

Create a folder, and a file named Greeting.java inside it:

public class Greeting {
    public static void main(String[] args) {
        String name = "Asha";
        System.out.println("Welcome to Java, " + name + "!");
    }
}

In the terminal, go to that folder and run:

javac Greeting.java      # creates Greeting.class (bytecode)
java Greeting            # runs it on the JVM (no .class extension!)

Output:

Welcome to Java, Asha!

Shortcut: run a single file directly#

Since Java 11 you can skip the javac step for single-file programs:

java Greeting.java

Java compiles it in memory and runs it. Perfect for practice.

Step 4: JShell, Java’s playground#

JShell (Java 9+) lets you type Java and see results immediately, with no class or main needed:

jshell

jshell> int a = 7
a ==> 7

jshell> a * 6
$2 ==> 42

jshell> "java".toUpperCase()
$3 ==> "JAVA"

jshell> /exit

Use JShell to test small ideas quickly.

Step 5: Choose an editor (IDE)#

Editor	Why
IntelliJ IDEA Community (free)	The most popular Java IDE: smart completion, refactoring, debugger. Recommended.
VS Code + “Extension Pack for Java”	Lightweight, good if you already use VS Code.
Eclipse	Older but still used in many companies.

In IntelliJ: New Project → Java → choose JDK 21 → Create. Right-click the src folder → New → Java Class, type Main, then type psvm and press Tab: IntelliJ writes the main method for you. Type sout + Tab for System.out.println().

Command-line arguments#

The String[] args in main receives words typed after the class name:

public class Args {
    public static void main(String[] args) {
        System.out.println("You passed " + args.length + " arguments");
        for (String a : args) {
            System.out.println("- " + a);
        }
    }
}

java Args.java red green

You passed 2 arguments
- red
- green

Reading compiler errors#

Java’s compiler tells you the file, the line and the problem:

Greeting.java:4: error: ';' expected
        System.out.println("Hi")
                                ^
1 error

Read the first error first: later errors are often caused by it. Line 4, a semicolon is missing.

Common mistakes#

• Running java Greeting.class (wrong) instead of java Greeting.
• An old Java version on the PATH: check java -version and javac -version show the same version.
• Saving the file as Greeting.java.txt on Windows (turn on “show file extensions”).

Exercise#

1. Install the JDK and confirm java -version works.
2. Write About.java that prints three facts about you, then compile and run it with javac and java.
3. In JShell, calculate how many seconds are in a day.
4. Modify Args.java to print the arguments in reverse order.

Hint for 4

Loop from args.length - 1 down to 0: for (int i = args.length - 1; i >= 0; i--).

Summary#

• Install a JDK (Temurin 21), then check with java -version.
• javac File.java compiles; java ClassName runs. java File.java does both for single files.
• JShell is a quick playground; IntelliJ IDEA is the most popular IDE.

Next: variables and data types, the building blocks of every program.