Creating and Running .java Files

Write your Java code in a text editor and save the file with a .java extension. Save the file using the UTF-8 character encoding.

In Java, the filename and class name must always match. For example, if you define a class named Greet, the filename must be Greet.java.

sample_Greet.java

public class Greet {
    public static void main(String[] args) {
        String name = "Ayanami Rei";
        int age = 14;

        System.out.println("Hello, " + name + "!");
        System.out.println("Age: " + age);

        if (age >= 20) {
            System.out.println("Adult.");
        } else {
            System.out.println("Minor.");
        }
    }
}

Java programs must always be written inside a class. Execution begins from the main method, which is the entry point of the program.

Greet.java

javac Greet.java
java Greet

Hello, Ayanami Rei!
Age: 14
Minor.

You can write comments (notes) in a .java file. Comments are ignored by the compiler, so they are useful for leaving descriptions or notes about the code.

sample_SampleComments.java

/**
 * A sample class.
 * Demonstrates how to write comments.
 */
public class SampleComments {

    // A method that returns the username.
    public static String getName() {
        return "Ikari Shinji";
    }

    public static void main(String[] args) {
        /*
          The following outputs a greeting message.
          The name is retrieved using the getName() method.
        */
        String name = getName();
        System.out.println("Hello, " + name + "!");
    }
}

SampleComments.java

javac SampleComments.java
java SampleComments

Hello, Ikari Shinji!

Because Java is a compiled language, two steps are required: "compile" and "run."

Step 1: Compile

Use the javac command to compile the .java file. If successful, a ClassName.class file is generated in the same directory. If no error message is displayed, compilation succeeded.

javac Greet.java

Step 2: Run

Use the java command with the class name (without extension) to run. Note that you specify the class name, not the filename.

java Greet

Hello, Ayanami Rei!
Age: 14
Minor.

To check the version, use the following command.

java --version

A .java file is simply a plain text file. You can create one by writing Java code in a text editor and saving it with a .java extension. No special tools are needed.

Because Java is a compiled language, two steps are required: compile with javac filename.java, then run with java ClassName. Compilation produces a class file (.class), which is then executed by the Java Virtual Machine (JVM).

An important rule in Java is that the filename and the public class name must always match. For example, Greet.java must contain the Greet class. A mismatch causes a compilation error.

For information on installing Java, see Setup.