javac and java

Learn to compile and run from the command line.

What is javac?

javac is the Java compiler. It turns your source file (.java) into a .class file containing bytecode that the JVM can read.

javac Main.java

When the command runs, Main.class appears in the same folder. If compilation fails, you get error messages; on success it finishes silently.

javac checks the source code: it catches syntax errors, type mismatches and missing semicolons.

The java command runs a compiled .class file on the JVM. You pass the class name without extension:

java Main

This command loads Main.class, finds the main method and runs it. The output appears on the screen.

This two-step flow follows the compile once, run anywhere philosophy. The same .class file runs on Windows, Linux and macOS.

Define a Main class with a main method. Print Running with println. This code must compile with javac Main.java and run with java Main.

Show example solution

public class Main {
    public static void main(String[] args) {
        System.out.println("Running");
    }
}