The JDK and Your First Program — Java Tutorial

Before you can write Java, you need the JDK — the Java Development Kit. It's easy to see "JDK", "JRE", and "JVM" thrown around interchangeably, but they're three different things with a clear relationship.

JDK vs. JRE vs. JVM

JVM (Java Virtual Machine) — the engine that actually runs compiled Java bytecode. This is what makes Java portable across operating systems.
JRE (Java Runtime Environment) — the JVM plus the standard library classes needed to run Java programs. If all you need is to run a Java app, this used to be enough.
JDK (Java Development Kit) — the JRE plus the tools needed to develop Java: the javac compiler, the java launcher, a debugger, and more.

Modern Java distributions (like Eclipse Temurin, Oracle's own JDK, or Amazon Corretto) bundle everything as one JDK download — you don't install the JRE separately anymore. Install a JDK (Java 17 or 21 are good, current long-term-support versions), and confirm it worked:

java -version
javac -version

Writing your first program

Every standalone Java program needs a class containing a main method — that's the entry point the JVM looks for when you run the program.

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

A few details worth internalizing immediately:

The file name must match the public class name exactly, including case: this file must be saved as Hello.java. Java enforces this at compile time.
public static void main(String[] args) is a fixed signature the JVM looks for — public so the JVM can call it from outside the class, static so it can be called without creating an object first, void because it returns nothing, and String[] args to receive command-line arguments.
System.out.println(...) prints a line to standard output, followed by a newline. System.out.print(...) does the same without the trailing newline.

Semicolons and braces

Java statements end with a semicolon, and code blocks (the body of a class, method, loop, or conditional) are wrapped in curly braces { }. Unlike Python, indentation is purely cosmetic to Java — the braces are what actually define scope. Most style guides still expect consistent indentation regardless, because unindented Java is nearly unreadable.

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

Every lesson from here builds directly off this skeleton — you'll always have a class and a main method (or a method called from it) to hold the code you're experimenting with.

JDK vs. JRE vs. JVM

JVM (Java Virtual Machine) — the engine that actually runs compiled Java bytecode. This is what makes Java portable across operating systems.

JRE (Java Runtime Environment) — the JVM plus the standard library classes needed to run Java programs. If all you need is to run a Java app, this used to be enough.

JDK (Java Development Kit) — the JRE plus the tools needed to develop Java: the javac compiler, the java launcher, a debugger, and more.

java -version
javac -version

Writing your first program

Every standalone Java program needs a class containing a main method — that's the entry point the JVM looks for when you run the program.

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

A few details worth internalizing immediately:

The file name must match the public class name exactly, including case: this file must be saved as Hello.java. Java enforces this at compile time.

public static void main(String[] args) is a fixed signature the JVM looks for — public so the JVM can call it from outside the class, static so it can be called without creating an object first, void because it returns nothing, and String[] args to receive command-line arguments.

System.out.println(...) prints a line to standard output, followed by a newline. System.out.print(...) does the same without the trailing newline.

Semicolons and braces

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

Every lesson from here builds directly off this skeleton — you'll always have a class and a main method (or a method called from it) to hold the code you're experimenting with.