What Actually Happens When You Compile and Run Java

The short answer#

Running a Java program takes two commands, and they are run by two different programs:

javac Hello.java     // compile: source  -> bytecode
java Hello           // run:     bytecode -> output

javac is the compiler. It reads the text you wrote and, if the text is legal Java, writes a new file called Hello.class. java is the launcher. It starts the Java Virtual Machine and executes that .class file.

Step 1 — javac turns source into bytecode#

Start with one file, Hello.java:

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

Before compiling, the directory holds one file. After javac Hello.java, it holds two:

Hello.java     119 bytes    your source
Hello.class    417 bytes    produced by javac

The .class file is bytecode. It is not the text you wrote, and it is also not machine code for your particular processor. It is an instruction set for an imaginary computer — the JVM — which every real machine then emulates.

You can see that it is a real, structured binary format rather than compressed text. Every .class file ever produced begins with the same four bytes:

ca fe ba be 00 00 00 41

Those first four bytes spell CAFEBABE — the format's magic number, a joke that has been in the file format since 1995. The 41 at the end is hexadecimal for 65, the class-file version written by Java 21.

Step 2 — java runs the bytecode#

Now the second command. Note carefully what you hand it:

java Hello

Hello, world!

You passed Hello, not Hello.class and not Hello.java. The launcher wants a class name, and it works out the filename itself. Typing the extension is probably the single most common first-day error:

$ java Hello.class
Error: Could not find or load main class Hello.class
Caused by: java.lang.ClassNotFoundException: Hello.class

Read that message literally and it makes sense: it went looking for a class named Hello.class, and there is no such class.

Why bother with two steps?#

Languages like C compile straight to machine code for one kind of processor, so a program built for Windows will not run on a Mac. Languages like Python skip compilation and interpret the source line by line every time, which costs speed.

Java splits the difference. javac does the expensive analysis once and produces bytecode that is not tied to any processor. Any machine with a JVM can then run that same .class file unchanged. This is what the old slogan write once, run anywhere is describing — the portable artifact is the compiled .class file, not the source.

The five errors you will actually hit first#

These are worth reading now, because you will meet all five, and each one is easier to fix when you recognize the wording.

1. The file name does not match the public class#

Saving public class Hello into a file called Greeter.java:

Greeter.java:1: error: class Hello is public, should be declared in a file named Hello.java
public class Hello {
       ^
1 error

A public class must live in a file of exactly the same name, capital letters included. Rename one or the other.

2. A missing semicolon#

Semi.java:3: error: ';' expected
        System.out.println("hi")
                                ^
1 error

Notice where the caret points: at the end of line 3, not at line 4. The compiler reports the position where the missing thing should have been, which is why a missing semicolon is often reported on the line above the one that looks wrong.

3. No main method#

This one compiles perfectly and then fails at run time — a class with no main is legal Java, it just cannot be used as a starting point:

Error: Main method not found in class NoMain, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

4. main exists but is not static#

Error: Main method is not static in class Inst, please define the main method as:
   public static void main(String[] args)

The JVM has to call main before any object exists, so it can only call a method that belongs to the class itself. That is what static means here.

5. main returns something#

In C a program's entry point returns an int. In Java it does not, and the JVM says so plainly:

Error: Main method must return a value of type void in class IntMain, please
define the main method as:
   public static void main(String[] args)

Compile-time or run-time?#

Sort every error you meet into one of these two columns and the fix usually suggests itself.

Where these go wrong#

• Editing and re-running without recompiling. java reads the .class file, so until you run javac again you are still running yesterday's program. If a change appears to have no effect at all, check this first.
• Typing java Hello.class. The launcher takes a class name. No extension, ever.
• Fixing errors from the bottom up. One real mistake often produces several complaints. Fix the first error, recompile, and watch most of the rest disappear.
• Assuming a compile error means the program crashed. It never even started. There is no output to look for and no .class file to run.
• Case. Hello.java and hello.java are different files, and Hello and hello are different classes. Windows will sometimes let you get away with this locally and then it breaks elsewhere.

Frequently asked questions#