How Java Works

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Hey there, and welcome!

In this quick breakdown, we’re going to explore how Java works — from writing your code to running your program.

We’ll walk step-by-step through the Java workflow using a simple visual guide. So whether you’re brand new to Java or just brushing up on the basics, this will help everything make sense.

Let’s dive in!

Step 1 – Writing Code in the Editor

Everything starts in the code editor — this is where you actually write your Java code.

Let’s say you write a simple program like printing “Hello, world!”

You save that code in a source file with a .java extension. So if your class is called HelloWorld, your file will be named HelloWorld.java.

At this stage, the code is just plain text — it hasn’t been converted into anything the computer can run yet.

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Step 2 – Compiling to Bytecode

Next, we need to compile that code.

Java comes with a compiler called javac — short for Java compiler. It takes your .java file and translates it into bytecode, which is saved in a .class file.

Now, here’s the cool part — bytecode is not specific to your computer or operating system. It’s written in a kind of “universal language” that the Java Virtual Machine, or JVM, can understand.

So your original code goes from human-friendly to machine-prepared.

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Step 3 – The Role of JVM and JRE

Once we have bytecode, we pass it to the JVM, or Java Virtual Machine.

This is where things get interesting.

The JVM is part of the JRE, or Java Runtime Environment. The JRE includes the JVM plus all the built-in Java libraries your program might need to run — like math functions, date and time utilities, or graphical tools.

The JVM reads the bytecode and prepares it for execution — but it doesn’t just interpret it line by line. Java also uses something called the Just-In-Time Compiler, or JIT.

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Step 4 – Just-In-Time Compilation

Here’s what the Just-In-Time Compiler does:

Instead of interpreting the bytecode every single time your program runs, the JIT compiler translates it into machine code — also called binary code — right when the program needs it.

This makes Java much faster than traditional interpreted languages, because the heavy lifting only happens once, and after that, the code runs almost as fast as native apps.

Think of it as real-time optimization. The JIT compiler watches how your program runs and then improves performance on the fly.

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Step 5 – Running the Program

Finally, once the bytecode is translated into machine code, the computer knows exactly what to do.

Your program starts running — whether it’s printing messages to the screen, doing calculations, or controlling a full desktop application.

All of this happens seamlessly because of how Java is designed — write it once, run it anywhere. That’s the power of the JVM and bytecode together.

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Wrap-Up

So let’s quickly recap how Java works:

• You start by writing code in a code editor, and save it in a .java file.
• That file is passed through the Java compiler, which turns it into bytecode — saved in a .class file.
• The JVM inside the JRE takes that bytecode and, with the help of the Just-In-Time compiler, converts it into machine code.
• Your program finally runs — using Java’s built-in libraries and executing at high speed.

This structure is what makes Java portable, efficient, and powerful — and it's why Java has been one of the world’s most used programming languages for decades.

Thanks for reading, and I’ll see you in the next lesson!