Is Java Compiled or Interpreted?

Introduction

A common question for Java beginners is whether Java is a compiled or interpreted language. The answer is both. Java uses a combination of compilation and interpretation to run programs. This process allows Java to work on different operating systems efficiently.

In this post, we’ll explain how Java uses both compilation and interpretation and provide a simple example to help you understand.

Compilation in Java

When you write a Java program, the first step is to compile the code. Java code is written in a human-readable form called source code. The Java compiler converts this source code into bytecode, an intermediate form that the computer can understand.

Example:

Let’s say you write a basic Java program called HelloWorld.java. This program prints the message "Hello, World!" to the screen.

Here is the code for the HelloWorld program:

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

This is how the program works:

• public class HelloWorld defines a class called HelloWorld.
• Inside the class, the main method is defined. This is where the program starts running.
• System.out.println("Hello, World!"); prints "Hello, World!" to the screen.

How to Compile:

To compile the Java program, you use the Java compiler (javac). Here’s how you compile the program:

javac HelloWorld.java

This command creates a file called HelloWorld.class, which contains the bytecode. The bytecode is the same for any operating system.

Interpretation in Java

Once you have compiled the Java code into bytecode, the next step is interpretation. The Java Virtual Machine (JVM) reads the bytecode and translates it into machine code that your operating system can understand.

The JVM is specific to the operating system you are using. For example, if you are on Windows, you have the Windows JVM. If you are on macOS, you have the macOS JVM. Each JVM can interpret the same bytecode and run the program without any changes.

How to Run the Program:

To run the program, use the following command:

java HelloWorld

This tells the JVM to interpret the HelloWorld.class bytecode and run the program. The output will be:

Hello, World!

Just-In-Time (JIT) Compilation

The JVM also uses a feature called Just-In-Time (JIT) compilation to make the program faster. Instead of interpreting the bytecode line by line every time, the JIT compiler converts frequently used code into machine code while the program runs. This makes the program run faster because machine code is more efficient than bytecode.

Why Java is Both Compiled and Interpreted

Java is both compiled and interpreted because:

1. The Java compiler first compiles the source code into bytecode.
2. The JVM then interprets the bytecode and converts it into machine code that the operating system can execute.

This process makes Java platform-independent, allowing the same program to run on different operating systems, and also improves performance with the help of the JIT compiler.

Conclusion

Java is a unique language that uses both compilation and interpretation. First, your Java code is compiled into bytecode, and then the JVM interprets the bytecode to run it on any platform. This combination allows Java to be both flexible and efficient. Understanding how this works is key to getting started with Java programming.