Java Compiler, Java Interpreter

📖 Theory

Two commands, two mysteries

Your first BookBridge session in the lab looks like this:

javac BookBridge.java

java BookBridge

Two different commands. The first one prints nothing at all if it succeeds. The second one actually runs your program. Most students type these for 3 years without asking what each one really does, and then the exam asks exactly that: explain the Java compiler and interpreter.

📖 Theory

The scriptwriter and the narrator

Think of a story written in Gujarati that must reach listeners in every state.

javac is the scriptwriter: it translates your story once into a standard script (bytecode) that no listener speaks natively.

The JVM is the local narrator: in each state, a narrator reads that standard script and performs it live in the local language. One script, many narrators. The writing happens once; the narration happens on every machine, every run.

📖 Theory

The two tools, formally

Java compiler (javac): translates human-readable source code (BookBridge.java) into bytecode (BookBridge.class), the standard instruction set of the Java Virtual Machine. This happens once, before running.

Java interpreter (the JVM): loads the .class file and executes bytecode instructions one by one on the actual machine. For code that runs repeatedly, its JIT (just-in-time) compiler translates hot bytecode into native machine code on the fly, so long-running programs speed up.

So Java is both compiled and interpreted: compiled to bytecode, interpreted (plus JIT) by the JVM.

⌨️ Practical

BookBridge.java, the first listing

public class BookBridge {
    public static void main(String[] args) {
        System.out.println("== BookBridge Library ==");
        System.out.println("Books ready to issue: 12");
    }
}

🪜 Follow along

From source to running program

1. Write BookBridge.java The public class name must exactly match the filename: public class BookBridge lives in BookBridge.java.
2. Compile: javac BookBridge.java javac checks the code and produces BookBridge.class (bytecode). Success prints nothing; errors stop everything here.
3. Run: java BookBridge Give the CLASS name, not the filename. The JVM loads BookBridge.class, finds main, and executes.

🧩 Quiz

What exactly does the javac command produce?

1. A .exe file of native machine code, ready to run directly
2. A .class file of bytecode, which needs a JVM to execute
3. Nothing; javac runs the program directly from the source file
4. A new JVM customised for this program

💭 Think first

Spot your friend's mistake

Your friend compiles successfully, then types java BookBridge.class and gets an error about not finding the class. Before tapping: what went wrong?

⚠️ Watch out

Where marks leak in this answer

Writing "Java is an interpreted language" alone loses the design point: it is compiled to bytecode first, then interpreted. Always present both stages.

And do not forget JIT in a full-marks answer: the JVM does not stay a slow line-by-line interpreter; it compiles frequently executed bytecode to native code during the run. JIT is the reason modern Java performance is close to C++ for long-running programs.

📖 Theory

Connect it backward and forward

This 2-stage pipeline is the machinery behind "write once, run anywhere" from the previous lesson: javac makes the travelling bytecode, the JVM is the local runner. Contrast with BCA204's Python, where you never ran a separate compile command, and BCA104's C, where compilation went straight to machine code. Every BookBridge program this semester goes through exactly these 2 commands.

✅ Summary

Key takeaways

• javac = the Java compiler: .java source in, .class bytecode out, once before running.
• The JVM = the Java interpreter: loads bytecode and executes it on the local machine.
• JIT inside the JVM compiles hot bytecode to native code at run time, for near-native speed.
• Java is both compiled (to bytecode) and interpreted (by the JVM); say both in the exam.
• Compile with the filename (javac BookBridge.java), run with the class name (java BookBridge).
• The public class name must match the filename exactly.
• Memory hook: the scriptwriter writes once, the narrator performs everywhere.