Java: compiled, then interpreted

By the end of this lesson

By the end you can show that some programs are partly compiled and partly interpreted, and explain why Java is the standard example.

Compile once, run anywhere

With ordinary compiled languages, the compiler produces machine code for one processor and operating system.

Java does something different:

The Java compiler does not produce machine code. It produces bytecode, saved in .class files.
Bytecode is not tied to any one processor.
At run time, the Java Virtual Machine (JVM) on that machine interprets the bytecode (and may compile hot sections to machine code with a JIT compiler).

So Java is partly compiled (source to bytecode) and partly interpreted (bytecode executed by the JVM).

Compile once to bytecode, then let each platform's JVM execute it.

Portability: the same .class file runs on any machine that has a JVM. That is the "compile once, run anywhere" idea.
Speed: bytecode is compact and closer to machine code than source, so the JVM has less work than a plain interpreter.
Optimisation: the JVM can JIT-compile frequently used methods, so repeated work runs at close to native speed.
Security: the JVM can check bytecode before running it, which is one reason Java is used in sandboxed environments.

The cost: the JVM must be installed on the machine, and it uses memory and adds a small start-up delay.

Worked example: Worked example: tracing a Java program's run

A student writes Hello.java and runs javac Hello.java, then java Hello in the console.

Describe what happens at each stage, and say which parts are compilation and which are interpretation.

Have a go first?

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Recall

Multiple choiceQuestion 3 · Apply

Answer the 3 questions above to continue.