Bytecodes

Java combines these ideas in a way that will take some explaining. To run a Java program the source file is first translated into a file of bytecodes.

A Java bytecode is a machine instruction for a Java processor. A file of bytecodes is a machine language program for a Java processor.

Here is a picture of the sample Java program Hello.java being translated into bytecodes. The file of bytecodes (machine language for the Java processor) is called Hello.class.

In this picture, the source program Hello.java is examined by a program called javac running on your computer. The javac program is a compiler (a translator) that translates the source program into a bytecode file called Hello.class.

Important Idea: The bytecode file will contain exactly the same bytecodes no matter what computer system is used.

The architecture of the processor that executes Java bytecodes is well-documented and is available to anyone. The Java compiler on a Macintosh will produce the exact same bytecodes as the Java compiler on an Intel system.