Why don't we use the .class extension with "java" command?

Question

Why don't we give the filename.class file after java command, instead of only filename?

Suppose we want to compile the test.java program, then we run javac test.java. It is ok!

After that it will produce test.class file but to run the program we run java test instead of java test.class. What is the reason for this?

Answer 1

Because you are not describing a file to run. You are telling Java which class contains the main method - and the class' name is (in your case) filename, not filename.class.

The fact that the bytecode is almost always contained in files on the filesystem is an implementation detail. The classpath you pass to the java command tells it where to look for classes, and then the main class argument tells it which class to use.

(It's different for javac, because this program specifically does take source files and compiles them into bytecode.)

Answer 2

You don't pass a file name to the java command either. You pass it a fully qualified class name. Something like com.yourcompany.yourapp.Main. Java then finds the .class file for this class name, by looking into all the directories and jar files in the classpath.