Compiling Java in the Shell(Optional)

 * Java programs get compiled into object code for an imaginary CPU called the "Java Virtual Machine"(JVM). Consequently, you can't execute compiled Java code directly, You must run program that simulates a JVM and let that simulated computer execute the Java code.

* That may seem a little convoluted, but the JVM simulator is easier to write than a "true" compiler. Consequently, JVM simulators can be built into other programs (such as web browsers), allowing Java code compiled on one machine to be executed on almost any other machine. By contrast, a true native-code compiler (e.g., g++) produces executable that can only be run on a single kind of computer.

* The command to compile Java code is "javac" ("c" for compiler) and the command to execute compiled java code is "java". So a typical sequence to compile and execute a single-file Java program would b

javac -g MyProgram.java

java MyhProgram

* Unlike most programming languages, Java includes some important restrictions on the file names used to store source code.

Java source code is stored in files ending with the extension ".java".

Each Java source code file must contain exactly one public class declaration.

* The base name of the file (the part before the extension) must be the same (including upper/lower case characters) as the name of the public class it contains.

So the command

javac -g MyProgram.java

* ocmpiles a file that must contain the code:

public 

class MyProgram ...

The output of this compilatioin will be a file named MyProgram.class ( and possibly some other .class files as well).

* If we have a program that consists of multiple files, we can simply compile each file in turn:

javac -g MyProgram.java

javac -g MyADT.java

but this might not be necessary. If one Java file imports another, then the imported file will be automatically compiled if no .class file for it exists.