Compiling and running Java programs (javac, java)

Compiling and running Java programs

Java uses a two‑step process: compilation to bytecode, then interpretation/JIT execution.

The compiler: javac

The Java compiler (javac) translates .java source files into .class bytecode files.

Basic usage:

javac MyProgram.java

If the program has multiple classes, you can compile them all:

javac *.java

Common options:

• -d <directory> – specify where to place generated .class files.
• -classpath / -cp – specify where to find required libraries.

The launcher: java

The Java launcher (java) starts the JVM and executes the bytecode.

Basic usage:

java MyProgram

You pass the class name (without .class). The JVM looks for a public static void main(String[] args) method in that class.

To run a program that resides in a package:

java com.example.MyProgram

You can pass command‑line arguments:

java MyProgram arg1 arg2

Inside the program, args contains ["arg1", "arg2"].

Classpath

The classpath tells javac and java where to find user‑defined classes and libraries. It can be set via the -cp option or the CLASSPATH environment variable.

Example:

javac -cp .:lib/* MyProgram.java
java -cp .:lib/* MyProgram

JAR files

JAR (Java Archive) files bundle multiple classes and resources. You can run an executable JAR with:

java -jar myapp.jar

This requires the JAR’s manifest to specify the main class.