In this blog post, we’ll explore how to compile and run Java code with packages, demonstrate the use of the classpath option for alternate directories, and delve into creating and executing Java Archive (JAR) files.

Organizing Code with Packages

Packages are Java’s way of structuring the codebase. They allow developers to group related classes and interfaces together. Let’s start by creating two simple classes, Person and Student, each in its own file within a package named com.example.

Person.java:

package com.example;

public class Person {
    public void greet() {
        System.out.println("Hello, I am a person!");
    }
}

Student.java:

package com.example;

import com.example.Person;  // Importing the Person class

public class Student {
    public static void main(String[] args) {
        Person person = new Person();
        person.greet();
        System.out.println("I am a student!");
    }
}

Compiling Java Code

To compile the code, open your terminal or command prompt, navigate to the directory containing the source files, and use the javac command.

javac com/example/Person.java com/example/Student.java

This command compiles both Person.java and Student.java. Ensure that the directory structure matches the package structure.

Running Java Code

After successful compilation, run the Student class using the java command.

java com.example.Student

The output should be:

Hello, I am a person! 
I am a student!

Using an Alternate Directory with Classpath

Sometimes, you may want to compile and run your code from a different directory. The classpath option (-cp or -classpath) allows you to specify the location of compiled classes.

# Compile in the current directory but output class files to the 'bin' directory
javac -d bin com/example/Person.java com/example/Student.java

# Run the Student class with the classpath pointing to the 'bin' directory
java -cp bin com.example.Student

Creating and Running JAR Files

JAR files package Java classes and resources into a single file for distribution. To create a JAR file, use the jar command.

cvf in jar Command:

• c Option: This option stands for “create.” It is used to create a new JAR file.
• v Option: This option stands for “verbose.” It prints detailed information about the JAR creation process.
• f Option: This option specifies the JAR file’s name. It must be followed by the filename of the JAR.

# Create a JAR file named 'myapp.jar' containing classes from the 'bin' directory
jar cvf myapp.jar -C bin .

# Run the JAR file
java -cp myapp.jar com.example.Student

• cvf: Specifies that we want to create a new JAR file (c), display detailed information (v), and specify the filename of the JAR (f).
• myapp.jar: The name of the JAR file that will be created.
• -C bin .: This is crucial. It tells the jar command to change to the bin directory (-C bin) before processing the files. The dot (.) at the end includes all files from the specified directory.