Understanding Java Packages: Creation, Usage, and Classpath Management

Package Creation

In a Java project, multiple .java files are organized using packages. Adding a package declaration at the beginning of a source file places the class into that package. For example, the following Human class is placed in the com.vamei.society package:

package com.vamei.society;

public class Human {
    /**
     * constructor
     */
    public Human(int h) {
        this.height = h;
        System.out.println("I'm born");
    }

    /**
     * accessor
     */
    public int getHeight() {
        return this.height;
    }

    /**
     * mutator
     */
    public void growHeight(int h) {
        this.height = this.height + h;
    }

    private int height;
}

The first line package com.vamei.society; indicates that the class belongs to the com.vamei.society package, where com.vamei is a reversed domain name used as a unique prefix.

Package Usage

After compiling, the Human.class file should be placed in the corresponding directory structure com/vamei/society/. Other classes can access it using an import statement:

import com.vamei.society.*;

public class TestAgain {
    public static void main(String[] args) {
        Human aPerson = new Human(180);
        System.out.println(aPerson.getHeight());
    }
}

The asterisk (*) imports all classes in the society package. Alternatively, a class can be referenced by its fully‑qualified name without an import:

public class TestAgain {
    public static void main(String[] args) {
        com.vamei.society.Human aPerson = new com.vamei.society.Human(180);
        System.out.println(aPerson.getHeight());
    }
}

Classpath Configuration

If the package directory is not on the current working path, the Java compiler and runtime need to be told where to find it using the -classpath option or the CLASSPATH environment variable. For example:

$javac -classpath /home/vamei/javapackage:. TestAgain.java
$java -classpath /home/vamei/javapackage:. TestAgain

This tells Java to search both the specified package directory and the current directory for classes.

Additional Notes

Java packages provide a namespace, allowing classes with the same name to coexist in different packages (e.g., com.vamei.society.Human vs. com.vamei.creature.Human). Classes without an explicit access modifier have default (package‑private) visibility, meaning they are accessible only within the same package.

The package mechanism is similar to import systems in other languages such as Python, and it contributes to Java’s “write once, run anywhere” promise by enabling portable .class files that run on any JVM.

Summary

Key points: package and import statements, default (package‑private) access, using fully‑qualified class names, configuring the classpath with -classpath or CLASSPATH, and the role of packages in Java’s portability.