Java Package

Intro

In collaborative software development, it’s common for different developers to create classes with the same name. For example:

• Developer Alice writes a class Cat
• Developer Bob also writes a class Cat
• Developer Charlie wants to use both versions of class Cat

Or consider this case:

• Developer David creates a custom Collections class.
• However, Java already provides a built-in Collections class in the java.util package.

Java uses packages to resolve these naming conflicts to eliminate ambiguity.

I. What Is a Package?

A package in Java is a namespace that organizes a set of related classes and interfaces. Every class belongs to a package. While class names like Cat or Dog may be common, the fully qualified class name includes the package path, making it unique.

e.g.

II. Declaring Packages in Java

To assign a class to a package, declare the package at the top of the file.

Cat.java by Developer Alice:

package dev.alice;

public class Cat {
    public void meow() {
        System.out.println("Meow");
    }
}

Cat.java by Developer Bob:

package dev.bob;

public class Cat {
    public void meow() {
        System.out.println("Meow");
    }
}

Collections.java by Developer David:

package custom.tools;

public class Collections {
    // custom implementation
}

Now the JVM distinguishes these classes by their fully qualified names: dev.alice.Cat, dev.bob.Cat, custom.tools.Collections, and java.util.Collections.

III. Directory Structure and Compilation

The directory hierarchy should reflect the package structure. For example:

project_root/
└── src/
    ├── dev/
    │   ├── alice/
    │   │   └── Cat.java
    │   └── bob/
    │       └── Cat.java
    └── custom/
        └── tools/
            └── Collections.java

If we omit the package declaration, the class belongs to the default package. This is strongly discouraged.

IV. Package Visibility

1. A class method or field with NO modifier (not public, protected, or private) is package-private — accessible only within the same package.

    package animals;
   
    public class Dog {
        void bark() {
            System.out.println("Woof!");
        }
    }
   

    package animals;
   
    public class Main {
        public static void main(String[] args) {
            Dog d = new Dog();
            d.bark(); // OK: Same package
        }
    }
   

   But if Main is in a different package, bark() will NOT be accessible.

2. Importing Classes

   To use classes from other packages, we can either:

   • Use the fully qualified name:

       package dev.alice;
     
       public class Rabbit {
           public void hop() {
               custom.tools.Collections utils = new custom.tools.Collections();
           }
       }
     

   • Use import

       package dev.alice;
     
       import custom.tools.Collections;
     
       public class Rabbit {
           public void hop() {
               Collections utils = new Collections();
           }
       }
     

3. How Java Resolves Class Names

   When the compiler encounters a class name like Cat, it checks in this order:

   • The current package.

   • All explicitly imported classes.

   • The java.lang package (automatically imported).

   If the class is not found in any of these, we get a compilation error.