Creating & Importing Packages in Java

In Java, packages are used to organize classes and interfaces into separate namespaces.

They help make large projects more manageable, reusable, and prevent class name conflicts.

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Creating a User-Defined Package

Step 1: Declare the Package

You must declare the package at the top of your Java file (before imports or class definitions).

// File: mypack/Message.java
package mypack;

public class Message {
    public void show() {
        System.out.println("Hello from mypack!");
    }
}

Note: The folder name must match the package name (mypack).

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Step 2: Compile the Program

Use the -d option with the compiler to generate the correct folder structure:

javac -d . Message.java

Explanation:

• d . → tells the compiler to place compiled files in a directory structure based on the package.
• . → current working directory.

After compiling, you’ll get this structure:

|-- mypack
    |-- Message.class

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Step 3: Import and Use the Package

Now, create another file to use the class from the package.

// File: TestPackage.java
import mypack.Message; // importing class

public class TestPackage {
    public static void main(String[] args) {
        Message msg = new Message();
        msg.show();
    }
}

Run it:

java TestPackage

Output:

Hello from mypack!

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Importing Packages

There are three ways to import classes from a package:

1. Import a Single Class

import mypack.Message;

2. Import All Classes in a Package

import mypack.*;

3. Use Fully Qualified Class Name

public class Main {
    public static void main(String[] args) {
        mypack.Message msg = new mypack.Message();
        msg.show();
    }
}

Works without import, but not recommended for large codebases.

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Points to Remember

• Always use package keyword at the top of the file.
• Folder name must match the package name.
• Use import to access classes from other packages.
• Compile using javac -d . to maintain proper directory structure.
• Classes you want to use outside the package must be public.