Pre Defined Packages

Java provides a rich set of pre-defined packages that contain classes and interfaces to handle various tasks such as input/output, networking, utilities, data structures, and more. These packages are grouped into libraries to promote code reusability and modular programming.

Understanding Pre-Defined Packages

A package in Java is a collection of classes, interfaces, and sub-packages that are logically grouped together. The java.lang package is automatically imported in every Java program, but other packages must be explicitly imported using the import statement.

Commonly Used Pre-Defined Packages:

1. java.lang – Provides core classes such as String, Math, Integer, Double, etc. It is automatically imported.
2. java.util – Contains utility classes like collections framework, date and time facilities, etc.
3. java.io – Handles input and output operations such as reading and writing files.
4. java.net – Provides classes for networking, such as Socket, URL, InetAddress.
5. java.awt – Abstract Window Toolkit, contains classes for GUI components.
6. javax.swing – Provides lightweight GUI components such as buttons, labels, and frames.
7. java.sql – Contains classes for JDBC, allowing database access and operations.
8. java.time – Includes classes for date and time operations.

Syntax for Importing Packages

import java.util.*; // Imports all classes in java.util
import java.io.File; // Imports only the File classCode language: JavaScript (javascript)

You can also use the fully qualified name to access a class without importing it:

java.util.Scanner scanner = new java.util.Scanner(System.in);Code language: JavaScript (javascript)

Example Program – Demonstrating Pre-Defined Packages

In this example, we will use classes from different packages to demonstrate how to use pre-defined packages in Java.

Problem Statement: LotusJavaPrince is creating a program to manage data input and output, perform mathematical calculations, and format dates. He wants to use predefined packages to implement these functionalities effectively.

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.lang.Math;

public class PackageDemo {
    public static void main(String[] args) {
        // Using java.util.Scanner for input
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        double number = scanner.nextDouble();

        // Using java.lang.Math for calculations
        double squareRoot = Math.sqrt(number);
        System.out.println("Square root of " + number + " is: " + squareRoot);

        // Using java.util.Date and java.text.SimpleDateFormat for date formatting
        Date currentDate = new Date();
        SimpleDateFormat formatter = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss");
        String formattedDate = formatter.format(currentDate);
        System.out.println("Current date and time: " + formattedDate);

        // Using java.io.File for file handling
        try {
            File file = new File("sample.txt");
            Scanner fileReader = new Scanner(file);
            System.out.println("File contents:");
            while (fileReader.hasNextLine()) {
                System.out.println(fileReader.nextLine());
            }
            fileReader.close();
        } catch (FileNotFoundException e) {
            System.out.println("File not found: " + e.getMessage());
        }

        scanner.close();
    }
}