Common Java Packages in Java

Java provides hundreds of built-in packages, but only a few are used regularly in real-world development.

These common Java packages offer essential classes for everyday programming, including strings, collections, I/O operations, networking, dates, concurrency, and more.

This guide explains the most important Java packages, what they contain, and when to use them — with simple examples.

1. java.lang

• Automatically imported in every Java program.
• Contains fundamental classes for basic language features.

Common Classes

• String
• Object
• Math
• System
• Thread
• Wrapper classes (Integer, Double, Boolean, etc.)

Example

public class Main {
    public static void main(String[] args) {
        System.out.println(Math.sqrt(16));
    }
}

2. java.util

Most widely used package for collections, dates, and utilities.

Common Classes

• ArrayList
• HashMap
• HashSet
• Collections
• Date
• Random
• Scanner

Example

import java.util.*;

class Demo {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Java");
        list.add("Python");
        System.out.println(list);
    }
}

3. java.io

Used for input/output operations, such as reading and writing files.

Common Classes

• File
• FileInputStream
• FileOutputStream
• BufferedReader
• PrintWriter

Example

import java.io.*;

class FileDemo {
    public static void main(String[] args) throws Exception {
        FileWriter fw = new FileWriter("test.txt");
        fw.write("Hello Java");
        fw.close();
    }
}

4. java.nio / java.nio.file

Modern, faster I/O system used for file handling and buffers.

Common Classes

• Path
• Files
• ByteBuffer

Example:

import java.nio.file.*;

class NioExample {
    public static void main(String[] args) throws Exception {
        Files.writeString(Path.of("data.txt"), "Hello NIO");
    }
}

5. java.net

Supports networking, such as HTTP, TCP, UDP communication.

Common Classes

• URL
• URLConnection
• Socket
• ServerSocket

6. java.time

Modern date and time API (Java 8+).

Common Classes

• LocalDate
• LocalTime
• LocalDateTime
• ZoneId
• Duration
• Period

Example:

import java.time.*;

class TimeDemo {
    public static void main(String[] args) {
        System.out.println(LocalDate.now());
    }
}

7. java.sql

Used for database connectivity (JDBC).

Common Classes

• Connection
• Statement
• PreparedStatement
• ResultSet
• DriverManager

8. java.math

Used for high-precision calculations.

Common Classes

• BigInteger
• BigDecimal

Example:

import java.math.*;

class MathDemo {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal("10.5");
        BigDecimal b = new BigDecimal("2.3");
        System.out.println(a.add(b));
    }
}

9. java.awt & javax.swing

Used for GUI programming.

• AWT → older GUI toolkit
• Swing → advanced, lightweight GUI framework

Common Classes (Swing)

• JFrame
• JButton
• JLabel

10. java.util.concurrent

Support for multithreading, concurrency, thread pools.

Common Classes

• ExecutorService
• Future
• Callable
• ConcurrentHashMap
• Locks (ReentrantLock)

Summary Table of Common Java Packages