Built-in Packages

Built-in packages are collections of predefined classes and interfaces provided by Java to handle common programming tasks such as language fundamentals, collections, I/O, networking, date/time, concurrency, and databases. They form the Java Standard Library and are available out of the box. This is a frequent interview topic, often asked alongside import, packages, and API usage.

Why Built-in Packages Exist

• Avoid reinventing common functionality
• Provide reliable, optimized implementations
• Promote standardization across Java programs
• Speed up development and reduce bugs

Most Important Built-in Packages (Must-Know)

1. java.lang — Core Language Support

Automatically imported (no import needed)

Common classes:

• Object
• String, StringBuilder, StringBuffer
• Math
• System
• Exception, RuntimeException
• Wrapper classes (Integer, Double, etc.)

String s = "Java";
System.out.println(Math.sqrt(16));
          

2. java.util — Utilities & Collections

Used for data structures and utilities

Common classes/interfaces:

• List, ArrayList, LinkedList
• Set, HashSet
• Map, HashMap
• Iterator
• Collections, Arrays
• Scanner

import java.util.*;

List<Integer> list = new ArrayList<>();
          

3. java.io — Input / Output (File-based, Stream-based)

Used for file handling and byte/character streams

Common classes:

• File
• FileReader, FileWriter
• InputStream, OutputStream
• BufferedReader, BufferedWriter

import java.io.*;

File file = new File("data.txt");
          

4. java.nio — New I/O (Non-blocking, Faster I/O)

Introduced for high-performance I/O

Key features:

• Buffers
• Channels
• Paths & Files API

import java.nio.file.*;

Path p = Paths.get("data.txt");
          

5. java.sql — Database Connectivity (JDBC)

Used for database operations

Common interfaces/classes:

• Connection
• Statement, PreparedStatement
• ResultSet
• DriverManager

import java.sql.*;

Connection con = DriverManager.getConnection(url, user, pwd);
          

6. java.time — Date & Time API (Java 8+)

Modern replacement for Date and Calendar

Common classes:

• LocalDate
• LocalTime
• LocalDateTime
• Period, Duration

import java.time.*;

LocalDate today = LocalDate.now();
          

7. java.net — Networking

Used for network programming

Common classes:

• URL
• URLConnection
• Socket
• ServerSocket

import java.net.*;

URL url = new URL("https://example.com");
          

8. java.util.concurrent — Multithreading Utilities

Used for high-level concurrency

Common classes:

• ExecutorService
• Executors
• Future
• Callable
• ConcurrentHashMap

import java.util.concurrent.*;

ExecutorService ex = Executors.newFixedThreadPool(2);
          

9. java.math — High-Precision Arithmetic

Used for financial and scientific calculations

Classes:

• BigInteger
• BigDecimal

import java.math.*;

BigDecimal bd = new BigDecimal("123.45");
          

Package Import Rules (Quick Recap)

• java.lang → auto-imported
• Other packages → must be imported explicitly
• Wildcard imports do not import sub-packages

import java.util.*;   // only util, not util.concurrent
          

Built-in Packages Summary Table (Interview-Friendly)

Common Beginner Mistakes

• Forgetting imports (except java.lang)
• Using old Date instead of java.time
• Overusing wildcard imports
• Confusing java.io with java.nio
• Not closing I/O or DB resources

Interview-Ready Answers

Short Answer

Built-in packages are predefined Java packages that provide commonly used classes and interfaces.

Detailed Answer

Java provides built-in packages such as java.lang, java.util, java.io, java.sql, and java.time to handle core language features, collections, I/O, databases, and date/time operations. These packages help developers write efficient, standardized, and maintainable code.

Key Takeaway

Built-in packages are the backbone of Java development. Mastering them allows you to write powerful applications faster, cleaner, and more reliably.