Overview of java.lang and java.util

1. Introduction

Java provides thousands of built-in classes organized into packages.
Two of the most important and commonly used packages are:

• java.lang
• java.util

Every Java program automatically uses classes from java.lang.
java.util provides utility classes that help with collections, dates, randomness, and more.

Understanding these two packages is essential because:

• Almost every Java program depends on them.
• They form the foundation of core Java programming.
• Most interview questions involve classes from these packages.

2. What is a Package in Java?

A package is a namespace that groups related classes and interfaces.

Benefits of packages:

• Avoid name conflicts
• Organize code
• Improve readability
• Control access

Example:

import java.util.ArrayList;
import java.util.Scanner;

Here, ArrayList and Scanner belong to java.util.

3. Overview of java.lang Package

java.lang is the most fundamental package in Java.

Important points:

• Automatically imported
• No need to write import java.lang.*
• Contains core language classes

Example:

String s = "Java";   // String belongs to java.lang
System.out.println(s); // System belongs to java.lang

You never import these explicitly.

4. Important Classes in java.lang

Some commonly used classes:

4.1 String

Represents immutable sequence of characters.

4.2 Object

Root class of all Java classes.

4.3 System

Provides system-level utilities like input/output.

4.4 Math

Contains mathematical operations.

4.5 Wrapper Classes

• Integer
• Double
• Boolean
• Character

4.6 Thread

Used for multithreading.

4.7 Runtime

Interacts with JVM runtime environment.

5. Example Usage of java.lang Classes

int max = Math.max(10, 20);
System.out.println(max);

Integer num = 100;
String str = num.toString();

All of these belong to java.lang.

6. Overview of java.util Package

java.util stands for Java Utilities.

Unlike java.lang, this package must be imported.

It provides:

• Collection framework
• Date and time utilities
• Random number generation
• Scanner class
• Optional class
• UUID generation

Example:

import java.util.ArrayList;
import java.util.Random;

7. Important Classes in java.util

7.1 Collections Framework

• ArrayList
• LinkedList
• HashSet
• TreeSet
• HashMap
• TreeMap
• Queue
• PriorityQueue

7.2 Scanner

Used for user input.

Scanner sc = new Scanner(System.in);

7.3 Random

Generates random numbers.

Random r = new Random();
int num = r.nextInt(100);

7.4 Date and Time

• Date
• Calendar
• TimeZone

(Modern Java uses java.time package instead.)

7.5 Optional

Helps avoid null pointer exceptions.

8. java.lang vs java.util

9. When to Use java.lang

You use it:

• Always (automatically)
• For basic data handling
• For strings and math operations
• For thread operations
• For wrapper classes

It forms the base layer of Java.

10. When to Use java.util

You use it when:

• You need dynamic data structures
• You need random numbers
• You need user input handling
• You need utility functions
• You need collections like List, Set, Map

It supports higher-level programming tasks.

11. Summary

• java.lang is automatically imported and contains core classes.
• java.util provides utility classes including collections and helper tools.
• java.lang is fundamental; java.util enhances functionality.
• Almost every Java program uses both packages.
• Mastering these packages is essential for strong Java fundamentals.

Written By: Shiva Srivastava