When learning Java, writing logic is important — but knowing how to use Java’s built-in libraries is equally important ☕

Java provides a massive collection of pre-written classes and packages called the Java Standard Library. These libraries save time, reduce effort, and make development much easier.

Instead of writing everything from scratch, Java allows developers to use powerful built-in classes for:

• Input & Output 📥
• Strings manipulation 🧵
• Mathematical operations 🔢
• Collections (ArrayList, HashMap, etc.) 📦
• Date and Time handling ⏰
• Wrapper classes 🔄

For beginners, it is important to focus on the most essential and commonly used standard libraries. This article covers exactly that — nothing overwhelming, just what you truly need to know at the beginning 🚀

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📦 What Is Java Standard Library?

Java Standard Library is a collection of ready-made classes grouped inside packages like:

• java.lang
• java.util
• java.io
• java.time
• java.math

Some packages are imported automatically, while others must be imported explicitly.

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🟢 1️⃣ java.lang Package (Automatically Imported)

This is the most fundamental package in Java.
It is imported automatically — no need to write import java.lang.*;

It includes:

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

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🔹 Example: String Class

public class StringDemo {
    public static void main(String[] args) {

        String name = "Java Programming";

        System.out.println("Length: " + name.length());
        System.out.println("Uppercase: " + name.toUpperCase());
        System.out.println("Substring: " + name.substring(0, 4));
    }
}

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🧠 Explanation

• length() → returns number of characters
• toUpperCase() → converts to uppercase
• substring() → extracts part of string

🖨 Output

Length: 16
Uppercase: JAVA PROGRAMMING
Substring: Java

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🔹 Example: Math Class

public class MathDemo {
    public static void main(String[] args) {

        System.out.println("Square Root: " + Math.sqrt(25));
        System.out.println("Power: " + Math.pow(2, 3));
        System.out.println("Random Number: " + Math.random());
    }
}

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🧠 Explanation

• Math.sqrt() → square root
• Math.pow() → power
• Math.random() → random value between 0.0 and 1.0

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🟢 2️⃣ java.util Package (Very Important)

This package provides utility classes.

Common beginner classes:

• Scanner
• ArrayList
• HashMap
• Arrays

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🔹 Example: Scanner (User Input)

import java.util.Scanner;

public class ScannerDemo {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        System.out.print("Enter your name: ");
        String name = sc.nextLine();

        System.out.println("Hello " + name);

        sc.close();
    }
}

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🧠 Explanation

• Scanner is used to take input
• nextLine() reads full line
• Always close scanner

⚠ Common mistake:
Forgetting sc.close();

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🔹 Example: ArrayList

import java.util.ArrayList;

public class ArrayListDemo {
    public static void main(String[] args) {

        ArrayList<String> list = new ArrayList<>();

        list.add("Java");
        list.add("Python");
        list.add("C++");

        System.out.println(list);
    }
}

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🧠 Explanation

• Dynamic array
• No fixed size
• add() method inserts element

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🔹 Example: HashMap

import java.util.HashMap;

public class HashMapDemo {
    public static void main(String[] args) {

        HashMap<Integer, String> map = new HashMap<>();

        map.put(1, "Ajay");
        map.put(2, "Rahul");

        System.out.println(map.get(1));
    }
}

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🧠 Explanation

• Stores key-value pairs
• put() inserts
• get() retrieves

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🟢 3️⃣ java.io Package (Basic File Handling)

Used for file operations.

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🔹 Example: File Writing

import java.io.FileWriter;
import java.io.IOException;

public class FileDemo {
    public static void main(String[] args) throws IOException {

        FileWriter fw = new FileWriter("sample.txt");
        fw.write("Hello Java");
        fw.close();

        System.out.println("File Written Successfully");
    }
}

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🧠 Explanation

• FileWriter writes to file
• close() is mandatory
• IOException must be handled

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🟢 4️⃣ Wrapper Classes

Used to convert primitive types into objects.

Example:

• int → Integer
• double → Double

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🔹 Example: Wrapper Class

public class WrapperDemo {
    public static void main(String[] args) {

        int num = 10;
        Integer obj = Integer.valueOf(num);

        System.out.println(obj);
    }
}

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🧠 Why Important?

Collections work with objects, not primitives.

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🟢 5️⃣ java.time Package (Date and Time)

Modern way to handle dates.

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🔹 Example

import java.time.LocalDate;
import java.time.LocalTime;

public class DateTimeDemo {
    public static void main(String[] args) {

        LocalDate date = LocalDate.now();
        LocalTime time = LocalTime.now();

        System.out.println("Date: " + date);
        System.out.println("Time: " + time);
    }
}

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❓ Common Confusions

❓ Why don’t we import java.lang?

Because it’s automatically imported.

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❓ Why use ArrayList instead of array?

ArrayList is dynamic.

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❓ Why use Scanner instead of System.in directly?

Scanner provides convenient methods.

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❌ Common Mistakes

❌ Forgetting import statement

ArrayList list = new ArrayList(); // ❌ Error without import

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❌ Forgetting to close Scanner or FileWriter

Can cause memory leaks.

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❌ Using == for String comparison

if(str1 == str2) // ❌ wrong

Correct:

if(str1.equals(str2)) // ✔

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📌 Essential Libraries Every Beginner Must Know

Package	Important Classes
java.lang	String, Math, System
java.util	Scanner, ArrayList, HashMap
java.io	FileWriter
java.time	LocalDate, LocalTime
Wrapper Classes	Integer, Double

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🎯 Why Standard Libraries Matter

Using standard libraries:

• Saves development time ⏳
• Improves reliability ✅
• Reduces bugs 🐞
• Makes code professional 💼

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✅ Conclusion

Java Standard Libraries are the backbone of practical Java programming 🚀

For beginners, mastering:

• String operations
• Scanner input
• ArrayList and HashMap
• Math utilities
• Basic file handling
• Date and time APIs

is essential for building real-world applications 💪

Once these core libraries are comfortable, learning advanced frameworks and APIs becomes much easier.