4. Java Strings, Exception Handling & Collections

Chapters 14-16: String Pool, StringBuilder, try-catch-finally, Checked vs Unchecked exceptions, ArrayList, HashSet & HashMap

Module 1: Strings, Immutability & StringBuilder

Understanding String memory pooling, immutability, and mutable StringBuilder operations.

🇬🇧 English Definition

String objects in Java are immutable sequence of characters managed inside String Constant Pool. StringBuilder provides mutable character sequences for efficient string concatenations.

🇮🇳 Hinglish Definition

String immutable (unchangeable) hoti hai aur String Pool me store hoti hai. Repeated modifications ke liye StringBuilder efficient hai.

Core Concepts & Rules

equals() vs ==: equals() compares string text content; == compares object memory references.
String Immutability: Modifying a string creates a new object instance leaving original untouched.

🔍 Line-by-Line Code Breakdown

Line 2-3: String a = new String('Java') and String b = new String('Java') create separate objects.
Line 4: a.equals(b) returns true (content match); a == b returns false (different references).

🎯 Interview Focus & Questions

Q: Why is String immutable in Java?
A: Security, thread-safety, caching hashcodes, and String Pool memory optimization.
Q: equals() vs ==?
A: equals() compares logical content; == compares memory reference addresses.

💻 Practical Java Code Example - Strings, Immutability & StringBuilder

public class StringDemo {
    public static void main(String[] args) {
        String a = new String("Java");
        String b = new String("Java");
        System.out.println(a.equals(b)); // true
        System.out.println(a == b);      // false
        
        StringBuilder sb = new StringBuilder();
        sb.append("Java").append(" Notes");
        System.out.println(sb.toString());
    }
}

📤 Compiled Execution Output:

true false Java Notes

Module 2: Exception Handling & Collections Framework

Managing runtime errors with try-catch-finally and utilizing Collections (ArrayList, HashSet, HashMap).

🇬🇧 English Definition

Exceptions represent abnormal conditions. Checked exceptions require compiler handling; Unchecked extend RuntimeException. Collections Framework provides ArrayList, HashSet, and HashMap data structures.

🇮🇳 Hinglish Definition

Error ko controlled way me handle karne ke liye try-catch-finally use karte hain. Collections Framework ArrayList, HashSet aur HashMap jaise data structures deta hai.

Core Concepts & Rules

try-catch-finally: try encloses risky code, catch handles matching exception, finally executes cleanup code.
List vs Set vs Map: List keeps ordered duplicates; Set stores unique elements; Map holds key-value pairs.

🔍 Line-by-Line Code Breakdown

Line 2-4: try-catch block handling ArithmeticException divide by zero error.
Line 7-10: HashMap storing ID to Name key-value mappings.

🎯 Interview Focus & Questions

Q: Checked vs Unchecked exception?
A: Checked exceptions are verified at compile-time; Unchecked extend RuntimeException and occur at runtime.
Q: ArrayList vs LinkedList?
A: ArrayList uses dynamic array with fast indexed access O(1); LinkedList uses doubly-linked nodes.

💻 Practical Java Code Example - Exception Handling & Collections Framework

import java.util.*;

public class ExceptionCollectionDemo {
    public static void main(String[] args) {
        try {
            int x = 10 / 0;
        } catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero");
        }
        
        Map<Integer, String> map = new HashMap<>();
        map.put(101, "Aman");
        System.out.println("Student: " + map.get(101));
    }
}

📤 Compiled Execution Output:

Cannot divide by zero Student: Aman