☕ Java Loops — Complete Deep Revision Guide

Bina loop ke:              Loop ke saath:
print("Hello");            for(int i=0; i<5; i++) {
print("Hello");                print("Hello");
print("Hello");            }
print("Hello");            // sirf 4 lines mein same kaam!
print("Hello");

for      → Jab pata ho kitni baar chalana hai
while    → Jab condition pe depend karna ho
do-while → Kam se kam EK baar zaroor chalana ho
for-each → Array/Collection ke har element pe kaam

🔵 Topic 1: for Loop

for (initialization ; condition ; update) {
         ↑                ↑           ↑
    Step 1: ek baar   Step 2: har   Step 4: body ke
    shuru mein        iteration se  baad update
                      pehle check
         ↓                ↓           ↑
                      TRUE → body    ← Step 3: body
                      FALSE → exit       execute

1. initialization  → sirf ek baar
2. condition check → true? → 3. body → 4. update → 2. again
                  → false? → loop khatam

for(;;)             // infinite loop - teeno skip
for(;i<5;)          // init aur update bahar
for(int i=0;;i++)  // condition nahi - infinite

public class ForLoop1 {
    public static void main(String[] args) {

        // Basic for loop - step by step trace
        System.out.println("Basic for loop:");
        for (int i = 1; i <= 5; i++) {
            System.out.println("i = " + i);
        }
        // Step trace:
        // i=1: 1<=5 true  → print 1 → i becomes 2
        // i=2: 2<=5 true  → print 2 → i becomes 3
        // i=3: 3<=5 true  → print 3 → i becomes 4
        // i=4: 4<=5 true  → print 4 → i becomes 5
        // i=5: 5<=5 true  → print 5 → i becomes 6
        // i=6: 6<=5 false → STOP

        System.out.println("\nBackward loop:");
        for (int i = 5; i >= 1; i--) {
            System.out.println("i = " + i);
        }

        System.out.println("\nStep of 2:");
        for (int i = 0; i <= 10; i += 2) {
            System.out.print(i + " ");
        }
    }
}

public class ForLoop2 {
    public static void main(String[] args) {

        // Sum of numbers 1 to 100
        int sum = 0;
        for (int i = 1; i <= 100; i++) {
            sum += i;  // sum = sum + i
        }
        System.out.println("Sum 1 to 100 = " + sum); // 5050

        // Multiplication table
        int num = 7;
        System.out.println("\nTable of " + num + ":");
        for (int i = 1; i <= 10; i++) {
            System.out.println(num + " x " + i + " = " + (num * i));
        }
    }
}

public class ForLoop3 {
    public static void main(String[] args) {

        // Optional parts demonstration
        System.out.println("--- Init outside loop ---");
        int i = 1;           // initialization bahar
        for (; i <= 3; i++) { // init part empty
            System.out.println("i = " + i);
        }

        System.out.println("\n--- Update inside body ---");
        for (int j = 1; j <= 3; ) {  // update part empty
            System.out.println("j = " + j);
            j++;             // update andar
        }

        System.out.println("\n--- Multiple init and update ---");
        for (int a = 0, b = 10; a <= b; a++, b--) {
            System.out.println("a=" + a + " b=" + b);
        }
    }
}

public class ForLoop4 {
    public static void main(String[] args) {

        // Factorial
        int  n         = 6;
        long factorial = 1;
        for (int i = 1; i <= n; i++) {
            factorial *= i;
        }
        System.out.println(n + "! = " + factorial); // 720

        // Prime check
        int     num     = 29;
        boolean isPrime = true;
        for (int i = 2; i <= num / 2; i++) {
            if (num % i == 0) {
                isPrime = false;
                break;
            }
        }
        System.out.println(num + " is prime: " + isPrime);

        // Fibonacci series
        System.out.println("\nFibonacci (10 terms):");
        int a = 0, b = 1;
        for (int i = 1; i <= 10; i++) {
            System.out.print(a + " ");
            int temp = a + b;
            a = b;
            b = temp;
        }
    }
}

public class ForLoop5 {
    public static void main(String[] args) {

        // Nested for loop - Star patterns
        System.out.println("Square Pattern:");
        for (int i = 1; i <= 4; i++) {       // rows
            for (int j = 1; j <= 4; j++) {   // columns
                System.out.print("* ");
            }
            System.out.println(); // new line after each row
        }

        System.out.println("\nRight Triangle:");
        for (int i = 1; i <= 5; i++) {
            for (int j = 1; j <= i; j++) {  // j goes till i
                System.out.print("* ");
            }
            System.out.println();
        }

        System.out.println("\nMultiplication Table:");
        for (int i = 1; i <= 5; i++) {
            for (int j = 1; j <= 5; j++) {
                System.out.printf("%4d", i * j); // formatted
            }
            System.out.println();
        }
    }
}

🟢 Topic 2: while Loop

while (condition) {
    // body
    // update zaroori warna infinite loop!
}

condition check → TRUE  → body → update → condition check again
               → FALSE → loop skip/exit

Agar condition shuru mein hi FALSE hai:
→ Body ZERO times execute hogi!
→ while loop skipped!

✅ Jab pata nahi kitni baar chalana hai
✅ User input pe depend kare
✅ File end tak padhna ho
✅ Game loop - jab tak game over nahi

public class WhileLoop1 {
    public static void main(String[] args) {

        // Basic while loop
        int i = 1;
        while (i <= 5) {
            System.out.println("i = " + i);
            i++;  // update zaroori! warna infinite loop
        }

        System.out.println("Loop ended, i = " + i); // i = 6

        // Zero times execution - condition pehle hi false
        int x = 10;
        System.out.println("\nCondition false from start:");
        while (x < 5) {         // 10 < 5 = FALSE
            System.out.println("This will NEVER print!");
            x++;
        }
        System.out.println("While body executed 0 times, x = " + x);
    }
}

public class WhileLoop2 {
    public static void main(String[] args) {

        // Digits of a number
        int num = 12345;
        System.out.println("Digits of " + num + ":");
        while (num > 0) {
            int digit = num % 10;   // last digit
            System.out.println("Digit: " + digit);
            num = num / 10;         // remove last digit
        }

        // Sum of digits
        int number    = 9876;
        int sumDigits = 0;
        int temp      = number;
        while (temp > 0) {
            sumDigits += temp % 10;
            temp /= 10;
        }
        System.out.println("\nSum of digits of " + number + " = " + sumDigits);
    }
}

public class WhileLoop3 {
    public static void main(String[] args) {

        // Reverse a number
        int num      = 12345;
        int reversed = 0;
        int original = num;

        while (num > 0) {
            int lastDigit = num % 10;
            reversed = reversed * 10 + lastDigit;
            num /= 10;
        }
        System.out.println("Original: " + original);
        System.out.println("Reversed: " + reversed);

        // Palindrome check
        int n = 121;
        int rev = 0, t = n;
        while (t > 0) {
            rev = rev * 10 + t % 10;
            t /= 10;
        }
        if (n == rev)
            System.out.println("\n" + n + " is Palindrome");
        else
            System.out.println("\n" + n + " is NOT Palindrome");

        // Power without Math.pow
        int  base   = 2, exp = 8;
        long result = 1;
        int  e      = exp;
        while (e > 0) {
            result *= base;
            e--;
        }
        System.out.println("\n" + base + "^" + exp + " = " + result);
    }
}

public class WhileLoop4 {
    public static void main(String[] args) {

        // Collatz Conjecture - while ka perfect use
        // Rule: even → n/2, odd → 3n+1, 1 pe stop
        int n     = 27;
        int steps = 0;
        System.out.print("Collatz: " + n);

        while (n != 1) {
            if (n % 2 == 0)
                n = n / 2;
            else
                n = 3 * n + 1;
            System.out.print(" → " + n);
            steps++;
            if (steps % 10 == 0) System.out.println(); // new line every 10
        }
        System.out.println("\nTotal steps: " + steps);

        // GCD using while
        int a = 48, b = 18;
        int x = a,  y = b;
        while (y != 0) {
            int r = x % y;
            x = y;
            y = r;
        }
        System.out.println("\nGCD of " + a + " and " + b + " = " + x);
    }
}

🔴 Topic 3: do-while Loop

do {
    // body - PEHLE execute hoti hai
} while (condition);  ← semicolon ZAROORI!
               ↑
         BAAD MEIN check

while:    condition → body → condition → body...
do-while: body → condition → body → condition...
          ↑
          MINIMUM EK BAAR body chalegi!

✅ Menu show karna (pehle dikhao, phir puchho continue karein?)
✅ User input validation (pehle lo, phir check karo)
✅ Game - pehle ek baar khelo, phir puchho dobara?
✅ OTP/Password - pehle enter karo, phir validate karo

public class DoWhile1 {
    public static void main(String[] args) {

        // Basic do-while
        int i = 1;
        do {
            System.out.println("i = " + i);
            i++;
        } while (i <= 5);

        System.out.println("After loop, i = " + i);

        // KEY DIFFERENCE - condition FALSE from start
        System.out.println("\n--- while (condition false) ---");
        int x = 10;
        while (x < 5) {                        // FALSE - never enters
            System.out.println("while: " + x);
        }
        System.out.println("while ran 0 times");

        System.out.println("\n--- do-while (condition false) ---");
        int y = 10;
        do {
            System.out.println("do-while: " + y); // EK BAAR chalega!
            y++;
        } while (y < 5);                          // FALSE - but already ran once
        System.out.println("do-while ran 1 time minimum!");
    }
}

public class DoWhile2 {
    public static void main(String[] args) {

        // Menu system - perfect do-while use case
        // (Simulated without Scanner for online compiler)
        int[] userChoices = {1, 2, 3, 4}; // simulating user input
        int   choiceIndex = 0;
        int   choice;

        do {
            // Menu dikhao
            System.out.println("\n=== FOOD MENU ===");
            System.out.println("1. Pizza  - Rs.299");
            System.out.println("2. Burger - Rs.149");
            System.out.println("3. Pasta  - Rs.199");
            System.out.println("4. Exit");

            // User ka choice (simulated)
            choice = userChoices[choiceIndex++];
            System.out.println("Your choice: " + choice);

            switch (choice) {
                case 1: System.out.println("Pizza ordered!");  break;
                case 2: System.out.println("Burger ordered!"); break;
                case 3: System.out.println("Pasta ordered!");  break;
                case 4: System.out.println("Goodbye!");        break;
                default: System.out.println("Invalid choice!");
            }

        } while (choice != 4); // jab tak exit nahi chuna
    }
}

public class DoWhile3 {
    public static void main(String[] args) {

        // Password validation simulation
        String   correctPassword = "java123";
        String[] attempts        = {"hello", "pass", "java123"}; // simulated
        int      attemptIndex    = 0;
        int      maxAttempts     = 3;
        int      attemptsUsed    = 0;
        boolean  success         = false;

        do {
            String entered = attempts[attemptIndex++];
            attemptsUsed++;
            System.out.println("Attempt " + attemptsUsed + ": " + entered);

            if (entered.equals(correctPassword)) {
                success = true;
                System.out.println("Password correct! Login successful!");
            } else {
                System.out.println("Wrong password! " +
                    (maxAttempts - attemptsUsed) + " attempts left");
            }

        } while (!success && attemptsUsed < maxAttempts);

        if (!success) {
            System.out.println("Account locked after " + attemptsUsed + " attempts!");
        }
    }
}

public class DoWhile4 {
    public static void main(String[] args) {

        // Number guessing game simulation
        int   secretNumber = 42;
        int[] guesses      = {10, 70, 42}; // simulated guesses
        int   guessIndex   = 0;
        int   guess;
        int   attempts     = 0;

        System.out.println("=== Number Guessing Game ===");
        System.out.println("Guess a number between 1-100");

        do {
            guess = guesses[guessIndex++];
            attempts++;
            System.out.println("\nGuess #" + attempts + ": " + guess);

            if      (guess < secretNumber)
                System.out.println("Too LOW! Go higher");
            else if (guess > secretNumber)
                System.out.println("Too HIGH! Go lower");
            else
                System.out.println("CORRECT! You got it!");

        } while (guess != secretNumber);

        System.out.println("Total attempts: " + attempts);
    }
}

🟡 Topic 4: for-each Loop

for (dataType element : arrayOrCollection) {
    // element use karo
}

// Normal for:
for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);  // index se access
}

// for-each:
for (int num : arr) {
    System.out.println(num);     // directly element
}

✅ Array aur Collection traverse ke liye
✅ Read-only access - simple aur clean
❌ Element modify karna array ko affect NAHI karta
❌ Index access nahi milta
❌ Reverse traverse nahi kar sakte
❌ Skip nahi kar sakte (every element visit hoga)

public class ForEach1 {
    public static void main(String[] args) {

        // Basic for-each with array
        int[] numbers = {10, 20, 30, 40, 50};

        System.out.println("Using for-each:");
        for (int num : numbers) {
            System.out.println("num = " + num);
        }

        // String array
        String[] fruits = {"Apple", "Banana", "Mango", "Orange"};
        System.out.println("\nFruits:");
        for (String fruit : fruits) {
            System.out.println("→ " + fruit);
        }

        // Sum using for-each
        int sum = 0;
        for (int num : numbers) {
            sum += num;
        }
        System.out.println("\nSum = " + sum);
    }
}

public class ForEach2 {
    public static void main(String[] args) {

        // IMPORTANT: for-each mein modify karna array affect NAHI karta!
        int[] arr = {1, 2, 3, 4, 5};

        System.out.println("Before modification:");
        for (int n : arr) {
            System.out.print(n + " ");
        }

        // array modify karne ki koshish
        for (int n : arr) {
            n = n * 10;  // sirf local copy badla, array nahi!
        }

        System.out.println("\nAfter 'modification' with for-each:");
        for (int n : arr) {
            System.out.print(n + " "); // original values!
        }

        // Sahi tarika - normal for loop se modify karo
        for (int i = 0; i < arr.length; i++) {
            arr[i] = arr[i] * 10;  // actual array modify hoga
        }

        System.out.println("\nAfter actual modification (normal for):");
        for (int n : arr) {
            System.out.print(n + " ");
        }
    }
}

public class ForEach3 {
    public static void main(String[] args) {

        // 2D array with for-each
        int[][] matrix = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}
        };

        System.out.println("2D Array:");
        for (int[] row : matrix) {         // outer: each row
            for (int elem : row) {         // inner: each element
                System.out.printf("%4d", elem);
            }
            System.out.println();
        }

        // Find max in array
        int[] scores = {45, 89, 23, 67, 91, 34, 78};
        int   max    = scores[0];
        for (int score : scores) {
            if (score > max) max = score;
        }
        System.out.println("\nScores: ");
        for (int s : scores) System.out.print(s + " ");
        System.out.println("\nMax Score: " + max);
    }
}

public class ForEach4 {
    public static void main(String[] args) {

        int[] arr = {5, 3, 8, 1, 9, 2, 7};

        // for-each: READ only kaam - clean & simple
        System.out.println("for-each (read only):");
        int total = 0;
        for (int n : arr) {
            total += n;
        }
        System.out.println("Total = " + total);

        // Normal for: index chahiye
        System.out.println("\nNormal for (with index):");
        for (int i = 0; i < arr.length; i++) {
            System.out.println("arr[" + i + "] = " + arr[i]);
        }

        // Count even numbers
        int evenCount = 0;
        for (int n : arr) {
            if (n % 2 == 0) evenCount++;
        }
        System.out.println("\nEven count: " + evenCount);

        // char array
        char[] vowels = {'a', 'e', 'i', 'o', 'u'};
        System.out.println("\nVowels:");
        for (char v : vowels) {
            System.out.print(v + " ");
        }
    }
}

⚡ Topic 5: break aur continue

break    → loop se BAHAR niklo abhi!
continue → is iteration SKIP karo, agli pe jao

for(i=1 to 10) {
    if(i==5) break; ← loop khatam, i=5 pe
    print(i)
}
// Output: 1 2 3 4

for(i=1 to 5) {
    if(i==3) continue; ← sirf i=3 skip
    print(i)
}
// Output: 1 2 4 5

public class BreakContinue1 {
    public static void main(String[] args) {

        // break - loop se bahar niklo
        System.out.println("--- break demo ---");
        for (int i = 1; i <= 10; i++) {
            if (i == 6) {
                System.out.println("Break at i=" + i);
                break;  // loop khatam
            }
            System.out.print(i + " ");
        }

        System.out.println("\n\n--- continue demo ---");
        for (int i = 1; i <= 10; i++) {
            if (i % 2 == 0) {
                continue;  // even numbers skip
            }
            System.out.print(i + " "); // sirf odd print
        }

        // break in while
        System.out.println("\n\n--- break in while ---");
        int n = 1;
        while (true) {        // infinite loop
            System.out.print(n + " ");
            if (n == 5) break; // yahan se bahar
            n++;
        }
    }
}

🏷️ Topic 6: Labelled break/continue IMPORTANT!

outer:                    ← label
for(int i=0; i<3; i++) {
    for(int j=0; j<3; j++) {
        if(j==1) break outer; ← OUTER loop band!
    }
}

break    → sirf inner loop band
continue → sirf inner loop ki current iteration skip

break outerLabel    → outer loop band
continue outerLabel → outer loop ki current iteration skip

public class LabelledLoop1 {
    public static void main(String[] args) {

        // Normal break - sirf inner loop
        System.out.println("--- Normal break (inner only) ---");
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                if (j == 2) break; // sirf inner loop band
                System.out.println("i=" + i + " j=" + j);
            }
        }

        // Labelled break - outer loop bhi band
        System.out.println("\n--- Labelled break (outer too) ---");
        outer:
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                if (j == 2) break outer; // OUTER loop band!
                System.out.println("i=" + i + " j=" + j);
            }
        }
        // sirf i=1,j=1 print hoga phir outer loop band!
    }
}

public class LabelledLoop2 {
    public static void main(String[] args) {

        // Labelled continue
        System.out.println("--- Normal continue ---");
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                if (j == 2) continue; // inner j=2 skip
                System.out.println("i=" + i + " j=" + j);
            }
        }
        // j=2 wali lines skip hongi but loop chalta rahega

        System.out.println("\n--- Labelled continue ---");
        outer:
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                if (j == 2) continue outer; // OUTER next iteration!
                System.out.println("i=" + i + " j=" + j);
            }
        }
        // j=2 aate hi outer loop next i pe jump karega
        // j=3 kabhi nahi chalega!
    }
}

public class LabelledLoop3 {
    public static void main(String[] args) {

        // Real use: 2D array mein search
        int[][] grid = {
            { 1,  2,  3,  4},
            { 5,  6,  7,  8},
            { 9, 10, 42, 12}  // 42 yahan hai
        };

        int     target = 42;
        boolean found  = false;

        System.out.println("Searching for " + target + " in grid:");

        search:                                  // label
        for (int i = 0; i < grid.length; i++) {
            for (int j = 0; j < grid[i].length; j++) {
                System.out.print("Checking ["+i+"]["+j+"]="+grid[i][j]+"  ");
                if (grid[i][j] == target) {
                    System.out.println("\nFound " + target +
                        " at position [" + i + "][" + j + "]");
                    found = true;
                    break search;               // dono loops band!
                }
            }
            System.out.println();
        }

        if (!found) System.out.println("Not found!");
    }
}

🚫 Topic 7: Infinite Loop aur Unreachable Code

public class InfiniteLoop1 {
    public static void main(String[] args) {

        // Infinite loop types
        // for(;;) { }          // ← teeno parts empty
        // while(true) { }      // ← always true
        // do { } while(true);  // ← always true

        // Controlled infinite loop with break
        int count = 0;
        for (;;) {              // infinite loop
            count++;
            System.out.println("Count: " + count);
            if (count == 5) {
                System.out.println("Breaking out!");
                break;          // yahan se bahar
            }
        }

        // Unreachable code - compiler error deta hai
        // for(;;) {
        //     System.out.println("infinite");
        // }
        // System.out.println("unreachable!"); // ← ERROR!
        // ISLIYE yeh comment kar diya

        System.out.println("Program ended normally");
    }
}

📋 Complete Revision Summary

⭐ Important Points — Exam Sheet

⚠️ Common Mistakes