Loops in Java

🟢 Beginner

📖 Definition

Loops repeatedly execute a block of Java code as long as a specified control condition remains true. Java supports standard for loops, while loops (pre-test), do-while loops (post-test), enhanced for-each loops (iterating over arrays/collections), and control keywords (break, continue, Labeled Loops).

🇮🇳 Hindi

Repetitive tasks ko automate karne ke liye loops ka use hota hai. Counter-based iterations ke liye for loop, condition-based iteration ke liye while loop, kam se kam ek baar guaranteed execute karne ke liye do-while loop, aur arrays/collections iterate karne ke liye enhanced for-each loop use karein.

🚩 Marathi

Code punha punha chalavnyasathi loops vaparatat. Counter sathi for, condition sathi while, aani arrays sathi for-each loop cha wapar hoto.

📝 1. Loop Types & Syntaxes

A. Standard Counter for Loop

for (int i = 1; i <= 5; i++) {
    System.out.println("Iteration: " + i);
}

B. Pre-Test while Loop

int count = 3;
while (count > 0) {
    System.out.println("Countdown: " + count);
    count--;
}

C. Post-Test do-while Loop (Guaranteed at least 1 execution)

int attempts = 0;
do {
    System.out.println("Processing login attempt #" + (attempts + 1));
    attempts++;
} while (attempts < 1);

D. Enhanced for-each Loop (Iterating Elements)

String[] languages = {"Java", "Python", "SQL", "C++"};
for (String lang : languages) {
    System.out.println("Language: " + lang);
}

🧠 2. Control Statements & Labeled Loops

  • break: Terminates the loop immediately.
  • continue: Skips the rest of the current iteration and jumps to the next loop cycle.
  • Labeled Loops (label:): Allows break or continue to exit outer nested loops directly.
outerLoop:
for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 3; col++) {
        if (row == 2 && col == 2) {
            break outerLoop; // Exits the outer loop directly!
        }
        System.out.println("Row: " + row + ", Col: " + col);
    }
}

💡 Complete Example: Processing Batch Transaction Data

public class BatchProcessor {
    public static void main(String[] args) {
        double[] transactionAmounts = {1500.00, -200.00, 4500.50, 0.00, 8900.00, -9999.00};
        double validTotal = 0.0;
        int processedCount = 0;

        System.out.println("=== PROCESSING TRANSACTION BATCH ===");

        for (double amount : transactionAmounts) {
            // Guard clause: Skip invalid negative transactions
            if (amount < 0) {
                System.out.println("Warning: Invalid negative amount (" + amount + ") skipped.");
                continue;
            }

            // Exit condition if zero sentinel encountered
            if (amount == 0.0) {
                System.out.println("Notice: Zero sentinel value encountered. Stopping batch.");
                break;
            }

            validTotal += amount;
            processedCount++;
            System.out.printf("Processed Transaction #%d : ₹%.2f%n", processedCount, amount);
        }

        System.out.println("====================================");
        System.out.printf("Total Valid Processed Amount : ₹%.2f%n", validTotal);
        System.out.println("Total Valid Transactions     : " + processedCount);
    }
}

👀 Output

=== PROCESSING TRANSACTION BATCH ===
Processed Transaction #1 : ₹1500.00
Warning: Invalid negative amount (-200.0) skipped.
Processed Transaction #2 : ₹4500.50
Notice: Zero sentinel value encountered. Stopping batch.
====================================
Total Valid Processed Amount : ₹6000.50
Total Valid Transactions     : 2

⚠️ Common Mistakes

  • Infinite Loops: Forgetting to update loop counter variables inside while or do-while loops (e.g. omitting i++).
  • Off-By-One Errors: Using i <= array.length instead of i < array.length when iterating indexed arrays.
  • Accidental Semicolon: Writing for (int i = 0; i < 5; i++); puts an empty statement in the loop body. The subsequent block { ... } executes only once after the loop finishes!

🛡️ Safety / Important Notes

Do NOT structurally add or remove items from a java.util.Collection inside an enhanced for-each loop; doing so will throw a ConcurrentModificationException! Use an Iterator or removeIf() method instead.

🌍 Real-World Usage

Batch database processing, streaming sensor records, rendering UI lists, retrying network connections, and traversing tree/graph data structures.

🧪 Try It Yourself

  1. Write a for loop that prints all odd numbers from 1 to 20.
  2. Write a while loop that calculates the sum of numbers from 1 to 100.

🎯 Mini Challenge

Write a program using nested for loops that prints a 5-by-5 multiplication grid table formatted cleanly with tab columns (\t).

🔗 Related Topics

🧭 Navigation

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