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Dashboard / Java notes / Concepts / Basics / Part i

06 - Loops

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#java/basics #l0 #concept #must-know #status/known

Loops

What is it?

A loop repeats a block of code multiple times. Instead of writing the same line 10 times, you write it once and tell the loop how many times to run it.

The while Loop

The most fundamental loop. Keeps running as long as the condition is true.

while (condition) {
    // code to repeat
}

Example - count from 1 to 5

int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++; // increment - without this, loop runs forever!
}
// Output: 1 2 3 4 5

The while (true) + break Pattern

Use this when you want to loop until the user gives valid input, or until some event happens inside the loop. You control when to stop using break.

Scanner scanner = new Scanner(System.in);

while (true) {
    System.out.println("Enter a positive number:");
    int number = Integer.parseInt(scanner.nextLine());

    if (number > 0) {
        break; // exits the loop
    }

    System.out.println("That's not positive. Try again.");
}
System.out.println("Thank you!");

Scanner scanner = new Scanner(System.in);

// 1. Create any variables you need before the loop
int sum = 0;

while (true) {
    // 2. Read input
    String input = scanner.nextLine();

    // 3. Check exit condition - break if done
    if (input.equals("done")) {
        break;
    }

    // 4. Check for invalid input - skip with continue
    int number = Integer.parseInt(input);
    if (number < 0) {
        continue; // goes back to the top of the loop
    }

    // 5. Handle valid input
    sum += number;
}

// 6. Code after the loop
System.out.println("Sum: " + sum);

break vs continue

Keyword What it does
break Exits the loop entirely - jumps to the code after the loop
continue Skips the rest of this iteration - jumps back to the top of the loop 
// continue example - skip even numbers
int i = 0;
while (i < 10) {
    i++;
    if (i % 2 == 0) {
        continue; // skip even numbers
    }
    System.out.println(i); // prints 1, 3, 5, 7, 9
}

The for Loop

Best used when you know exactly how many times you want to loop:

for (int i = 0; i < 5; i++) {
    System.out.println(i); // prints 0, 1, 2, 3, 4
}

Breaking it down:

for ( initializer ; condition ; update ) { body }
  int i = 0        i < 5       i++
  • Initializer: runs once at the start (int i = 0)
  • Condition: checked before every iteration; loop stops when false (i < 5)
  • Update: runs after each iteration (i++)

while vs for - when to use which

Use while when… Use for when…
You don’t know how many iterations You know the exact count
You’re waiting on user input Iterating over a range or list
The exit condition is complex Simple counter-based loops

Nested Loops

A loop inside a loop. The inner loop completes all its iterations for each single iteration of the outer loop:

for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 3; col++) {
        System.out.print(row + "," + col + " ");
    }
    System.out.println(); // newline after each row
}
// Output:
// 1,1  1,2  1,3
// 2,1  2,2  2,3
// 3,1  3,2  3,3

Gotchas

  • Infinite loop: forgetting to increment i or forgetting break in a while(true) loop - your program hangs forever
  • Off-by-one error: i < 5 runs 5 times (0–4). i <= 5 runs 6 times (0–5). Think carefully about < vs <=
  • Variables declared inside a loop are reset every iteration - declare accumulators (like sum) before the loop
  • continue inside a for loop still executes the update (i++) - it only skips the body

Related: Condition, Input, Methods, Lists

Linked references: