Chapter 15: for Loop in Java

This chapter is only about the for loop: repeating work with a compact header that controls initialization, condition, and update. The while and do-while loops are Chapters 16 and 17.

1. Topic title

The for loop: for (init; condition; update) and the enhanced for-each

2. What it means

Classic for:

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

Three parts in the header, separated by semicolons:

1. Initialization — runs once before the first check (int i = 0).
2. Condition — checked before each iteration; if false, the loop stops.
3. Update — runs after each iteration’s body (i++ here).

The loop variable (i) exists only inside the for block if you declare it in the init (as above).

Enhanced for (“for-each”):

int[] scores = {10, 20, 30};
for (int value : scores) {
    System.out.println(value);
}

Reads: “for each value in scores.” No index unless you add one manually. Arrays get a deeper treatment in Part 4; here you only need the syntax to read collections you already have.

3. Why it is used

for shines when you know how many steps you want, or you want the index (i) for printing row numbers, walking a string by position, or driving a times table. The header keeps all loop control in one line—easier to scan than a while with scattered update.

4. Real-world example

Printing a receipt line list: you have five items; you loop i from 0 to 4 and print i + 1 as the line number plus the item name from a parallel array (again, Part 4 makes arrays comfortable—for now, imagine the data exists).

5. Java code example

public class ForLoopDemo {
    public static void main(String[] args) {
        for (int i = 0; i < 3; i++) {
            System.out.println("classic i = " + i);
        }
 
        for (int row = 1; row <= 3; row++) {
            for (int col = 1; col <= 3; col++) {
                System.out.print("(" + row + "," + col + ") ");
            }
            System.out.println();
        }
 
        int total = 0;
        for (int n = 1; n <= 100; n++) {
            total += n;
        }
        System.out.println("sum 1..100 = " + total);
 
        String[] days = {"Mon", "Tue", "Wed"};
        for (String d : days) {
            System.out.println("for-each day: " + d);
        }
    }
}

Sample output: nested block prints a 3×3 grid of coordinates; last line shows sum 1..100 = 5050.

6. Explanation of code

• First loop: i is 0, 1, 2 — condition i < 3 fails when i becomes 3.
• Nested for: inner completes for each outer value; col resets every outer iteration.
• Summation: total declared outside the loop so it survives after the loop ends; n is loop-local.
• Enhanced for: each d is one element of days; no index variable.

7. Common mistakes

Off-by-one: i <= array.length when indexing 0..length-1 → ArrayIndexOutOfBoundsException. Prefer i < arr.length when using index i.

Wrong update: i-- when you meant i++ → infinite loop or wrong direction.

Declaring int i twice in nested loops is fine (for (int i...) inner shadows outer—confusing; use row/col names).

Forgetting braces and adding a second line you thought was in the loop—same lesson as if.

8. Best practices

Use meaningful names: row, col, attempt, not only i in non-trivial loops.

Prefer i < n style over i <= n-1 when both mean the same—pick one style per project.

Keep loop bodies small; extract a method if the body grows huge.

Use enhanced for when you only need values, not indices.

9. Small practice task

Print a multiplication line for 7: 7 x 1 = 7 through 7 x 10 = 70 using one classic for with multiplier from 1 to 10.

Beginner tip

Trace the first and last iteration on paper: what is i at start, what is i when the loop exits? If you cannot answer, the condition is unclear.