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Java for Loops in Depth: Counting, for-each, Nested Loops and Patterns

Go deeper with Java for loops: counting up and down, custom steps, multiple loop variables, the enhanced for-each loop and its limits, looping over a Map, nested loops and star patterns, and the classic sum, max and count patterns.

Beginner11 min readUpdated
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The Loops lesson introduced the for loop. This lesson covers the variations you will write every day, and the classic patterns that appear in exercises and interviews.

Counting in different ways#

Java
public class Main {
    public static void main(String[] args) {
        for (int i = 10; i > 0; i -= 3) {   // count down in steps of 3
            System.out.print(i + " ");
        }
        System.out.println();

        for (int i = 1; i <= 100; i *= 2) {  // powers of two
            System.out.print(i + " ");
        }
        System.out.println();
    }
}
Output
10 7 4 1 
1 2 4 8 16 32 64

Several loop variables#

Separate several variables with commas. Walking from both ends towards the middle is a common use:

Java
public class Main {
    public static void main(String[] args) {
        String word = "level";
        boolean palindrome = true;

        for (int left = 0, right = word.length() - 1; left < right; left++, right--) {
            if (word.charAt(left) != word.charAt(right)) {
                palindrome = false;
            }
        }
        System.out.println(word + (palindrome ? " is" : " is not") + " a palindrome");
    }
}
Output
level is a palindrome

The enhanced for loop in detail#

The for-each loop gives you each element, not its position:

Java
public class Main {
    public static void main(String[] args) {
        int[] prices = {120, 45, 300};

        int total = 0;
        for (int p : prices) {
            total += p;
        }
        System.out.println("Total: " + total);

        for (int p : prices) {
            p += 10;           // changes only the loop variable!
        }
        System.out.println(prices[0]);

        for (int i = 0; i < prices.length; i++) {
            prices[i] += 10;   // changes the array
        }
        System.out.println(prices[0]);
    }
}
Output
Total: 465
120
130

The loop variable is a copy of each element (for primitives) or of each reference (for objects). Assigning to it does not change the array or collection. Use an index loop when you need to replace elements or know the position.

Looping over a Map#

A Map has no single "element", so loop over its entrySet(), which gives key-value pairs:

Java
import java.util.Map;
import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> stock = new TreeMap<>(Map.of("pens", 40, "books", 12, "bags", 5));

        for (Map.Entry<String, Integer> entry : stock.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }

        stock.forEach((item, count) -> System.out.println(item + " -> " + count));
    }
}
Output
bags: 5
books: 12
pens: 40
bags -> 5
books -> 12
pens -> 40

A TreeMap keeps its keys sorted; see HashMap and TreeMap. The forEach with an arrow is a lambda, covered in Lambdas.

Nested loops#

A loop inside a loop runs the inner loop completely for each step of the outer one:

Java
public class Main {
    public static void main(String[] args) {
        for (int row = 1; row <= 3; row++) {
            for (int col = 1; col <= 4; col++) {
                System.out.print(row * col + "\t");
            }
            System.out.println();
        }
    }
}
Output
1	2	3	4	
2	4	6	8	
3	6	9	12

Printing patterns#

Java
public class Main {
    public static void main(String[] args) {
        int height = 4;
        for (int row = 1; row <= height; row++) {
            System.out.println(" ".repeat(height - row) + "*".repeat(2 * row - 1));
        }
    }
}
Output
   *
  ***
 *****
*******

String.repeat(n) (Java 11) repeats a string n times, which removes the need for two inner loops.

The core loop patterns#

Java
public class Main {
    public static void main(String[] args) {
        int[] values = {14, 3, 27, 8, 19};

        int sum = 0;
        int largest = values[0];
        int evenCount = 0;

        for (int v : values) {
            sum += v;
            if (v > largest) largest = v;
            if (v % 2 == 0) evenCount++;
        }

        System.out.println("Sum: " + sum);
        System.out.println("Largest: " + largest);
        System.out.println("Even numbers: " + evenCount);
    }
}
Output
Sum: 71
Largest: 27
Even numbers: 2

Accumulate, track the best so far, and count matches: these three patterns solve most loop problems. Streams can express them in one line, such as Arrays.stream(values).sum().

Common mistakes#

Practice#

Practice exercise

Print this right-angled number triangle using nested loops:

Text
1
1 2
1 2 3
1 2 3 4
Show solution
Java
public class Triangle {
    public static void main(String[] args) {
        for (int row = 1; row <= 4; row++) {
            StringBuilder line = new StringBuilder();
            for (int n = 1; n <= row; n++) {
                if (n > 1) line.append(' ');
                line.append(n);
            }
            System.out.println(line);
        }
    }
}
Output
1
1 2
1 2 3
1 2 3 4

StringBuilder builds the line efficiently; see StringBuilder.

Practice exercise

Given int[] marks = {56, 91, 78, 34, 88};, print the average as a decimal and how many marks are 75 or above.

Show solution
Java
public class Marks {
    public static void main(String[] args) {
        int[] marks = {56, 91, 78, 34, 88};
        int total = 0;
        int high = 0;
        for (int m : marks) {
            total += m;
            if (m >= 75) high++;
        }
        System.out.println("Average: " + (double) total / marks.length);
        System.out.println("75 or above: " + high);
    }
}
Output
Average: 69.4
75 or above: 3

Key takeaway#

Finished this lesson?

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