Enhanced for loop in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of enhanced for loop with array

What is the output of the following Java code using an enhanced for loop?

Java

public class Main {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3};
        int sum = 0;
        for (int num : numbers) {
            sum += num;
        }
        System.out.println(sum);
    }
}

A Compilation error

B 123

C 0

D 6

💡 Hint

Think about how the enhanced for loop adds each number to sum.

The enhanced for loop iterates over each element in the array 'numbers'. It adds each element to 'sum'. The sum of 1 + 2 + 3 is 6, so the output is 6.

❓ Predict Output

intermediate

Enhanced for loop with a list

What will be printed by this Java code using an enhanced for loop over a List?

Java

import java.util.List;
import java.util.ArrayList;
public class Main {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>();
        fruits.add("apple");
        fruits.add("banana");
        fruits.add("cherry");
        for (String fruit : fruits) {
            System.out.print(fruit.charAt(0));
        }
    }
}

A applebananacherry

B a b c

C abc

D Compilation error

💡 Hint

Look at what character is printed for each fruit.

The enhanced for loop goes through each fruit in the list. For each fruit, it prints the first character (charAt(0)). So it prints 'a' from apple, 'b' from banana, and 'c' from cherry, resulting in 'abc'.

❓ Predict Output

advanced

Enhanced for loop with modification inside loop

What is the output of this Java code that tries to modify array elements inside an enhanced for loop?

Java

public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3};
        for (int x : arr) {
            x = x * 2;
        }
        for (int x : arr) {
            System.out.print(x + " ");
        }
    }
}

A 1 2 3

B 2 4 6

C Compilation error

D Runtime exception

💡 Hint

Does changing 'x' inside the loop change the original array?

The enhanced for loop variable 'x' is a copy of each element. Changing 'x' does not change the original array elements. So the array remains {1, 2, 3}, and the second loop prints '1 2 3 '.

❓ Predict Output

advanced

Enhanced for loop with multidimensional array

What will this Java program print when using an enhanced for loop over a 2D array?

Java

public class Main {
    public static void main(String[] args) {
        int[][] matrix = {{1, 2}, {3, 4}};
        int total = 0;
        for (int[] row : matrix) {
            for (int val : row) {
                total += val;
            }
        }
        System.out.println(total);
    }
}

A 10

B Compilation error

C 24

D 0

💡 Hint

Add all numbers in the 2D array.

The outer enhanced for loop iterates over each row (which is an int array). The inner loop iterates over each value in that row. Adding 1 + 2 + 3 + 4 equals 10, so the output is 10.

🧠 Conceptual

expert

Behavior of enhanced for loop with Iterable and Iterator

Which statement about the enhanced for loop in Java is true?

A It requires manual handling of the loop index variable.

B It internally uses an Iterator to loop over elements of Iterable collections.

C It allows modifying the collection elements directly during iteration.

D It can only be used with arrays, not with collections.

💡 Hint

Think about how enhanced for loops work with collections like ArrayList.

The enhanced for loop works with arrays and any class that implements Iterable. Internally, it uses an Iterator to get elements one by one. It does not allow modifying the collection elements directly, and it does not require manual index handling.

Practice

1. What is the main purpose of the enhanced for loop in Java?

easy

A. To create infinite loops

B. To iterate over all elements in an array or collection easily

C. To declare variables with enhanced types

D. To replace if-else statements

Solution

1. Step 1: Understand the enhanced for loop usage

   The enhanced for loop is designed to simplify iterating over arrays or collections without manual index management.

2. Step 2: Compare options with the loop's purpose

   Only To iterate over all elements in an array or collection easily correctly describes this purpose; others describe unrelated concepts.

3. Final Answer:

   To iterate over all elements in an array or collection easily -> Option B

4. Quick Check:

   Enhanced for loop = easy iteration [OK]

Hint: Enhanced for loop = easy way to visit each item [OK]

Common Mistakes:

• Thinking it declares variables
• Confusing with infinite loops
• Using it to replace conditional statements

2. Which of the following is the correct syntax for an enhanced for loop iterating over an integer array nums?

easy

A. foreach (int num : nums) { System.out.println(num); }

B. for int num in nums { System.out.println(num); }

C. for (int num : nums) { System.out.println(num); }

D. for (num : nums) { System.out.println(num); }

Solution

1. Step 1: Recall Java enhanced for loop syntax

   The correct syntax uses: for (type variable : arrayOrCollection) { ... }

2. Step 2: Check each option

   for (int num : nums) { System.out.println(num); } matches the correct syntax. Options A, B, and C use incorrect keywords or missing parentheses.

3. Final Answer:

   for (int num : nums) { System.out.println(num); } -> Option C

4. Quick Check:

   Correct syntax uses ':' inside parentheses [OK]

Hint: Remember: for (type var : collection) { } [OK]

Common Mistakes:

• Using 'in' instead of ':'
• Omitting parentheses
• Using 'foreach' keyword in Java

3. What will be the output of the following code?

int[] numbers = {1, 2, 3};
int sum = 0;
for (int n : numbers) {
    sum += n;
}
System.out.println(sum);

medium

A. 6

B. 123

C. 0

D. Compilation error

Solution

1. Step 1: Understand the loop operation

   The loop adds each element of the array (1, 2, 3) to the variable sum.

2. Step 2: Calculate the sum

   sum = 0 + 1 + 2 + 3 = 6

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum of 1, 2, 3 = 6 [OK]

Hint: Sum all elements inside the loop [OK]

Common Mistakes:

• Concatenating numbers instead of adding
• Forgetting to initialize sum
• Syntax errors in loop

4. Identify the error in the following code snippet:

String[] fruits = {"apple", "banana", "cherry"};
for (String fruit : fruits) {
    System.out.println(fruits);
}

medium

A. Inside the loop, it should print 'fruit' instead of 'fruits'

B. The loop variable should be 'fruits' not 'fruit'

C. The array declaration is incorrect

D. Enhanced for loop cannot be used with arrays

Solution

1. Step 1: Analyze the loop variable and print statement

   The loop variable is 'fruit', representing each element. Printing 'fruits' prints the whole array reference each time.

2. Step 2: Correct the print statement

   To print each fruit, use 'System.out.println(fruit);' inside the loop.

3. Final Answer:

   Inside the loop, it should print 'fruit' instead of 'fruits' -> Option A

4. Quick Check:

   Print loop variable, not array name [OK]

Hint: Print the loop variable, not the array name [OK]

Common Mistakes:

• Printing the array variable inside the loop
• Confusing loop variable with array name
• Incorrect array declaration

5. Given a list of integers, which enhanced for loop code correctly counts how many numbers are greater than 10?

List<Integer> nums = List.of(5, 12, 7, 20, 10);
int count = 0;
// Which code correctly counts numbers > 10?

hard

A. for (Integer n : nums) { if (n >= 10) count++; }

B. for (int n : nums) { if (n >= 10) count++; }

C. for (n : nums) { if (n > 10) count++; }

D. for (int n : nums) { if (n > 10) count++; }

Solution

1. Step 1: Understand the data and requirement

   List.of(5,12,7,20,10). Numbers greater than 10: 12,20 (count=2).

2. Step 2: Recall enhanced for loop with List<Integer>

   Supports int n via automatic unboxing of Integer to int.

3. Step 3: Check each option

   A: >=10 counts 12,20,10 (3) incorrect.
   B: int n >10 correct syntax, condition, count=2.
   C: Missing type for n: compile error.
   D: >=10 incorrect condition.

4. Final Answer:

   for (int n : nums) { if (n > 10) count++; } -> Option D

5. Quick Check:

   int n works with List<Integer> via unboxing [OK]

Hint: int n works for List<Integer> via autounboxing [OK]

Common Mistakes:

• Using >= instead of >
• Missing type declaration for loop variable
• Wrong comparison operator