Loop execution flow in Java - Mini Project: Build & Apply

Loop Execution Flow in Java

📖 Scenario: Imagine you are managing a small bookstore. You want to count how many books you have in stock and print each book's name one by one.

🎯 Goal: You will create a Java program that uses a for loop to go through a list of book titles and print each title. This will help you understand how loops execute step-by-step.

📋 What You'll Learn

Create a String array called books with exactly these titles: "Java Basics", "Data Structures", "Algorithms", "Design Patterns"

Create an integer variable called count and set it to 0

Use a for loop with an integer variable i to iterate over the books array

Inside the loop, print the current book title using System.out.println(books[i])

Inside the loop, increase count by 1 each time

After the loop, print the total number of books using System.out.println("Total books: " + count)

💡 Why This Matters

🌍 Real World

Loops are used in many real-world programs to process lists of items, like books, customers, or products, one by one.

💼 Career

Understanding loop execution flow is essential for software developers to write efficient and correct code that handles repeated tasks.

1

Create the books array

Create a String array called books with these exact titles: "Java Basics", "Data Structures", "Algorithms", "Design Patterns"

Java

# Create the books array with the given titles
// Your code here

Hint

Use curly braces {} to list the book titles inside the array.

2

Create the count variable

Create an integer variable called count and set it to 0 inside the main method, below the books array.

Java

public class Main {
    public static void main(String[] args) {
        String[] books = {"Java Basics", "Data Structures", "Algorithms", "Design Patterns"};
        // Create count variable and set to 0
        // Your code here
    }
}

Hint

Use int count = 0; to create the variable.

3

Use a for loop to print each book and count them

Use a for loop with an integer variable i to iterate over the books array. Inside the loop, print the current book title using System.out.println(books[i]) and increase count by 1.

Java

public class Main {
    public static void main(String[] args) {
        String[] books = {"Java Basics", "Data Structures", "Algorithms", "Design Patterns"};
        int count = 0;
        // Use a for loop to print each book and increase count
        // Your code here
    }
}

Hint

Use for (int i = 0; i < books.length; i++) to loop through the array.

4

Print the total number of books

After the for loop, print the total number of books using System.out.println("Total books: " + count).

Java

public class Main {
    public static void main(String[] args) {
        String[] books = {"Java Basics", "Data Structures", "Algorithms", "Design Patterns"};
        int count = 0;
        for (int i = 0; i < books.length; i++) {
            System.out.println(books[i]);
            count++;
        }
        // Print the total number of books
        // Your code here
    }
}

Hint

Use System.out.println("Total books: " + count); to print the total.

Practice

1. What does a for loop in Java primarily do?

easy

A. Executes code only once

B. Stops the program immediately

C. Repeats a block of code a specific number of times

D. Creates a new variable

Solution

1. Step 1: Understand the purpose of a for loop and compare options

   A for loop is designed to repeat a block of code multiple times based on a condition. Only "Repeats a block of code a specific number of times" correctly describes it.

2. Final Answer:

   Repeats a block of code a specific number of times -> Option C

3. Quick Check:

   for loop = repeat code [OK]

Hint: For loops repeat code a set number of times [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing loops with variable declarations
• Believing loops stop the program

2. Which of the following is the correct syntax for a basic while loop in Java?

easy

A. while (condition); { // code }

B. while condition { // code }

C. while: (condition) { // code }

D. while (condition) { // code }

Solution

1. Step 1: Recall Java while loop syntax and check options

   The correct syntax requires parentheses around the condition and braces for the code block. Only "while (condition) { // code }" matches: while (condition) { // code }.

2. Final Answer:

   while (condition) { // code } -> Option D

3. Quick Check:

   while loop syntax = parentheses + braces [OK]

Hint: While loops need parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around condition
• Adding semicolon after while condition
• Using colon instead of parentheses

3. What will be the output of this Java code?

int count = 0;
while (count < 3) {
  System.out.print(count + " ");
  count++;
}

medium

A. 0 1 2

B. 1 2 3

C. 0 1 2 3

D. 3 2 1

Solution

1. Step 1: Analyze the loop condition and trace iterations

   The loop runs while count is less than 3, starting from 0. It prints 0, then increments to 1; prints 1, increments to 2; prints 2, increments to 3; then stops because 3 is not less than 3.

2. Final Answer:

   0 1 2 -> Option A

3. Quick Check:

   Prints 0,1,2 then stops [OK]

Hint: Count starts at 0 and stops before 3 [OK]

Common Mistakes:

• Including 3 in output
• Starting count from 1
• Printing in reverse order

4. Identify the error in this loop code:

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

medium

A. Missing increment of i inside the loop

B. Incorrect loop condition syntax

C. Missing semicolon after while

D. Variable i should be declared inside the loop

Solution

1. Step 1: Check loop control variable and consequences

   The variable i is initialized but never changed inside the loop. Without incrementing i, the condition i < 5 is always true, causing an infinite loop.

2. Final Answer:

   Missing increment of i inside the loop -> Option A

3. Quick Check:

   Loop needs variable update to stop [OK]

Hint: Always update loop variable to avoid infinite loops [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Misplacing semicolons
• Declaring variable inside loop unnecessarily

5. Consider this code snippet:

int sum = 0;
for (int i = 1; i <= 5; i++) {
  if (i == 3) continue;
  sum += i;
}
System.out.println(sum);

What is the output and why?

hard

A. 15 because all numbers 1 to 5 are added

B. 12 because number 3 is skipped in addition

C. 10 because only numbers less than 3 are added

D. Error due to incorrect use of continue

Solution

1. Step 1: Trace the loop and effect of continue

   The loop runs from 1 to 5. When i equals 3, continue skips the addition for that iteration. Sum = 1 + 2 + 4 + 5 = 12 because 3 is skipped.

2. Final Answer:

   12 because number 3 is skipped in addition -> Option B

3. Quick Check:

   continue skips 3, sum = 12 [OK]

Hint: Continue skips current loop iteration [OK]

Common Mistakes:

• Adding 3 despite continue
• Thinking continue causes error
• Summing only numbers less than 3