Use cases in Java - Mini Project: Build & Apply

Use Cases in Java

📖 Scenario: You are building a simple program to manage a library system. The system needs to keep track of books and their availability.

🎯 Goal: Create a Java program that stores book titles and their availability status, then prints out the available books.

📋 What You'll Learn

Create a dictionary-like data structure to store book titles and availability

Add a configuration variable to represent the availability status to filter

Use a loop to find all books that are available

Print the list of available books

💡 Why This Matters

🌍 Real World

Managing collections of items with their properties is common in library systems, inventory management, and many business applications.

💼 Career

Understanding how to use maps and loops to filter data is a fundamental skill for software developers working with data structures.

1

Create the initial data structure

Create a HashMap<String, Boolean> called books with these exact entries: "The Hobbit" as true, "1984" as false, "Pride and Prejudice" as true.

Java

// Your code here

Hint

Use HashMap<String, Boolean> and put method to add entries.

2

Add availability filter variable

Add a boolean variable called availableStatus and set it to true to represent the availability status to filter.

Java

import java.util.HashMap;

public class Library {
    public static void main(String[] args) {
        HashMap books = new HashMap();
        books.put("The Hobbit", true);
        books.put("1984", false);
        books.put("Pride and Prejudice", true);
        // Your code here
    }
}

Hint

Declare a boolean variable named availableStatus and assign true.

3

Find available books using a loop

Use a for loop with variables title and isAvailable to iterate over books.entrySet(). Inside the loop, check if isAvailable equals availableStatus. If yes, add title to a new ArrayList<String> called availableBooks.

Java

import java.util.HashMap;
import java.util.ArrayList;

public class Library {
    public static void main(String[] args) {
        HashMap books = new HashMap();
        books.put("The Hobbit", true);
        books.put("1984", false);
        books.put("Pride and Prejudice", true);
        boolean availableStatus = true;
        ArrayList availableBooks = new ArrayList();
        // Your code here
    }
}

Hint

Use books.entrySet() to loop through the map entries.

4

Print the available books

Print the availableBooks list using System.out.println.

Java

import java.util.HashMap;
import java.util.ArrayList;
import java.util.Map;

public class Library {
    public static void main(String[] args) {
        HashMap books = new HashMap();
        books.put("The Hobbit", true);
        books.put("1984", false);
        books.put("Pride and Prejudice", true);
        boolean availableStatus = true;
        ArrayList availableBooks = new ArrayList();
        for (Map.Entry entry : books.entrySet()) {
            String title = entry.getKey();
            Boolean isAvailable = entry.getValue();
            if (isAvailable == availableStatus) {
                availableBooks.add(title);
            }
        }
        // Your code here
    }
}

Hint

Use System.out.println(availableBooks); to print the list.

Practice

1. What is the main purpose of a use case in Java programming?

easy

A. To debug syntax errors

B. To write the actual Java code

C. To compile the Java program

D. To describe how users interact with the program

Solution

1. Step 1: Understand the definition of use case

   A use case explains how users will use or interact with a program, focusing on user actions and system responses.

2. Step 2: Differentiate use case from coding tasks

   Writing code, compiling, or debugging are programming tasks, not the purpose of use cases.

3. Final Answer:

   To describe how users interact with the program -> Option D

4. Quick Check:

   Use case = user interaction description [OK]

Hint: Use cases focus on user actions, not code details [OK]

Common Mistakes:

• Confusing use cases with code writing
• Thinking use cases are for debugging
• Assuming use cases are about compiling

2. Which of the following is the correct way to write a simple use case step in Java documentation?

easy

A. if(user == null) { throw new Exception(); }

B. User clicks the 'Login' button to access the system.

C. System.out.println('Login successful');

D. int loginButton = 1;

Solution

1. Step 1: Identify use case style

   Use cases are written as simple, clear steps describing user actions, not code.

2. Step 2: Check options for descriptive text

   User clicks the 'Login' button to access the system. describes a user action clearly; others are code snippets, not use case steps.

3. Final Answer:

   User clicks the 'Login' button to access the system. -> Option B

4. Quick Check:

   Use case step = user action description [OK]

Hint: Use case steps are plain user actions, not code [OK]

Common Mistakes:

• Writing code instead of user actions
• Using technical syntax in use case steps
• Mixing code and use case descriptions

3. Given this use case description:

1. User enters username and password.
2. System validates credentials.
3. System grants access if valid.
4. System shows error if invalid.

What is the expected system behavior if the user enters wrong password?

medium

A. System shows error

B. System grants access

C. System crashes

D. System ignores input

Solution

1. Step 1: Analyze the use case steps

   Step 4 says system shows error if credentials are invalid.

2. Step 2: Apply to wrong password case

   Wrong password means invalid credentials, so system shows error.

3. Final Answer:

   System shows error -> Option A

4. Quick Check:

   Invalid credentials = show error [OK]

Hint: Invalid login means error message shown [OK]

Common Mistakes:

• Choosing system grants access for wrong password
• Assuming system crashes on error
• Ignoring error handling step

4. Identify the error in this use case step:

Step 2: System checks if user password is correct.
Step 3: If password is wrong, system grants access.

medium

A. Step 3 should deny access if password is wrong

B. Step 2 should accept any password

C. Step 3 should crash the system

D. No error, steps are correct

Solution

1. Step 1: Review step 3 logic

   Granting access on wrong password is a security error.

2. Step 2: Correct expected behavior

   System should deny access or show error if password is wrong.

3. Final Answer:

   Step 3 should deny access if password is wrong -> Option A

4. Quick Check:

   Wrong password = deny access [OK]

Hint: Wrong password must never grant access [OK]

Common Mistakes:

• Allowing access on wrong password
• Ignoring security in use case steps
• Assuming system crashes on wrong input

5. You want to write a use case for a Java program that allows users to reset their password. Which sequence of steps best fits this use case?

hard

A. User logs in -> System checks password -> System grants access -> User resets password

B. System resets password automatically -> User notified -> User logs in

C. User requests password reset -> System sends reset link -> User sets new password -> System confirms update

D. User changes username -> System updates profile -> User logs out

Solution

1. Step 1: Identify correct password reset flow

   Reset involves user request, system sending link, user setting new password, and confirmation.

2. Step 2: Compare options to expected flow

   User requests password reset -> System sends reset link -> User sets new password -> System confirms update matches this flow clearly; others describe unrelated or incorrect sequences.

3. Final Answer:

   User requests password reset -> System sends reset link -> User sets new password -> System confirms update -> Option C

4. Quick Check:

   Password reset = request, link, new password, confirm [OK]

Hint: Password reset needs user action and confirmation steps [OK]

Common Mistakes:

• Confusing login with password reset
• Assuming system resets password without user input
• Mixing username change with password reset