Use cases in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method call with record use case

What is the output of the following Java code using records to represent a use case?

Java

record User(String name, int age) {}

public class Main {
    public static void main(String[] args) {
        User user = new User("Alice", 30);
        System.out.println(user.name() + " is " + user.age() + " years old.");
    }
}

A Alice is 30 years old.

B User[name=Alice, age=30]

C Alice 30

D Compilation error

💡 Hint

Look at how record components are accessed using methods with the same name.

Records automatically create methods with the same name as their components to access values. So user.name() returns "Alice" and user.age() returns 30.

🧠 Conceptual

intermediate

Purpose of use cases in software development

Which of the following best describes the main purpose of use cases in software development?

A To define detailed database schemas for the application

B To write unit tests for each method in the code

C To specify the hardware requirements for deployment

D To describe interactions between users and the system to achieve goals

💡 Hint

Think about how use cases help understand what users want to do with the system.

Use cases describe how users interact with the system to accomplish specific goals, helping developers understand requirements.

🔧 Debug

advanced

Identify the error in this use case implementation

What error will this Java code produce when run?

Java

public class UseCase {
    private String action;
    public UseCase(String action) {
        this.action = action;
    }
    public void execute() {
        System.out.println("Executing " + action);
    }
    public static void main(String[] args) {
        UseCase uc = null;
        uc.execute();
    }
}

A Compilation error: cannot find symbol execute

B NullPointerException at runtime

C No output, program ends silently

D IllegalStateException at runtime

💡 Hint

Check what happens when you call a method on a null reference.

Calling a method on a null object reference causes a NullPointerException at runtime.

📝 Syntax

advanced

Correct syntax for defining a use case class with a method

Which option shows the correct syntax to define a Java class named UseCase with a method run that prints "Running use case"?

A

public class UseCase {
    public void run() {
        System.out.println("Running use case");
    }
}

B

public UseCase {
    void run() {
        System.out.println("Running use case");
    }
}

C

public class UseCase {
    void run() {
        System.out.println("Running use case");
    }
}

D

public class UseCase {
    public run() {
        System.out.println("Running use case");
    }
}

💡 Hint

Remember Java method declarations require return type and access modifier is optional but recommended.

Option A correctly declares a public class with a public void method run with proper syntax.

🚀 Application

expert

Number of use case objects created and their output

Consider this Java code creating use case objects. How many objects are created and what is printed?

Java

public class UseCase {
    private final String name;
    public UseCase(String name) {
        this.name = name;
    }
    public void execute() {
        System.out.println("Use case: " + name);
    }
    public static void main(String[] args) {
        UseCase[] cases = new UseCase[3];
        for (int i = 0; i < cases.length; i++) {
            cases[i] = new UseCase("Case" + (i + 1));
        }
        for (UseCase uc : cases) {
            uc.execute();
        }
    }
}

A 1 object created; prints Use case: Case3

B 3 objects created; prints Use case: Case3 three times

C 3 objects created; prints Use case: Case1\nUse case: Case2\nUse case: Case3

D Compilation error due to array initialization

💡 Hint

Look at the loop creating new UseCase objects and the loop printing them.

The code creates 3 UseCase objects with names Case1, Case2, Case3 and prints each name once.

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