Throwing custom exceptions in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of custom exception throwing

What will be the output when this Java code runs?

Java

class MyException extends Exception {
    public MyException(String message) {
        super(message);
    }
}

public class Test {
    public static void check(int num) throws MyException {
        if (num < 0) {
            throw new MyException("Negative number: " + num);
        } else {
            System.out.println("Number is " + num);
        }
    }

    public static void main(String[] args) {
        try {
            check(-5);
        } catch (MyException e) {
            System.out.println("Caught: " + e.getMessage());
        }
    }
}

A Caught: Negative number: -5

B Number is -5

C Exception in thread "main" MyException: Negative number: -5

D Compilation error due to missing throws declaration

💡 Hint

Look at how the exception is thrown and caught in the try-catch block.

The method throws a custom checked exception when the number is negative. The main method catches it and prints the message.

🧠 Conceptual

intermediate

Understanding checked vs unchecked exceptions

Which statement correctly describes the difference between checked and unchecked exceptions in Java?

A Checked exceptions must be declared or caught; unchecked exceptions do not need to be declared or caught.

B Neither checked nor unchecked exceptions need to be declared or caught.

C Both checked and unchecked exceptions must always be caught or declared.

D Unchecked exceptions must be declared or caught; checked exceptions do not need to be declared or caught.

💡 Hint

Think about compiler requirements for exception handling.

Checked exceptions require explicit handling or declaration, while unchecked exceptions (RuntimeExceptions) do not.

🔧 Debug

advanced

Identify the error in custom exception throwing

What error will this code produce when compiled?

Java

class MyException extends Exception {
    public MyException(String message) {
        super(message);
    }
}

public class Test {
    public static void check(int num) {
        if (num < 0) {
            throw new MyException("Negative number: " + num);
        }
    }
}

A Compilation error: missing constructor in MyException

B Runtime error: MyException thrown but not caught

C Compilation error: unhandled exception type MyException

D No error, code compiles and runs fine

💡 Hint

Check if the method declares the exception it throws.

The method throws a checked exception but does not declare it with 'throws', causing a compile error.

📝 Syntax

advanced

Correct syntax for throwing a custom exception

Which option shows the correct way to throw a custom checked exception in Java?

A throw new MyException;

B throw MyException("Error occurred");

C throw MyException;

D throw new MyException("Error occurred");

💡 Hint

Remember how to create new objects in Java.

You must use 'new' and provide parentheses with any required arguments when throwing exceptions.

🚀 Application

expert

Result of nested custom exception handling

What will be printed when this Java program runs?

Java

class CustomException extends Exception {
    public CustomException(String msg) {
        super(msg);
    }
}

public class Main {
    public static void methodA() throws CustomException {
        throw new CustomException("Error in methodA");
    }

    public static void methodB() {
        try {
            methodA();
        } catch (CustomException e) {
            System.out.println("Caught in methodB: " + e.getMessage());
            throw new RuntimeException("Runtime in methodB");
        }
    }

    public static void main(String[] args) {
        try {
            methodB();
        } catch (RuntimeException e) {
            System.out.println("Caught in main: " + e.getMessage());
        }
    }
}

A Caught in methodB: Error in methodA

B

Caught in methodB: Error in methodA
Caught in main: Runtime in methodB

C Caught in main: Runtime in methodB

D Exception in thread "main" java.lang.RuntimeException: Runtime in methodB

💡 Hint

Follow the flow of exceptions through the try-catch blocks.

methodA throws CustomException caught in methodB, which then throws RuntimeException caught in main, printing both messages.