Creating custom exception class in Java - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of custom exception message

What is the output of this Java program that uses a custom exception class?

Java

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

public class Test {
    public static void main(String[] args) {
        try {
            throw new MyException("Custom error occurred");
        } catch (MyException e) {
            System.out.println(e.getMessage());
        }
    }
}

A Custom error occurred

B MyException: Custom error occurred

C Compilation error

D Exception in thread "main" MyException: Custom error occurred

💡 Hint

Look at what getMessage() returns from the Exception class.

The getMessage() method returns only the message string passed to the exception, so it prints exactly "Custom error occurred".

❓ Predict Output

intermediate

Value of variable after catching custom exception

What is the value of variable 'result' after running this code?

Java

class MyException extends Exception {}

public class Test {
    public static int testMethod() throws MyException {
        throw new MyException();
    }

    public static void main(String[] args) {
        int result = 0;
        try {
            result = testMethod();
        } catch (MyException e) {
            result = -1;
        }
        System.out.println(result);
    }
}

A 0

B -1

C Compilation error

D Exception in thread "main" java.lang.MyException

💡 Hint

Consider what happens when the exception is thrown and caught.

The method throws MyException, so the try block does not assign to result. The catch block assigns -1 to result, which is printed.

📝 Syntax

advanced

Identify syntax error in custom exception class

Which option contains a syntax error in defining a custom exception class?

A

public class MyException extends Exception {
    public MyException() {}
}

B

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

C

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

D

public class MyException extends Exception {
    public MyException(String message) {
        this.message = message;
    }
}

💡 Hint

Check how the superclass Exception stores the message.

Option D tries to assign to 'this.message' which is not accessible; the correct way is to call super(message).

🧠 Conceptual

advanced

Purpose of creating a custom exception class

Why would a developer create a custom exception class instead of using existing Java exceptions?

A To avoid using try-catch blocks in the code

B To automatically fix errors during runtime

C To provide more specific error information related to the application domain

D To improve the performance of exception handling

💡 Hint

Think about how exceptions help communicate problems.

Custom exceptions help clearly indicate specific problems related to the application's logic or domain, making error handling clearer.

🔧 Debug

expert

Why does this custom exception code fail to compile?

This code tries to define a custom checked exception but fails to compile. What is the cause?

Java

class MyException extends Exception {
    public MyException() {
        System.out.println("Error");
    }
}

public class Test {
    public static void main(String[] args) {
        throw new MyException();
    }
}

A You cannot throw checked exceptions without declaring them in method signature

B MyException constructor does not call super(), causing a compile error

C The catch block must catch Exception, not MyException

D System.out.println cannot be used inside exception constructors

💡 Hint

Check the method main's throws declaration and checked exceptions rules.

MyException extends Exception, which is checked. The main method does not declare throws MyException, so throwing it causes a compile error.