Throwing custom exceptions in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a custom exception class named MyException.

Java

public class MyException extends [1] {
}

Drag options to blanks, or click blank then click option'

ARuntimeException

BString

CExceptionHandler

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Extending a non-exception class like String or int.

Forgetting to extend any class.

The custom exception class should extend RuntimeException or Exception. Here, RuntimeException is used.

2fill in blank

medium

Complete the constructor of the custom exception to accept a message.

Java

public class MyException extends RuntimeException {
    public MyException([1] String message) {
        super(message);
    }
}

Drag options to blanks, or click blank then click option'

Aprivate

Bfinal

Cpublic

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using access modifiers like private or public on parameters.

Omitting the parameter type.

The constructor parameter can be declared final to prevent modification, but it's optional. Here, final is used as a good practice.

3fill in blank

hard

Fix the error in throwing the custom exception with a message.

Java

throw new MyException([1]);

Drag options to blanks, or click blank then click option'

Anew MyException()

BException

C"Error occurred"

DMyException

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the class name instead of a message.

Not passing any argument.

When throwing the exception, you must pass a string message to the constructor, like "Error occurred".

4fill in blank

hard

Fill both blanks to throw a custom exception when a number is negative.

Java

if (number [1] 0) {
    throw new MyException([2]);
}

Drag options to blanks, or click blank then click option'

A<

B"Negative number not allowed"

C>

D"Number is positive"

Attempts:

3 left

💡 Hint

Common Mistakes

Using > instead of < for the condition.

Throwing exception with wrong message.

The condition checks if the number is less than zero, then throws the exception with the message "Negative number not allowed".

5fill in blank

hard

Fill all three blanks to define and throw a custom exception with a message including the invalid value.

Java

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

int value = -5;
if (value [1] 0) {
    throw new MyException("Invalid value: " + [2] + [3]);
}

Drag options to blanks, or click blank then click option'

A<

Bvalue

C"!"

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using > instead of < in the condition.

Not concatenating the message parts correctly.

The code checks if value is less than zero, then throws MyException with a message showing the invalid value followed by an exclamation mark.

Practice

1. What is the main purpose of throwing a custom exception in Java?

easy

A. To speed up the program execution.

B. To automatically fix errors in the program.

C. To create a specific error type with a clear message for better error handling.

D. To avoid writing any error handling code.

Solution

1. Step 1: Understand what custom exceptions do

   Custom exceptions let programmers define specific error types that describe particular problems clearly.

2. Step 2: Identify the purpose of throwing them

   Throwing a custom exception signals a specific error condition, making it easier to catch and handle that error properly.

3. Final Answer:

   To create a specific error type with a clear message for better error handling. -> Option C

4. Quick Check:

   Custom exceptions improve error clarity = A [OK]

Hint: Custom exceptions clarify errors for better handling [OK]

Common Mistakes:

• Thinking exceptions fix errors automatically
• Believing exceptions speed up code
• Assuming exceptions remove need for error handling

2. Which of the following is the correct syntax to throw a custom exception named MyException?

easy

A. throw new MyException();

B. throw MyException();

C. throw exception MyException();

D. throw exception new MyException();

Solution

1. Step 1: Recall Java syntax for throwing exceptions

   In Java, to throw an exception, you use the keyword throw followed by new and the exception class constructor.

2. Step 2: Match the syntax with the options

   Only throw new MyException(); uses the correct syntax: throw new MyException();

3. Final Answer:

   throw new MyException(); -> Option A

4. Quick Check:

   Throw syntax = throw new Exception() [OK]

Hint: Always use 'throw new ExceptionName()' to throw exceptions [OK]

Common Mistakes:

• Omitting the 'new' keyword
• Adding extra keywords like 'exception'
• Using parentheses without 'new'

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

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 not allowed");
        } else {
            System.out.println("Number is " + num);
        }
    }

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

medium

A. Negative number not allowed

B. Compilation error

C. Number is -5

D. No output

Solution

1. Step 1: Analyze the check method behavior

   If the input number is less than 0, it throws a MyException with message "Negative number not allowed"; otherwise, it prints the number.

2. Step 2: Follow the main method execution

   Main calls check(-5), which triggers the exception because -5 < 0. The exception is caught and its message is printed.

3. Final Answer:

   Negative number not allowed -> Option A

4. Quick Check:

   Exception message printed = C [OK]

Hint: Exception message prints when thrown and caught [OK]

Common Mistakes:

• Expecting the number to print despite exception
• Thinking code causes compilation error
• Ignoring exception catch block

4. Identify the error in the following code snippet that throws a custom exception:

class MyException extends Exception {}

public class Demo {
    public static void test() {
        throw new MyException();
    }
}

medium

A. No error; code is correct.

B. Missing 'throws' declaration in method signature.

C. Incorrect syntax for throwing exception.

D. Cannot extend Exception class.

Solution

1. Step 1: Check method signature for checked exceptions

   Since MyException extends Exception (a checked exception), the method must declare it with throws MyException.

2. Step 2: Identify missing throws declaration

   The method test() throws MyException but does not declare it, causing a compilation error.

3. Final Answer:

   Missing 'throws' declaration in method signature. -> Option B

4. Quick Check:

   Checked exceptions require 'throws' declaration [OK]

Hint: Add 'throws ExceptionName' when throwing checked exceptions [OK]

Common Mistakes:

• Forgetting 'throws' in method signature
• Thinking all exceptions are unchecked
• Assuming extending Exception is invalid

5. You want to create a checked custom exception InvalidAgeException that should be thrown when age is less than 18. Which of the following code snippets correctly defines and throws this exception inside a method validateAge?

hard

A. class InvalidAgeException extends Exception {
public InvalidAgeException() {}
}

void validateAge(int age) {
if (age < 18) throw new InvalidAgeException();
}

B. class InvalidAgeException {
public InvalidAgeException(String msg) { super(msg); }
}

void validateAge(int age) {
if (age < 18) throw new InvalidAgeException("Age must be 18 or older");
}

C. class InvalidAgeException extends RuntimeException {
public InvalidAgeException() {}
}

void validateAge(int age) {
if (age < 18) throw new InvalidAgeException();
}

D. class InvalidAgeException extends Exception {
public InvalidAgeException(String msg) { super(msg); }
}

void validateAge(int age) throws InvalidAgeException {
if (age < 18) throw new InvalidAgeException("Age must be 18 or older");
}

Solution

1. Step 1: Define a checked exception with message constructor

   class InvalidAgeException extends Exception { public InvalidAgeException(String msg) { super(msg); } } void validateAge(int age) throws InvalidAgeException { if (age < 18) throw new InvalidAgeException("Age must be 18 or older"); } correctly extends Exception and defines a constructor that accepts a message, calling super(msg).

2. Step 2: Throw exception with message and declare it

   The method validateAge throws InvalidAgeException when age < 18 and declares it with throws InvalidAgeException.

3. Final Answer:

   class InvalidAgeException extends Exception { public InvalidAgeException(String msg) { super(msg); } } void validateAge(int age) throws InvalidAgeException { if (age < 18) throw new InvalidAgeException("Age must be 18 or older"); } -> Option D

4. Quick Check:

   Checked exception needs constructor, throw, and throws declaration [OK]

Hint: Checked exceptions need constructor, throw, and throws declaration [OK]

Common Mistakes:

• Not extending Exception for checked exceptions
• Missing throws declaration in method
• Not calling super(message) in constructor