Throwing custom exceptions in Java

Introduction

Sometimes, you want to create your own error messages to explain problems clearly in your program. Custom exceptions help you do that.

When you want to show a specific error that built-in exceptions don't cover.

When you want to make your program easier to understand by naming errors clearly.

When you want to handle special cases differently in your program.

When you want to pass extra information about an error.

When you want to separate your error handling from general errors.

Syntax

Java

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

// To throw the exception:
throw new MyException("Error message here");

You create a new class that extends Exception or RuntimeException.

Use throw keyword to throw your custom exception.

Examples

This example creates an AgeException to check if age is less than 18.

Java

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

// Throwing it
if (age < 18) {
    throw new AgeException("Age must be 18 or older");
}

This example uses RuntimeException so you don't have to declare it in method signature.

Java

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

// Throwing it
if (number < 0) {
    throw new NegativeNumberException("Number cannot be negative");
}

Sample Program

This program defines a custom exception AgeException. It checks if age is less than 18 and throws the exception if so. The main method tries two ages and catches the exception to print the message.

Java

public class CustomExceptionDemo {
    static class AgeException extends Exception {
        public AgeException(String message) {
            super(message);
        }
    }

    public static void checkAge(int age) throws AgeException {
        if (age < 18) {
            throw new AgeException("Age must be 18 or older");
        } else {
            System.out.println("Age is valid: " + age);
        }
    }

    public static void main(String[] args) {
        try {
            checkAge(16);
        } catch (AgeException e) {
            System.out.println("Caught exception: " + e.getMessage());
        }

        try {
            checkAge(20);
        } catch (AgeException e) {
            System.out.println("Caught exception: " + e.getMessage());
        }
    }
}

Important Notes

Custom exceptions make your code clearer and easier to debug.

Checked exceptions (extend Exception) must be declared or caught.

Unchecked exceptions (extend RuntimeException) do not need to be declared or caught.

Summary

Custom exceptions let you create your own error types with clear messages.

Use throw new YourException() to signal an error.

Catch your custom exceptions to handle errors gracefully.

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