Java Custom Exceptions
In this page:
Why Custom Exceptions?
Custom exceptions let you represent domain-specific failures with a meaningful name, like InsufficientFundsException, instead of forcing callers to interpret a generic RuntimeException message string.
Example: Why Custom Exceptions?
class InsufficientFundsException extends RuntimeException {
InsufficientFundsException(String message) {
super(message);
}
}
public class Main {
public static void main(String[] args) {
try {
throw new InsufficientFundsException("Not enough balance");
} catch (InsufficientFundsException e) {
System.out.println(e.getMessage());
}
}
}Login to try C/C++/Java/PHP code in the editor
Creating Unchecked Custom Exceptions
Extending RuntimeException creates an unchecked custom exception, meaning callers aren't compiler-forced to catch or declare it — appropriate for programming errors that shouldn't normally be recovered from mid-flow.
Example: Creating Unchecked Custom Exceptions
class InvalidAgeException extends RuntimeException { // unchecked: no forced catch
InvalidAgeException(String message) {
super(message);
}
}
public class Main {
public static void main(String[] args) {
throw new InvalidAgeException("Age must be positive");
}
}Login to try C/C++/Java/PHP code in the editor
Creating Checked Custom Exceptions
Extending Exception directly creates a checked custom exception, forcing every caller to explicitly handle or re-declare it — appropriate for expected, recoverable failure conditions like a file genuinely not existing.
Example: Creating Checked Custom Exceptions
class InvalidFileException extends Exception { // checked: must be handled or declared
InvalidFileException(String message) {
super(message);
}
}
public class Main {
static void open() throws InvalidFileException {
throw new InvalidFileException("File not found");
}
public static void main(String[] args) {
try {
open();
} catch (InvalidFileException e) {
System.out.println(e.getMessage());
}
}
}Login to try C/C++/Java/PHP code in the editor
Adding Custom Fields and Methods
You can add extra fields to a custom exception, like an error code or the invalid value that caused the failure, giving catch blocks far more to work with than a bare message string.
Example: Adding Custom Fields and Methods
class InvalidAgeException extends RuntimeException {
int invalidValue;
InvalidAgeException(String message, int invalidValue) {
super(message);
this.invalidValue = invalidValue;
}
}
public class Main {
public static void main(String[] args) {
try {
throw new InvalidAgeException("Invalid age", -5);
} catch (InvalidAgeException e) {
System.out.println(e.getMessage() + ": " + e.invalidValue);
}
}
}Login to try C/C++/Java/PHP code in the editor
Throwing and Catching Custom Exceptions
Once defined, a custom exception is thrown with throw new MyException(...) just like a built-in one, and caught the same way with a catch block naming that specific type.
Example: Throwing and Catching Custom Exceptions
class InvalidAgeException extends RuntimeException {
InvalidAgeException(String message) {
super(message);
}
}
public class Main {
static void checkAge(int age) {
if (age < 0) {
throw new InvalidAgeException("Age cannot be negative");
}
}
public static void main(String[] args) {
try {
checkAge(-1);
} catch (InvalidAgeException e) {
System.out.println("Caught: " + e.getMessage());
}
}
}Login to try C/C++/Java/PHP code in the editor
Chapter Quiz — Complete all 8 topics to unlock
0/8 topics done
Complete these topics first: