Creating Custom Exceptions in Java

Direct Answer

Create a custom exception by extending the exception type that matches the API contract—often RuntimeException for an unchecked application failure or Exception for a checked one. Give it a clear domain name and the constructors callers need, commonly message and message-plus-cause constructors that delegate to super. Add structured fields only when handlers use them.

Code Example

// Unchecked domain exception -- the common choice
public class InsufficientFundsException extends RuntimeException {
    public InsufficientFundsException(String message) {
        super(message);
    }
    public InsufficientFundsException(String message, Throwable cause) {
        super(message, cause);   // preserve the underlying cause
    }
}

// Usage -- the type name itself explains the failure
if (amount > balance) {
    throw new InsufficientFundsException(
        "withdrawal " + amount + " exceeds balance " + balance);
}

Extend Exception instead of RuntimeException if you want callers to be forced to handle it.

Common Mistakes & Pitfalls

• Omitting the (message, cause) constructor — you can't chain a wrapped exception without it.
• Extending Throwable or Error — almost always wrong; extend RuntimeException/Exception.
• Vague names — MyException says nothing; name it for the domain failure.
• Adding heavy logic — exceptions should carry information, not run business logic.