ERR51-JG. Prefer user-defined exceptions over more general exception types

Because an exception is caught by its type, it is better to define exceptions for specific purposes than to use the general exception types for a variety of purposes. Throwing the general exception types makes code hard to understand and maintain and defeats much of the advantage of the Java exception-handling mechanism.

Noncompliant Code Example

This noncompliant code example attempts to distinguish between different exceptional behaviors by looking at the exception's message:

try {
    doSomething();
} catch(Exception e) {
  String msg = e.getMessage();
  switch (msg) {
    case "stack underflow":
      // ... 
      break;
    case "connection timeout":
      // ... 
      break;
    case "security violation":
      // ... 
      break;
    default throw e;
  }
}

If doSomething throws an exception, whose type is a subclass of Exception, with the message "stack underflow," then the appropriate action will be taken in the exception handler. However, any change to the literals involved will break the code. For example, if a maintainer should edit the throw expression to read throw Exception("Stack Underflow");, the exception will be rethrown by the code of the noncompliant code example rather than handled. Also, an exception may be thrown with no message.

Compliant Solution

It is better to user specific existing exception types or define new, special-purpose exception types:

public class StackUnderflowException extends Exception {
  StackUnderflowException () {
    super();
  }
  StackUnderflowException (String msg) {
    super(msg);
  }
}

// ...

try {
    doSomething();
} catch(StackUnderflowException sue) {
  // ...
} catch(TimeoutException te) {
  // ...
} catch(SecurityException se) {
  // ...
} catch(Exception e) {
  // ...
  throw e;
}

Applicability

Exceptions are used to handle exceptional conditions. If an exception is not caught, the program will be terminated. An exception that is incorrectly caught or is caught at the wrong level of recovery will likely cause incorrect behavior.

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