Will catching an exception catch a parent class of that exception

Question

In Java I have a method catching an exception 'ChildException' that extends 'NewException'. If that method calls another method that throws a 'NewException' and let's say something happens and throws it; will the caller method that catches a child class of the exception thrown catch it?

public MethodCatchingChildException
{
  try
  {
     //stuff
     callingMethodThrowingNewException();
     //stuff
  }
  catch (ChildException e)
  {
     //stuff
  }
}

So will the exception from callingMethodThrowingNewException get caught in MethodCatchingChildException?

Answer 1

A catch clause will catch any exception that is assignment-compatible with the declared type of the exception. In the case you describe, an instance of NewException (that is not a ChildException will not be caught by that catch clause because you cannot assign a NewException object to a ChildException variable.

The rules are spelled out in section 14.20.1 of the Java Language Specification :

If execution of the try block completes abruptly because of a throw of a value V, then there is a choice:

• If the run-time type of V is assignment compatible with ( §5.2 ) a catchable exception class of any catch clause of the try statement, then the first (leftmost) such catch clause is selected. The value V is assigned to the parameter of the selected catch clause, and the Block of that catch clause is executed, and then there is a choice:

  • If that block completes normally, then the try statement completes normally.

  • If that block completes abruptly for any reason, then the try statement completes abruptly for the same reason.

• If the run-time type of V is not assignment compatible with a catchable exception class of any catch clause of the try statement, then the try statement completes abruptly because of a throw of the value V.

Answer 2

Think of it as an instanceof test

eg

if (e instanceof ChildException) {
 ...
}

so specifying a class type will catch the class and its subclasses.

Answer 3

It will only catch the Exception you specify or it's subclass. Just write it so it catches the parent Exception and you're safe.