Question

How come I can have

public void someMethod() {
    throw new UnsupportedOperationException();
}

but not

public void someMethod() {
    throw new throw new IOException();
}

The first is fine, the second generates the compile error “unhandled exception type IOException”.

Answer 1

All things that can be thrown are Throwable. There are two types of Throwable:

• Exception
• Error

One subclass of Exception is RuntimeException, which is “unchecked” – meaning you don’t have to declare or catch them. These are generally for “programming errors”, like NullPointerException or ArrayOutOfBoundsException.

Errors are also “unchecked” and are for “unrecoverable” situations, such as OutOfMemoryError etc.

Any Throwable not a subclass of Error or RuntimeException is “checked” and must be declared as thrown or caught.