Java: What is the preferred Throwable to use in a private utility class constructor?

Question

Effective Java (Second Edition), Item 4, discusses using private constructors to enforce noninstantiability. Here's the code sample from the book:

public final class UtilityClass {
    private UtilityClass() {
        throw new AssertionError();
    }
}

However, AssertionError doesn't seem like the right thing to throw. Nothing is being "asserted", which is how the API defines the use of AssertionError.

Is there a different Throwable that's typically in this situation? Does one usually just throw a general Exception with a message? Or is it common to write a custom Exception for this?

It's pretty trivial, but more than anything I guess I'm just curious about it from a style and standards perspective.

Answer 1

There is an assertion: "I'm asserting that this constructor will never be called". So, indeed, AssertionError is correct here.

Answer 2

A broken assertion means that you've broken a contract specification of your code. So it's the right thing here.

However, as I assume you'll be privately instantiating an instance, it will also call the constructor and cause an error- unless you have another constructor?

Answer 3

What about IllegalAcessError ? :)

Answer 4

No no no, with all due respect to Josh Bloch, never throw an AssertionError unless it's from an assertion. If you want an AssertionError here, throw it with assert(false). Then someone reading the code can find it later.

Even better, define your own exception, say CantInstantiateUtilityClass. then you'll have code that says

try {
    // some stuff
} catch (CantInstantiateUtilityClass e) {
    // react
}

so that the reader of the catcher knows what happened.

Answer 5

UnsupportedOperationException sounds like the best fit, though a checked exception would be even better, since it might warn someone erroneously instantiating the class at compile time.

Answer 6

I like including Bloch's comment:

// Suppress default constructor for noninstantiability

Or better yet putting it in the Error:

private UtilityClass()
{
    throw new AssertionError("Suppress default constructor for noninstantiability");
}