Support for lazy evaluation of arguments in Preconditions

A seemingly innocent Precondition that looks like this…

// Make sure 'user' is null
Preconditions.checkState(user == null, "User is not null. User id: %s", user.getId());

…actually throws an NPE when the precondition is met (user == null) since user.getId() is eagerly evaluated. The obvious workaround is to provide an argument like…

Preconditions.checkState(user == null, "User is not null. User id: %s", new Object() {
    @Override
    public String toString() {
        return user.getId();
    }
});

…but it feels clunky at best.

It would be very slick if Preconditions would support lazy evaluation of parameters, for example by special treatment of Supplier<?> arguments, so one could write something like

Preconditions.checkState(user == null, "User is not null. User id: %s", (Supplier<?>) () -> user.getId());

The toString implementation of Supplier is pretty meaningless so I don’t this would infringe on other use-cases or cause any back compat issues.

Another alternative could be to use a dedicated interface, such as LazyArgument and have a factory method in Preconditions to allow for something like

Preconditions.checkState(user == null, "User is not null. User id: %s", Preconditions.lazyArg(() -> user.getId()));

[kevinb9n]

(We should promote flogger more.)

I try to never say never, but the lambda approach is probably unlikely to fly.

They might not be what you want, but I’ll leave this open while we think about failState & failArgument a sec.

[kevinb9n]

Sorry I missed that.

We think that

checkState(user == null, "User is not null. User id: %s", lazy(() -> user.getId()));

is not sufficiently better code than

if (user != null) {
  throw new IllegalStateException("User is not null. User id: " + user.getId());
}

And in fact, anyone can read the latter code and know exactly what it does, which isn’t the case at top.

Guava has never wanted to be a different way to do things. We have always strived to add only features that provide better ways than the universal idioms. Preconditions provides value just by being usable most of the time; it doesn’t need to be “all of the time”.