Object validation using builder pattern

Question

Let's say I have an object Foo. I also have an FooBuilder which is to be used to construct the Foo objects. When and where should I validate the Foo object's data?

Foo foo = new FooBuilder()
    .withX("specific data for X")
    .withY("specific data for Y")
    .build();

Let's add to the equation that the validation may contain lookups in for example a database. Should the builder perform a validation inside the build method? Or should there be a specific validate method in the foo object? Or maybe the validation is best off completely abstracted from both the Foo object and the FooBuilder?

Answer 1

Ideally it should not be possible to create an invalid Foo object at all. Therefore having a validation method in Foo is not the best choice.

The validation should be done as early as possible. This could be in the build() method or - if possible - even earlier within the setter methods of the builder.

If you should put the actual implementation of your validator into the builder or into a separate class depends its complexity. If it requires things like database lookups, as you mentioned, it probably makes sense to create a separate validation class.

I like to add that the builder is sometimes considered an antipattern indicating that the class is probably too complex and should be split into multiple smaller classes.