When to use Single method Interfaces in Java

Question

I have seen in many libraries like Spring which use a lot of interfaces with single methods in them like BeanNameAware, etc.

Answer 1

You create interfaces not according to the number of methods in it but to define behaviour expected by components of your systems to deliver a single responsibility to their neighbors. If you follow this simple principle/rule, you might or might not end up with single method interfaces, depending on the responsibility you are defining. I like to keep tests stupid simple and the application very flexible so I usually have many of those