Nested class is a class that is defined within another class. This post explains different types of nested classes.

Interfaces and abstract classes are non-instantiable i.e. we can't directly create instances out of them. Regular classes are instantiable by default, so how to enforce non-instantiability?

Overriding common methods (equals, hashCode, toString, and clone) of the Object class and Comparable.compareTo() method, should be done according to the general contracts provided in the documentation to ensure proper behaviour of these methods.

Serialization is the process of converting an object into a stream of bytes which can be deserialized back to a copy of the object.

Some objects are required to be singleton such as "builder, facade, state, logging, ...etc". Singleton pattern can be used to enforce the creation of exactly one instance of those objects.

Classes that have many optional parameters can use builder pattern to create instances of these classes.

Service provider interface (SPI) enables developers to extend applications functionalities by adding services implementation, e.g. Jar files, to classpath without modifying the application code base.