https://stackoverflow.com/a/7486111/17273668 ; from what I have seen here to make a class "static" , we have to make it final with private constructor and static fields and methods.

Is there any difference between making the constructor private and making the class abstract?

CodePudding user response：

There is a huge difference between making a constructor private, or making a class abstract. Making a constructor private means that the constructor can only be invoked within the class itself or its nested classes, which means it cannot be called from outside. Making a class abstract means that it can only be instantiated by subclassing it. If a class is abstract, but has a non-private constructor (the default constructor is public), it means it can be subclassed by classes in other compilation units.

When it comes to utility classes, making the class final with a private constructor is - in my opinion - the better choice. The alternative is to make it abstract with a private constructor, but I think that is abusing the term abstract. Making something abstract raises an expectation of having concrete subclasses, which is not what you do with a utility class.

CodePudding user response：

An abstract class can be extended by sub classes, a private constructor (if it is the only constructor) prevents sub-classing (exception: nested classes). The only way to instantiate a class with private constructor is by implementing a static factory method in the class itself (e.g. Optional.of).