4th: The ability to harden non-instancing through a private constructor

When you need to write a class that contains only static methods and static fields, Like java.lang.math,java.util.arrays, organize the values of the basic type or the related methods on the array type, or, like java.util.Collections, organize static methods on objects that implement a particular interface.

Such a tool class does not want to be instantiated, and the instance has no meaning for it.

An attempt to force a class to be instantiated by making it an abstract class does not work. Because the class can be quilt-like, and subclasses can be instantiated.

We explicitly declare the constructor of the class as private, which is guaranteed to be inaccessible outside of the class.

 Public class Utilityclass {        private  Utilityclass () {            thrownew  Assertionerror ();//Throw an error is not required, but this can guarantee that the class cannot be instantiated inside the class        }}

Such classes are guaranteed not to be quilts, because subclasses always call the superclass constructor explicitly or implicitly in the constructor, and just say that the class's private constructor cannot be accessed outside the class.

4th: The ability to harden non-instancing through a private constructor