Item 4: Enforce noninstantiability with a private constructor

Occasionally you’ll want to write a class that is just a grouping of static methods and static fields. Such classes have acquired a bad reputation because some people abuse them to avoid thinking in terms of objects, but they do have valid uses.

• [purpose]They can be used to group related methods on primitive values or arrays, in the manner of java.lang.Math or java.util.Arrays .
  • to group static methods
  • implement a particular interface
  • to group methods on a final class
  • e.g. an utility classes were not designed to be instantiated

• [mechanism] explicitly or implicitly: In the absence of explicit constructors, however, the compiler provides a public, parameterless default constructor.

• [method] a class can be made noninstantiable by including a private constructor
  • Because the explicit constructor is private, it is inaccessible outside of the class.
  • this is counterintuitive, It is therefore wise to include a comment.

  • [side effect] this idiom also prevents the class from being subclassed.

      // Noninstantiable utility class
      public class UtilityClass {
          // Suppress default constructor for noninstantiability
          private UtilityClass() {
              throw new AssertionError();
          }
          ... // Remainder omitted
      }