Utility classes are classes that only include static members and are not designed to be instantiated, for example java.lang.Math or java.util.Arrays.

In the absence of explicit constructors, however, the compiler provides a public, parameterless default constructor. To a user, this constructor is indistinguishable from any other. It is not uncommon for a published API to accidentally include a public constructor for a class intended to be uninstantiable.

To prevent users from instantiating classes that are not designed to be instantiated, you can add a private constructor:

public class UtilityClass {
  private UtilityClass() {}
}

See:

• Effective Java 3rd Edition §4