如果你定义的类仅仅是包含了一些静态的方法和静态的字段，这些类一般是一些工具类，这些一般是设计为不能被实例化的。

1. Attempting to enforce noninstantiability by making a class abstract does not work.

2. a class can be made noninstantiable by including a private constructor

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