Java之建立物件>4.Enforce noninstantiability with a private constructor

如果你定義的類僅僅是包含了一些靜態的方法和靜態的欄位，這些類一般是一些工具類，這些一般是設計為不能被例項化的。

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
}