参考：有效的Java 2nd Edition，第4项“使用私有构造函数强制执行非示例性”
public final class MyClass { //final not required but clearly states intention //private default constructor ==> can't be instantiated //side effect: class is final because it can't be subclassed: //super() can't be called from subclasses private MyClass() { throw new AssertionError() } //... public static void doSomething() {}}
