If a class supplies at least one constructor but does not supply a no-argument constructor, it is illegal to construct objects without supplying arguments.
如果类中至少包含一个构造器，但是又没有一个不带参数的构造器。那么在生成类实例的时候必须有参数。
例如Employee类中存在构造器Employee(String name, double salary, int y, int m, int d)，那么e = new Employee()会报错。
Please keep in mind that you get a free no-argument constructor only when your class has no other constructors. If you write your class with even a single constructor of your own and you want the users of your class to have the ability to create an instance by a call to new ClassName(), then you MUST provide a no-argument constructor.
如果不存在自定义的构造器，那么可以使用new ClassName构造一个不带参数的实例，相当于 Java 缺省条件下自带一个不带参数的构造器。但是只要包含自定义构造器，这个缺省自带的构造器就不存在，被覆盖了。