The Constructor with No Arguments

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.

若是類中至少包含一個構造器，可是又沒有一個不帶參數的構造器。那麼在生成類實例的時候必須有參數。

例如Employee類中存在構造器Employee(String name, double salary, int y, int m, int d)，那麼e = new Employee()會報錯。

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.

若是不存在自定義的構造器，那麼能夠使用new ClassName構造一個不帶參數的實例，至關於 Java 缺省條件下自帶一個不帶參數的構造器。可是隻要包含自定義構造器，這個缺省自帶的構造器就不存在，被覆蓋了。