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.

如果類中至少包含一個構造器，但是又沒有一個不帶參數的構造器。那么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)，那么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.

如果不存在自定義的構造器，那么可以使用new ClassName構造一個不帶參數的實例，相當于 Java 缺省條件下自帶一個不帶參數的構造器。但是只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。

轉載于:https://www.cnblogs.com/yaos/p/7089334.html

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