It calls the primary constructor with Nil (which represents an empty List object).
它调用主构造函数，并且传入Nil (Nil表示了一个空的List对象)。
Scala distinguishes between a primary constructor and zero or more auxiliary constructors.
Scala可以区分主构造函数和0个或多个辅助构造函数。
The primary constructor takes a label argument and a list of callbacks that are invoked when the button's click method is invoked.
主构造函数接受一个label(标签)参数和一个callbacks(回调)的list(列表)，这些回调函数会在按钮的click方法被调用时被调用。
The constructor invoked may be either another auxiliary constructor or the primary constructor, and it must be the first statement in the auxiliary constructor's body.
被调用的构造函数可以是另外一个辅助构造函数或者主构造函数，而且它必须出现在辅助构造函数主体的第一句。
Any such additional constructors must call the primary constructor.
任何此类附加构造函数都必须调用主构造函数。
One is the primary constructor, whose parameters appear in parentheses just after the type name.
一种是主构造函数，其参数出现在紧靠类型名称后面的括号中。
Because all auxiliary constructors eventually invoke the primary constructor, logic checks and other initializations done in the body will be performed consistently for all instances created.
因为所有的辅助构造函数最终都会调用主构造函数，它主体中进行的逻辑检查和其它初始化工作会在所有实例被创建的时候执行。
Because all auxiliary constructors eventually invoke the primary constructor, logic checks and other initializations done in the body will be performed consistently for all instances created.
因为所有的辅助构造函数最终都会调用主构造函数，它主体中进行的逻辑检查和其它初始化工作会在所有实例被创建的时候执行。
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