例如，这是怎么回事：
class Cat(name: String, val age: Int) {
def this() = this("Garfield", 20)
}
val someCat = new Cat
someCat.age
res0: Int = 20
不同于：
class Cat(name: String = "Garfield", val age: Int = 20)
val someCat = new Cat
someCat.age
res0: Int = 20
Note: I have seen answers to other questions(e.g here) that discuss the differences between Java & Scala in the implementation for auxiliary constructors. But I am mostly trying to understand why do we need them in Scala, in the first place.
辅助构造函数不仅可以提供默认值，还可以带来更多好处。例如，以下示例可以接受不同类型的参数：
如果主构造函数包含实现详细信息，您还可以隐藏它：
This allows you to have
dataclearly be the backing structure for your class while avoiding cluttering your class with the arguments to one of your auxiliary constructors.辅助构造函数的另一个用途是将Java代码迁移到Scala，而不会破坏依赖关系。