当作为构造函数参数时，这些可变或者不可变的变量会在一个对象被实例化的时候被初始化。
When used as constructor parameters, the mutable or immutable variables specified will be initialized when an object is instantiated.
要创建一个线程局部变量，请实例化类ThreadLocal的一个对象。
To create a thread-local variable, you instantiate an object of class ThreadLocal.
这意味着，如果一个变量被声明为接口类型，其值可参考，即实例化任何实现接口的类的任何对象。
This means that if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface.
这意味着，如果一个变量被声明为接口类型，其值可参考，即实例化任何实现接口的类的任何对象。
This means that if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface.
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