We call this interface virtual because there is no "real" service provider for it.
我们将调用此虚拟接口，因为没有为它提供真正的服务提供程序。
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Research has shown that most virtual invocations have only a single target for all receiver objects, and JIT compilers can generate more-efficient code for a direct call than for a virtual invocation.
A virtual method invocation, for example, involves looking at the class of the receiver object for the call to discover which actual target implements the virtual method for the receiver object.