- 上一篇：电子商务英文文献和中文翻译
- 下一篇：搅拌釜内混合液体的分离涡模拟英文文献和中文翻译
That’s exactly what polymorphism allows you to do. However, most programmers who come from a procedural programming background have a bit of trouble with the way polymorphism works.
The difficulty with Music.java can be seen by running the program. The output is Wind.play(). This is clearly the desired output, but it doesn’t seem to make sense that it would work that way.
It receives an Instrument reference. So how can the compiler possibly know that this Instrument reference points to a Wind in this case and not a Brass or Stringed? The compiler can’t. To get a deeper understanding of the issue, it’s helpful to examine the subject of binding.
Method-call binding, connecting a method call to a method body is called binding. When binding is performed before the program is run (by the compiler and linker, if there is one), it’s called early binding. You might not have heard the term before because it has never been an option with procedural languages. C compilers have only one kind of method call, and that’s early binding.
The confusing part of the preceding program revolves around early binding, because the compiler cannot know the correct method to call when it has only an Instrument reference.
The solution is called late binding, which means that the binding occurs at run time, based on the type of object. Late binding is also called dynamic binding or run-time binding. When a language implements late binding, there must be some mechanism to determine the type of the object at run time and to call the appropriate method. That is, the compiler still doesn’t know the object type, but the method-call mechanism finds out and calls the correct method body. The late-binding mechanism varies from language to language, but you can imagine that some sort of type information must be installed in the objects.
All method binding in Java uses late binding unless the method is static or final (private methods are implicitly final). This means that ordinarily you don’t need to make any decisions about whether late binding will occur—it happens automatically.
Why would you declare a method final? As noted in the last chapter, it prevents anyone from overriding that method. Perhaps more important, it effectively “turns off” dynamic binding, or rather it tells the compiler that dynamic binding isn’t necessary. This allows the compiler to generate slightly more efficient code for final method calls. However, in most cases it won’t make any overall performance difference in your program, so it’s best to only use final as a design decision, and not as an attempt to improve performance.
Producing the right behavior, once you know that all method binding in Java happens polymorphically via late binding, you can write your code to talk to the base class and know that all the derived-class cases will work correctly using the same code. Or to put it another way, you “send a message to an object and let the object figure out the right thing to do.”
The classic example in OOP is the “shape” example. This is commonly used because it is easy to visualize, but unfortunately it can confuse novice programmers into thinking that OOP is just for graphics programming, which is of course not the case.
The shape example has a base class called Shape and various derived types: Circle, Square, Triangle, etc. The reason the example works so well is that it’s easy to say “a circle is a type of shape” and be understood. The inheritance diagram shows the relationships:
Fig. 1 Shape
The upcast could occur in a statement as simple as: Shape s = new Circle(), here, a Circle object is created, and the resulting reference is immediately assigned to a Shape, which would seem to be an error (assigning one type to another); and yet it’s fine because a Circle is a Shape by inheritance. So the compiler agrees with the statement and doesn’t issue an error message.