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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. Look at the tune( ) method:
It receives an Instrument handle. So how can the compiler possibly know that this Instrument handle 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 useful to examine the subject of 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 above program revolves around early binding because the compiler cannot know the correct method to call when it has only an Instrument handle.
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 a method has been declared final. This means that you ordinarily 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 importantly, it effectively “turns off” dynamic binding, or rather it tells the compiler that dynamic binding isn’t necessary. This allows the compiler to generate more efficient code for final method calls.
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:
The upcast could occur in a statement as simple as:
Here, a Circle object is created and the resulting handle 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.
When you call one of the base class methods (that have been overridden in the derived classes):
Again, you might expect that Shape’s draw( ) is called because this is, after all, a Shape handle, so how could the compiler know to do anything else? And yet the proper Circle.draw( ) is called because of late binding (polymorphism).
The following example puts it a slightly different way:
The base class Shape establishes the common interface to anything inherited from Shape – that is, all shapes can be drawn and erased. The derived classes override these definitions to provide unique behavior for each specific type of shape.
The main class Shapes contains a static method randShape( ) that produces a handle to a randomly-selected Shape object each time you call it. Note that the upcasting happens in each of the return statements, which take a handle to a Circle, Square, or Triangle and send it out of the method as the return type, Shape. So whenever you call this method you never get a chance to see what specific type it is, since you always get back a plain Shape handle.
main( ) contains an array of Shape handles filled through calls to randShape( ). At this point you know you have Shapes, but you don’t know anything more specific than that (and neither does the compiler). However, when you step through this array and call draw( ) for each one, the correct type-specific behavior magically occurs, as you can see from one output example:
Of course, since the shapes are all chosen randomly each time, your runs will have different results. The point of choosing the shapes randomly is to drive home the understanding that the compiler can have no special knowledge that allows it to make the correct calls at compile time. All the calls to draw( ) are made through dynamic binding.
Now let’s return to the musical instrument example. Because of polymorphism, you can add as many new types as you want to the system without changing the tune( ) method. In a well-designed OOP program, most or all of your methods will follow the model of tune( ) and communicate only with the base-class interface. Such a program is extensible because you can add new functionality by inheriting new data types from the common base class. The methods that manipulate the base-class interface will not need to be changed at all to accommodate the new classes.
Consider what happens if you take the instrument example and add more methods in the base class and a number of new classes. Here’s the diagram:
All these new classes work correctly with the old, unchanged tune( ) method. Even if tune( ) is in a separate file and new methods are added to the interface of Instrument, tune( ) works correctly without recompilation. Here is the implementation of the above diagram:
The new methods are what( ), which returns a String handle with a description of the class, and adjust( ), which provides some way to adjust each instrument.
In main( ), when you place something inside the Instrument3 array you automatically upcast to Instrument3.
You can see that the tune( ) method is blissfully ignorant of all the code changes that have happened around it, and yet it works correctly. This is exactly what polymorphism is supposed to provide. Your code changes don’t cause damage to parts of the program that should not be affected. Put another way, polymorphism is one of the most important techniques that allow the programmer to “separate the things that change from the things that stay the same.”
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