10.1-10.3 Oct. 2

(10.3) How can polymorphism be accomplished using interfaces?

An interface name can be used as the type of a reference. Such a reference variable can refer to any object of any class that implements that interface. Because all classes implement the same interface, they have methods with common signatures, which can be dynamically bound.

why is compile time binding considered more efficient than dynamic binding?

Compile time binding is considered more efficient than dynamic binding. Compile time binding occurs before the program is executed and therefore does not delay the execution progress of the program. Dynamic binding occurs while the program is running and therefore does affect the runtime efficiency of the program.

how does inheritance support polymorphism?

In Java, a reference variable declared using a parent class can be used to refer to an object of the child class. If both classes contain a method with the same signature, the parent reference can be polymorphic.

Which pay method is invoked by the following line from the payday method of the Staff class ?

It depends. The pay method invocation is polymorphic. The actual method that is invoked is determined at run time and is based on the class of the object referenced by the current (according to the value of count) element of the staff list.

Suppose the class MusicPlayer is the parent of the class CDPlayer. Is the following sequence of statements allowed in Java? Explain. MusicPlayer mplayer = new MusicPlayer(); CDPlayer cdplayer = new CDPlayer(); cdplayer = mplayer;

No, the third statement is not valid. A MusicPlayer is not necessarily a CDPlayer. It is not legal to perform cdplayer = mplayer without using an explicit cast operation. Consider that the mplayer variable could potentially represent many different kinds of music players: CD players, record players, mp3 players, etc. Suppose at the time of the assignment statement, it represents an mp3 player. Then, you would be assigning an mp3 player object to a CDPlayer variable. That, most likely, doesn't make sense and would cause problems if it was allowed to happen.

What is a polymorphism?

Polymorphism is the ability of a reference variable to refer to objects of various types at different times. A method invoked through such a reference is bound to different method definitions at different times, depending on the type of the object referenced.

Why is the StaffMember class in the Firm example declared as abstract?

The StaffMember class is abstract because it is not intended to be instantiated. It serves as a placeholder in the inheritance hierarchy to help organize and manage the objects polymorphically.

Why is the pay method declared in the StaffMember class, given that it is abstract and has no body at that level?

The pay method has no meaning at the StaffMember level, so is declared as abstract. But by declaring it there we guarantee that every object of its children will have a pay method. This allows us to create an array of StaffMember objects, which is actually filled with various types of staff members, and pay each one. The details of being paid are determined by each class as appropriate.

SR 10.6 How is overriding related to polymorphism?

When a child class overrides the definition of a parent's method, two versions of that method exist. If a polymorphic reference is used to invoke the method, the version of the method that is invoked is determined by the type of the object being referred to, not by the type of the reference variable.

suppose MuiscPlayer is the parent of the class CDPlayer. Is the following sequence of statements allowed in Java? Explain. MusicPlayer mplayer = new MusicPlayer(); CDPlayer cdplayer = new CDPlayer(); mplayer = cdplayer;

Yes, the statements are valid. Since a CDPlayer is a MusicPlayer, it is legal to assign an object of class CDPlayer to a variable of class MusicPlayer.

(10.3) Suppose that the Speaker interface and the Philosopher and Dog classes are as described in this section. Are the following sequences of statements allowed in Java? Explain. [A] Speaker current = new Speaker(); [B] Speaker current = new Dog(); [C] Speaker first, second; first = new Dog(); second = new Philosopher(); first.speak(); first = second; [D] Speaker first = new Dog(); Philospher second = new Philosopher(); second.pontificate(); first = second; [E] Speaker first = new Dog(); Philospher second = new Philosopher(); first = second; second.pontificate(); first.pontificate();

[A] Invalid—Speaker is an interface and interfaces do not have constructors. [B] Valid—the Dog class implements Speaker. [C] Valid—note that all the classes involved implement Speaker. [D] Valid—Philosopher implements Speaker, so it is legal to assign a philosopher object to a speaker variable. [E] Invalid—first is declared to be a Speaker, so we cannot invoke the Philosopher method pontificate through first.