Let's explore some different ways in which polymorphism presents itself. Consider the following example of the union design pattern:
/** * An interface that represents an operation on two doubles */ public interface IBinaryOp { double apply( double x, double y); // all interface methods are public and abstract by default }
/** * An IBinaryOp that adds two numbers */ public class AddOp implements IBinaryOp { public double apply( double x, double y) { return x+y; } }
/** * An IBinaryOp that multiplies two numbers */ public class MultOp implements IBinaryOp { public double apply( double x, double y) { return x*y; }
public String getDescription() { return "MultOp is a multiplying function."; } }
Exercise 2.1
Is the following legal code? IBinaryOp bop = new IBinaryOp();
Exercise 2.2
Is the following legal code? IBinaryOp bop = new AddOp();
Exercise 2.3
Given the above declaration and assignment of bop, is the following assignment then possible? bop = new MultOp();
Exercise 2.4
Suppose we have bop = new AddOp(); , what is the result of bop.apply(5,3) ?
Exercise 2.5
Suppose we now say bop = new MultOp(), what is the result of bop.apply(5,3) now?
Exercise 2.6
Suppose we have some variable, called myOp of type IBinaryOp what is the result of myOp.apply(5,3)?
Exercise 2.7
Suppose we have bop = new MultOp(), is it legal to call bop.getDescription() ?
Exercise 2.8
Is the following legal code? AddOp aop = new AddOp()
Exercise 2.9
Given the declaration in the previous exercise, is the following legal? Aop = new MultiOp()
Exercise 2.10
Suppose we have definitions of aop and bop from above. Is the following legal? That is, can we compile and run the following statement without error? bop = aop;
Exercise 2.11
Is the converse legal as well? That is, using the above definitions, can we compile and run the following statement? aop = bop;
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