We can assign a subclass instance to a supertype but

10. Example – Suppose the Dog class provides the method shown
on the right where a Dog can barkAt another Dog . Define some
dogs:
public String barkAt(Dog d ) {
return this .getName() +
.
36

37

11. Example – Consider the classes defined on the left and the static methods on the right.
class A {
public String toString() {
return "A";
}
}
class B extends A {
public String toString() {
return "B";
}
}
class C extends B {}
public static void m( A a ) {
System.out.println( a );
}
public static void m2( C c ) {
System.out.println( c );
}
State whether the method calls below will: (a) compile, (b) generate a run-time error, or (c) what output will it
produce?
a. m( new A() );
b. m( new B() );
c. m( new C() );
d. m2( new C() );
e. m2( new B() );
38

f.
g. Homework
h.
26. Define these terms in your own words:
i.
a. Reference type
b. Supertype reference (or polymorphic reference)
c. Polymorphism
j.
27. True or False
k.
a. You can always refer to a supertype instance with a subtype reference.
b. You can always refer to a subtype instance with a supertype reference.
c. You can always pass an instance of a subclass to a method with a parameter defined as a supertype.
d. You can sometimes pass an instance of a superclass to a parameter defined as a subclass.
l.
m.
n.

o. Section 11.8 – Dynamic Binding
p.
1. Consider the example below. First, we create a WolfDog instance but give it a (supertype) Dog reference. When
the bark method is called, the actual bark method that runs is determined by the instance of the object not the
reference type . Thus, the WolfDog ’s bark method is called (red arrow). Next, the run method is called. Since the
WolfDog class does not define a run method, the JVM finds a run method in the Dog superclass (blue arrow).
Finally, the Dog reference, sam is assigned to an instance of a Dog and so the Dog class’s bark method is called
(green arrow). Thus, dynamic binding means that the the actual method that runs is determined at run-time by
the instance of the object, not the reference type.
q.
r.
s.
2. The JVM goes through a process like this to implement dynamic binding
t.
u. Get the type of the instance.
v. If ( instance contains a definition for the behavior )
w. Execute behavior
x. Else
y. Find behavior in superclass & execute
z.
aa. Thus, the method is bound to the call at run-time ( dynamically ).
ab.
3. Non-software example – In the figure on the right, Saxophone ,
Trumpet , and Trombone are subclasses of Horn and each one
overrides the play method in a way that is appropriate for the
specific type of instrument (instance). Thus, each plays notes