Answers to Self-Review Questions - Java Software Solutions: Foundations of Program Design - page 773

Chapter 9 Inheritance

9.1 Creating Subclasses
SR 9.1 A child class is derived from a parent class using inheritance. The meth-

ods and variables of the parent class automatically become a part of the
child class, subject to the rules of the visibility modifiers used to declare
them.

SR 9.2 Because a new class can be derived from an existing class, the character-

istics of the parent class can be reused without the error-prone process
of copying and modifying code.

SR 9.3 Each inheritance derivation should represent an is-a relationship: the

child is-a more specific version of the parent. If this relationship does
not hold, then inheritance is being used improperly.

SR 9.4 The protected modifier establishes a visibility level (like public and pri-

vate) that takes inheritance into account. A variable or method declared
with protected visibility can be referenced by name in the derived class,
while retaining some level of encapsulation. Protected visibility allows
access from any class in the same package.

SR 9.5 The super reference can be used to call the parent's constructor, which

cannot be invoked directly by name. It can also be used to invoke the
parent's version of an overridden method.

SR 9.6

public class SchoolBook2 extends Book2
{

private int ageLevel;

//---------------------------------------------------
// Constructor: Sets up the schoolbook with the
// specified number of pages and age level (assumed
// to be between 4 and 16 inclusive).
//---------------------------------------------------
public SchoolBook2 ( int numPages, int age)
{

super (numPages);
ageLevel = age;
}

//---------------------------------------------------
// Returns a string that describes the age level.
//---------------------------------------------------
public String level ()
{

if (ageLevel <= 6)
return "Pre-school";