Transcript ppt - Montana State University

Inheritance and Class Hierarchies
Chapter 3
Chapter Objectives
 To understand inheritance and how it facilitates code
reuse
 To understand how Java determines which method to
execute when there are multiple methods with the same
name in a class hierarchy
 To learn how to define and use abstract classes as base
classes in a hierarchy
 To study class Object and its methods and to learn how
to override them
 To learn how to “clone” an object and to understand the
difference between a true clone (deep copy) and a
shallow copy
Chapter Objectives (continued)
 To understand why Java does not implement multiple
inheritance and to see how you can gain some of the
advantages of multiple inheritance through interfaces
and delegation
 To become familiar with a class hierarchy for drawable
shapes
 To be introduced to an object factory and to learn how
to use it
 To understand how to create packages and to learn more
about visibility
Introduction to Inheritance and
Class Hierarchies
 Popularity of OOP is that it enables programmers to
reuse previously written code saved as classes
 All Java classes are arranged in a hierarchy, starting
with Object, which is the superclass of all Java classes
 Inheritance in OOP is analogous to inheritance in humans
 Inheritance and hierarchical organization allow you to
capture the idea that one thing may be a refinement or
extension of another
Is-a Versus Has-a Relationships
 One misuse of inheritance is confusing the has-a
relationship with the is-a relationship
 The has-a relationship means that one class has the
second class as an attribute
 We can combine is-a and has-a relationships
 The keyword extends specifies that one class is a
subclass of another
A Superclass and a Subclass
 Consider two classes: Computer and Laptop
 A laptop is a kind of computer and is therefore a
subclass of computer
Initializing Data Fields in a
Subclass and the No-Parameter
Constructor
 Private data fields belonging to a base class must be
initialized by invoking the base class’s constructor with
the appropriate parameters
 If the execution of any constructor in a subclass does
not invoke a superclass constructor, Java automatically
invokes the no-parameter constructor for the superclass
 Initializes that part of the object inherited from the
superclass before the subclass starts to initialize its
part of the object
Protected Visibility for Superclass
Data Fields
 Private data fields are not accessible to derived classes
 Protected visibility allows data fields to be accessed
either by the class defining it or any subclass
 In general, it is better to use private visibility because
subclasses may be written by different programmers
and it is always good practice to restrict and control
access to the superclass data fields
Method Overriding
 If a derived class has a method found
within its base class, that method will
override the base class’s method
 The keyword super can be used to gain
access to superclass methods
overridden by the base class
 A subclass method must have the same
return type as the corresponding
superclass method
Method Overloading
 Method overloading: having multiple methods with the
same name but different signatures in a class
 Constructors are often overloaded
 Example:
 MyClass(int inputA, int inputB)
 MyClass(int inputA, int inputB, double inputC)
Polymorphism
 A variable of a superclass type can reference an object
of a subclass type
 Polymorphism means many forms or many shapes
 Polymorphism allows the JVM to determine which
method to invoke at run time
 At compile time, the Java compiler can’t determine what
type of object a superclass may reference but it is
known at run time
Abstract Classes, Assignment, and
Casting in a Hierarchy
 An interface can declare methods but does not provide
an implementation of those methods
 Methods declared in an interface are called abstract
methods
 An abstract class can have abstract methods, data
fields, and concrete methods
 Abstract class differs from a concrete class in that
 An abstract class cannot be instantiated
 An abstract class can declare abstract methods,
which must be implemented in its subclasses
Abstract Classes and Interfaces
 Like an interface, an abstract class can’t be instantiated
 An abstract class can have constructors to initialize its
data fields when a new subclass is created
 Subclass uses super(…) to call the constructor
 May implement an interface but it doesn’t have to define
all of the methods declared in the interface
 Implementation is left to its subclasses
Abstract Class Number and the
Java Wrapper Classes
Summary of Features of Actual
Classes, Abstract Classes, and
Interfaces
Class Object, Casting and Cloning
 Object is the root of the class hierarchy; every class
has Object as a superclass
 All classes inherit the methods defined in class Object
but may be overridden
The Method toString
 You should always override the toString method if you
want to represent an object’s state
 If you do not override it, the toString method for class
Object will return a string…just not the string you want
or are expecting
Operations Determined by Type of
Reference Variable
 A variable can reference an object whose type is a
subclass of the variable type
 The type of reference, not the type of the object
referenced, determines what operations can be
performed
 Java is a strongly typed language so the compiler always
verifies that the type of the expression being assigned
is compatible with the variable type
Casting in a Class Hierarchy
 Java provides casting to enable us to process one object
referenced by one type through a reference variable of
its actual type
 Casting does not change the object referenced; it
creates an anonymous reference to that object
 Downcast: cast a higher type to a lower type
 The instanceof operator can guard against
ClassCastException errors
 You can downcast an interface reference to the specific
implementation type
Java 5.0 Reduces Need for Casting
 Two new features that reduce the need for casting:
 Autoboxing/unboxing
 Generics
 Autoboxing/unboxing eases the conversion between a
primitive type and its corresponding wrapper type
The Method Object.equals
 The Object.equals method has a parameter of type
Object
 Compares two objects to determine whether they are
equal
 You must override the equals method if you want to be
able to compare two objects of a class
Cloning
 The purpose of cloning in object-oriented programming is
analogous to cloning in biology
 Create an independent copy of an object
 Initially, both objects will store the same information
 You can change one object without affecting the other
 Will cause both e1.name and e2.name to reference “Jim”
The Shallow Copy Problem
 The statement e1.setAddressLine1("Room 224"); creates
a new String object that is referenced by
e1.address.line1 and e2.address.line1
The Object.clone method
 Java provides the Object.clone method to help solve the
shallow copy problem
 The initial copy is a shallow copy as the current object’s
data fields are copied
 To make a deep copy, you must create cloned copies of
all components by invoking their respective clone
methods
The Object.clone method
(continued)
 After e1.setAddressLine1("Room 224"); only e1.address.line1
references the new String object.
Employee.clone()
Address.clone()
Multiple Inheritance, Multiple
Interfaces, and Delegation
 Multiple inheritance: the ability to extend more than one
class
 Multiple inheritance is a language feature that is
difficult to implement and can lead to ambiguity
 Therefore, Java does not allow a class to extend
more than one class
Using Multiple Interfaces to
Emulate Multiple Inheritance
 If we define two interfaces, a class can implement both
 Multiple interfaces emulate multiple inheritance
Using Multiple Interfaces to
Emulate Multiple Inheritance
(continued)
Implementing Reuse Through
Delegation
 You can reduce duplication of modifications and reduce
problems associated with version control through a
technique known as delegation
 In delegation, a method of one class accomplishes an
operation by delegating it to a method of another class
Packages
 The Java API is organized into packages
 The package to which a class belongs is declared by the
first statement in the file in which the class is defined
using the keyword package followed by the package name
 All classes in the same package are stored in the same
directory or folder
 All the classes in one folder must declare themselves to
be in the same package
 Classes that are not part of a package may access only
public members of classes in the package
The No-Package-Declared
Environment and Package Visibility
 There exists a default package
 Files that do specify a package are considered part
of the default package
 If you don’t declare packages, all of your packages
belong to the same, default package
 Package visibility sits between private and protected
 Classes, data fields, and methods with package
visibility are accessible to all other methods of
the same package but are not accessible to
methods outside of the package
 Classes, data fields, and methods that are
declared protected are visible to all members of
the package
Visibility Supports Encapsulation
 The rules for visibility control how encapsulation occurs
in a Java program
 Private visibility is for members of a class that should
not be accessible to anyone but the class, not even the
classes that extend it
 Package visibility allows the developer of a library to
shield classes and class members from classes outside
the package
 Use of protected visibility allows the package developer
to give control to other programmers who want to
extend classes in the package
Table 3.3
 Private Visibility
 Classes : Applicable to inner classes. Accessible only
to members of the class in which it is declared.
 Class Members: Visible only within this class.
 Default (or package) Visibility
 Classes : Visible to classes in this package.
 Class Members : Visible to classes in this package.
 Protected Visibility
 Classes : Applicable to inner classes. Visible to
classes in this package and to classes outside the
package that extend the class in which it is declared.
 Class Members : Visible to classes in this package and
to classes outside the package that extend this class.
 Public Visibility
 Classes: Visible to all classes.
 Class Members : Visible to all classes. The class
defining the member must also be public.
Visibility Supports Encapsulation
(continued)
A Shape Class Hierarchy
A Shape Class Hierarchy
(continued)
A Shape Class Hierarchy
(continued)
Object Factories
 An object factory is a method that creates instances of
other classes
 Object factories are useful when:
 The necessary parameters are not known or must be
derived via computation
 The appropriate implementation of an interface or
abstract class should be selected as the result of
some computation
Object Factories (continued)
Chapter Review
 Inheritance and class hierarchies to capture the idea
that one thing may be a refinement or extension of
another
 Encapsulation and inheritance impose structure on
object abstractions
 The keyword interface defines an interface
 The keyword abstract defines an abstract class or
method
 Delegation gains some of the advantages of multiple
inheritance
 Visibility is influenced by the package in which a class is
declared