Chapter 7: Class Inheritance

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Chapter 7 Class Inheritance

• Superclasses and Subclasses
• Keywords super and this
• Overriding methods
• The Object Class
• Modifiers protected, final and abstract
• Casting Objects
• Numeric Wrapper Classes
• Interfaces
• Inner Classes
• Class Design Guidelines

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Superclasses and Subclasses
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Creating a Subclass

• Creating a subclass extends properties and
  methods from the superclass. You can also
• Add new properties
• Add new methods
• Override the methods of the superclass

Cylinder Class
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Using the Keyword super
The keyword super refers to the superclass of the
class in which super appears. This keyword can be
used in two ways

• To call a superclass constructor
• To call a superclass method

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Example 7.1Testing Inheritance

• Objective Create a Cylinder object and explore
  the relationship between the Cylinder and Circle
  classes.

TestCylinder
Run
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Example 7.3 Overriding Methods in the Superclass

• The Cylinder class overrides the findArea()
  method defined in the Circle class.

Test Modifying Methods
Run
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The Keyword this

• The keyword this refers to the current object.
• The keyword can be used in two ways

• To call a class constructor
• To pass the current class as an argumentto a
  method

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Example of Using this

• class Circle
• private double radius
• Circle(double radius)
• this.radius radius
• Circle()
• this(1.0)
• public double findArea()
• return radiusradiusMath.PI

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The Object Class

• The Object class is the root of all Java classes.
• The toString() method returns a string
  representation of the object.
• The equals() method compares thecontents of two
  objects.

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The protected Modifier

• The protected modifier can be applied on data and
  methods in a class.
• A protected data or a protected method in a
  public class can be accessed by any class in the
  same package or its subclasses, even if the
  subclasses are in a different package.

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The final Modifier

• The final class cannot be extended
• final class Math ...
• The final variable is a constant
• final static double PI 3.14159
• The final method cannot bemodified by its
  subclasses.

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The abstract Modifier

• The abstract class
• Cannot be instantiated
• Should be extended and implemented in subclasses
• The abstract method
• Method signature withoutimplementation

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Casting Objects

• It is always possible to convert a subclass to a
  superclass.
• For this reason, explicit casting can be
  omitted. For example,
• Circle myCircle myCylinder
• is equivalent to
• Circle myCircle (Circle)myCylinder

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Casting fromSuperclass to Subclass

• Explicit casting must be used when casting an
  object from a superclass to a subclass.
• This type of casting may not always succeed.
• Cylinder myCylinder (Cylinder)myCircle

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The instanceof Operator

• Use the instanceof operator to test whether an
  object is an instance of a class
• Circle myCircle new Circle()
• if (myCircle instanceof Cylinder)
• Cylinder myCylinder (Cylinder)myCircle
• ...

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Example 7.4Casting Objects

• Objective Use implicit casting to assign circles
  and cylinders to an array and then use explicit
  casting to access data and methods in the objects
  when processing the array.

TestCasting
Run
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Class Inheritance Hierarchy
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Numeric Wrapper Classes

• Boolean
• Character
• Short
• Byte
• Integer
• Long
• Float
• Double

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The Integer Classand The Double Class

• Constructors
• Class Constants MAX_VALUE, MIN_VALUE
• Conversion Methods

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Class Design Guidelines

• Hide private data and private methods.
• Choose informative names and followconsistent
  styles.
• A class should describe a single entity or aset
  of similar operations.
• Group common data fields and operations shared by
  other classes.

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Example 7.5Designing Abstract Classes

• Objective This example gives a generic class for
  matrix arithmetic. This class implements matrix
  addition and multiplication common for all types
  of matrices.

GenericMatrix
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Example 7.6Extending Abstract Classes

• Objective This example gives two programs that
  utilize the GenericMatrix class for integer
  matrix arithmetic and rational matrix arithmetic.

TestIntegerMatrix
Run
TestRationalMatrix
Run
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Interfaces

• What Is an Interface?
• Creating an Interface
• Implementing an Interface

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Creating an Interface

• modifier interface InterfaceName
• constants declarations
• methods signatures

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Example of Creating an Interface

• public interface CompareObject
• public static final int LESS -1
• public static final int EQUAL 0
• public static final int GREATER 1
• public int compare(CompareObject otherObject)

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Example 7.7 Using Interfaces

• Objective Use the generic sorting method defined
  in the CompareObject interface to sort an array
  of circles in increasing order of their radii and
  an array of cylinders in increasing order of
  their volumes.

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Example 7.9 (cont.)
TestSortCircleCylinder
Run
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Inner Classes

• Inner class A class is a member of another
  class.
• Advantages In some applications, you can use an
  inner class to make programs simple.
• An inner class can reference the data and methods
  defined in the outer class in which it nests, so
  you do not need to pass the reference of the
  outer class to the constructor of the inner class.

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Inner Classes (cont.)

• Inner classes can make programs simple and
  concise. As you see, the new class is shorter and
  leaner.
• Many Java development tools use inner classes to
  generate adapters for handling events.
  Event-driven programming is introduced in Chapter
  8, "Getting Started with Graphics Programming.
• An inner class is only for supporting the work of
  its containing outer class, and it cannot be used
  by other classes.