Software Design and Interfaces

1
Software Design and Interfaces

• Chapter 12 part 2
• Dr. James Jiang

Advanced Inheritance Concepts
2
The Object Class

• Every class in Java is a subclass
• Except the Object Class
• All classes are extensions of the Object class
• Use or override Object class methods

3
The toString() Method

• Converts primitive type to a string
• Using the Object class toString() method provides
  unpredictable/unuseful results

public String toString() return(entertainer
, featuring typeOfMusic music. Fee
is fee per event)
OUTPUT Bob, featuring country music. Fee is 600
per event
4
The equals() Method

• Object class equals() method requires a single
  argument
• Returns a boolean if objects are equal
• Only if one is a reference to the other
• To compare objects based on their contents, write
  your own equals() method

someObject.equals(someOtherObjectOfTheSameType
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Dog class does not override the equals() method
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Overriding the equals() method

• Boolean equals(Dog anotherDog)
• boolean result
• if(getName().equals(anotherDog.getName()))
• result true
• else
• result false
• return result

7
Inheritance and Good Software Design

• Inheritance makes programming easier!
• Use existing components as-is OR with slight
  modifications

8
Advantages of Extendable Superclasses

• Subclass creators save development time
• Subclass creators save testing time
• Superclass code is reliable
• Programmers understand superclass
• Superclass code maintains integrity when subclass
  extends the superclass

9
Multiple Inheritance

• Multiple inheritance capability to inherit from
  more than one class

Employee
Product
Who does super() refer to?
Patent
10
Multiple inheritance is prohibited in Java
11
Interfaces

• Alternative to multiple inheritance
• Interface looks like a class
• All methods must be abstract
• All data must be static and final

Any class that implements Working must contain
a work() method
public interface Working public void
work()
12
WorkingDog Class

• public class WorkingDog extends Dog implements
  Working
• public WorkingDog(String nm)
• super(nm)
• public void work()
• speak()
• System.out.println(I can herd cows)

13
Testing the WorkingDog Class
public class DemoWorkingDog public static
void main(String args) WorkingDog
mySheltie new WorkingDog(Simon)
mySheltie.work()
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Comparing Abstract classes and Interfaces

• Cant instantiate concrete objects
• Abstract classes can contain nonabstract methods
• Class can inherit from only one abstract
  superclass

• Cant instantiate concrete objects
• All methods within an interface must be abstract
• Class can implement multiple interfaces

INTERFACE
ABSTRACT CLASS
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When to use an abstract class?

• Create an abstract class when you want to provide
  data or methods that subclasses can inherit AND
  the subclasses will need to override some
  specific methods

• Example
• CardGame class
• shuffle()
• deal()
• listRules()
• keepScore()

Hearts, Solitaire, and Poker subclasses inherit
the shuffle() method and override deal(),
listRules(), and keepScore()
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When to use an interface?

• Create an interface when you know what actions
  you want to include, but you want every user to
  define behavior separately

• Example
• MusicalInstrument class
• playNote()
• outputSound()

Piano, Violin, and Drum classes must implement
playNote() and outputSound() methods differently