L4-Abstract Classes,Interfaces and OOD principles

Intended Learning Outcomes

• To understand abstract type.
• To enforce a design protocol by using abstract classes or interfaces.
• To know the similarities and differences between an abstract class and an
  interface.
• To become familiar with the process of program development.
• To discover classes using CRC cards.
• To understand the impacts of coupling to a system.
• To learn the relationship types: association, aggregation, composition,
  realization and generalization.(UML class diagram)
• To understand design principles and guidelines

Interface Syntax

• Similar to a class but contains only constants and abstract methods.
• Using abstract modifier.

public interface InterfaceName{
	constant declarations;
	method signatures;
	}
public interface Stack{
	public static final int MAX_SIZE=100;
	public abstract int pop();
	public abstract void push(int e);
	}

• Interface class is a special class:
  • interfaces contain abstract methods which don't define method bodies, you can't create instances from an interface using the new as usual, but you can use an interface to declare a variable.

Stack s1;//no instance,only reference var
Stack[] s2=new Stack[10];

• Properties of Interface(these modifiers can be omitted)

  • All Data fields: constant--public static final
  • All methods:abstract--public abstract

• Using Interface(relying another class to implement the abstract class)

public interface Colorable{
	abstract public void color();
	}
class Apple implements Colorable{
	@override
	public void color(){
	System.out.println("red");
	}
}
class Banana implements Colorable{
	@override
	public void color(){
	System.out.println("Yellow");
	}
}

The abstract Modifier

• Abstract also can be used in class
  • Having both ordinary class and interface.
  • Having both abstract and non-abstract method
  • Being extended and the subclass override the abstract method from parent class with a concrete method.
  • Can't be instantiated.(can't use new operator)(can usr reference variable)

public abstract class shape{
	protected char drawingchar='*';
	public abstract void draw();//abstract method
	public char getDrawingChar(){//non-abstract method
		return drawingchar;
		}
	}

• Abstract Class(define)

  • an abstract class can contains no abstract methods
  • But a class that includes abstract methods must be an abstract class(but not all abstract)

• Why Abstract Type?

  Abstract types are useful that they can be used to define and enforce a
  protocol ; a set of operations which all objects that implement the protocol

  must support.

Interfaces VS. Abstract Classes

Abstract Class	Interfaces
no restrictions of variables	All variables must be public static final
no restrictions of methods	All methods must be public abstract
Constructors are invoked by subclasses through constructor chaining. An abstract class cannot be instantiated using the new operator.	No constructors. An interface cannot be instantiated using the new operator.
single Inheritance,extend one class only	multiple inheritance more than one interface

Object-oriented-Design Principles

• software development process
  

  • Requirement Specification:understand the problem
  • System analysis: analyze the business process, capture the essential elements
  • System design: decompose the problem, establish relationships
  • Implementation: coding,testing,debugging
  • Testing:ensure the code meet the requirement
  • Deployment: available for use
  • Maintainance: provide upgrades of the product to fix newly discovered bugs

• CRC code

• 
  

• Coupling

  • Association, aggregation, composition, realization and generalization all describe the coupling between two classes.
  • The difference is the degree of coupling.
  • In general, lower degree of coupling implies higher stability of the system.
  • Low coupling is often a desirable system property.

• Association

  • Object of one class is connecte to object of another and there is a channel between them through which message can be sent.
  • send message each other
  • one change another change too
  • the connection relationship is usually implemented using data fields (Strong connection between 2 classes)
    

• Composition and Aggregation

  • they represent a whole-part relationship (between two classes.)
  • whole contains part, but part can't contain whole
  • Composition stronger than Aggregation(because:adds lifetime responsibility--the part must be created and destroyed with whole together.)
  • Usually represented as a data field in the 'owner' class.
    

• Inner class

  • If Department is used in the University class only,it is usually declared as an inner class of University.(表示这个子类只属于它，其他类中没有该子类）
  • There are three types of inner class:

1. Member inner class - declared within another class
2. Local inner class - declared within the body of a method
3. Anonymous inner class - declared within the body of a method without name
   example ： member inner class.

public class University { 
 private Department[] depts;
 ...
 private class Department {
 ...
 }
}

• Generalization (Extend relationship)

  • models the inheritance relationship(between 2 classes)
  1. Generalized class(superclass)
  2. Specialized class(subclass)
     

• Realization

  • Represents the is-a-kind-of relationship(Implementation of interface)
    

Class design in 4 steps

1. Identify classes for the system.
   Ordinary classes, abstract classes, interfaces
2. Describe attributes and methods in each class.
   Using Modifiers public, protected, private and static
3. Establish relationships among classes.
   Association, generalization,realization, ... etc.
4. Create classes.