Class
Unit 9, Inheritance
Software template in OOP
Software models, objects (or instances), are made from a class, as an achitectural blueprint is used to build houses.
Class, building block of OOP.
The 4 tenets of OOP are
☐ Abstraction - Pulling out big ideas, forming classes
☐ Polymorphism - Objects specializing responses to same request
☐ Inheritance - Reuse of class as basis for another class
☐ Encapsulation - Subsuming logic, data inside class or object
Object or Instance
Unit 9, Inheritance
Software model made from class. Object is instatiated, or 'newed up' based on class.
SoccerPlayer player = new SoccerPlayer();
SoccerPlayer is the class, and player represents the object.
Instance may have instance variables, instance methods. Instance variables may be protected by accessibility modifier and may only be viewed or changed by methods called getters and setters.
public class SoccerPlayer {
private String name;
...
public String getName() {
return this.name;
}
An object is a reference to an area of memory.
Inheritance
Unit 9, Inheritance
Reuse by one class, subclass, of methods and properties of another class, superclass.
Inheritance is designated with keyword extends in Java.
Class, ChocolateChipCookie inherits from class
PlainCookie. The subclass parameterized constructor calls super constructor.
public class PlainCookie {
private double qtyFlour;
// Other attributes, constructor, methods
}
public class ChocChipCookie
extends
PlainCookie {
private double qtyChocChips;
public ChocChipCookie(double qtyChocChips,
double qtyFlour,...)
{
super(qtyFlour, ...);
... }
Class Hierarchy
Unit 9, Inheritance
Set of classes related via Inheritance.
PlainCookie🍪
↧
ChocChipCookie 🍪
↧
NutChocChipCookie🍪
PlainCookie is parent class of ChocChipCookie, which in turn is parent class of
NutChocChipCookie.
IS-A relationship between subclasses and superclass. NutChocChipCookie IS-A ChocChipCookie, which IS-A PlainCookie.
Superclass, Subclass
Unit 9, Inheritance
Superclass or parent class reused by subclass or child class, via Inheritance.
PlainCookie🍪
↧
ChocChipCookie 🍪
↧
NutChocChipCookie🍪
PlainCookie is the parent class of ChocChipCookie, the parent class of
NutChocChipCookie.
IS-A relationship exists between subclasses and superclass. NutChocChipCookie IS-A ChocChipCookie, which IS-A PlainCookie.
Override
Unit 9, Inheritance
Subclass can redefine method from parent class. When method called from instance of subclass, method defined in subclass is called, thus overriding method in superclass.
public class PlainCookie {
....
public void displayBakeTemp() {
System.out.println("385 Fahrenheit");
}
public class ChocChipCookie {
....
public void displayBakeTemp() {
System.out.println("350 Fahrenheit");
}
PlainCookie cookie = new Cookie();
PlainCookie chocCookie = new ChocChipCookie();
chocCookie.displayBakeTemp(); //350 Fahrenheit displayed!
Polymorphism
Unit 9, Inheritance
When objects (related via inheritance) respond in different ways to same request. Overriding methods is how Java does Polymorphism!
public class PlainCookie {
public void displayBakeTemp() {
System.out.println("385 Fahrenheit");
}
public class ChocChipCookie extends PlainCookie{
public void displayBakeTemp() {
System.out.println("350 Fahrenheit");
}
PlainCookie cookie = new Cookie();
PlainCookie chocCookie = new ChocChipCookie();
chocCookie.displayBakeTemp(); // 350 Fahrenheit is displayed!
cookie.displayBakeTemp(); // 385 Fahrenheit is displayed!
Upcasting
Unit 9, Inheritance
Casting to superclass, further UP in class hierarchy. Also known as generalization or widening.
public class PlainCookie {....}
public class ChocChipCookie extends PlainCookie{ ....}
PlainCookie chocCookie = new ChocChipCookie();
Implicit upcast done here, to superclass.
int quantityChips = ((ChocChipCookie)chocCookie).getQtyChocChips();
Upcasting can result in hiding specialized class' attributes/methods - can be resolved by casting to specialized class.
PlainCookie chocCookie = new ChocChipCookie();
Downcasting
Unit 9, Inheritance
Specialization or narrowing, casts instance of superclass to instance of subclass.
In Java, downcasting can cause Class Cast Exception - exception thrown when real object does not match class object being cast to.
public class PlainCookie {....}
public class ChocChipCookie extends PlainCookie{.... }
This causes incompatible types (classes) error,
ChocChipCookie cookie = new Cookie();
Explicit cast removes incompatible types issue, but causes Class Cast Exception at runtime.
ChocChipCookie cookie = (ChocChipCookie)new Cookie();
Downcasting is not allowed in Java, upcasting is!
Class Cast Exception
Unit 9, Inheritance
Exception thrown at runtime when instance of superclass downcast to subclass.
public class PlainCookie {....}
public class ChocChipCookie extends PlainCookie{.... }
This causes an incompatible types (classes) error,
ChocChipCookie cookie = new Cookie();
Explicit cast removes incompatible types issue, but causes Class Cast Exception runtime.
ChocChipCookie cookie = (ChocChipCookie)new Cookie();
Implicit Call To Super
Unit 9, Inheritance
When classes are related in a hierarchy, superclass constructor called, even without specific code.
public class PlainCookie {....
public PlainCookie() {}
}
public class ChocChipCookie extends PlainCookie{....
public ChocChipCookie() {}
}
public class NutChocChipCookie extends ChocChipCookie{.... public NutChocChipCookie() {}
}
NutChocChipCookie cookie = new NutChocChipCookie();
PlainCookie constructor called,
followed by ChocChipCookie constructor, and then NutChocChipCookie constructor.
Explicit Call To Super
Unit 9, Inheritance
When classes are related in hierarchy, may be necessary to call superclass, so values assigned to attributes. This is explicitely calling constructor of superclass.
public class PlainCookie {....
public PlainCookie(int flour, ....) {... }
}
public class ChocChipCookie extends PlainCookie{....
public ChocChipCookie(int chocChips, int flour, ...) {
super(qtyFlour, ...);...
}
}
