Object Review
> Click or hit Control-Enter to run Example.main above
Who’s Frustrated?
Probably everyone. What do you do when you’re frustrated?
Review: Polymorphism
Polymorphism: the provision of a single interface to entities of different types.
Subtype Polymorphism
public class Pet {
public void printMe() {
System.out.println("I'm a pet");
}
}
public class Dog extends Pet {
public void printMe() {
System.out.println("I'm a dog");
}
}In Java, every object is really an instance of at least two types:
-
Each
Petis also anObject -
Each
Dogis also aPetand also anObject
Object Upcasting
public class Pet { }
public class Dog extends Pet {
public String toString() {
return "Dog";
}
}
public class Example {
public static void main(String[] unused) {
Dog chuchu = new Dog();
Pet xyz = new Pet();
Example.printAnything(chuchu);
Example.printAnything(xyz);
}
public static void printAnything(Object toPrint) {
System.out.println(toPrint.toString());
}
}Java will upcast object types automatically.
> Click or hit Control-Enter to run Example.main above
But Instances Retain Their Types
public class Pet { }
public class Dog extends Pet {
public String toString() {
return "Still a Dog";
}
}
public class Example {
public static void main(String[] unused) {
Dog chuchu = new Dog();
Object chuchuAsObject = chuchu;
System.out.println(chuchuAsObject);
Pet chuchuAsPet = chuchu;
System.out.println(chuchuAsPet);
}
}> Click or hit Control-Enter to run Example.main above
Object Downcasting
public class Pet { }
public class Dog extends Pet {
public String toString() {
return "Still a Dog";
}
}
public class Example {
public static void main(String[] unused) {
Object chuchu = new Dog();
Example.printAnything(chuchu);
Pet chuchuAsPet = (Pet) chuchu; // chuchu is a Pet, so this works
Example.printAnything(chuchuAsPet);
}
}We can also cast instances down but only if the instance is actually the appropriate subtype.
Java checks the cast at runtime to make sure that it is appropriate.
> Click or hit Control-Enter to run Example.main above
Type Testing: instanceof
public class Pet { }
public class Dog extends Pet { }
public class Cat extends Pet { }
public class Example {
public static void main(String[] unused) {
Pet chuchu = new Dog();
Pet xyz = new Cat();
System.out.println(chuchu instanceof Dog); // Prints true
System.out.println(chuchu instanceof Pet); // Prints true
System.out.println(chuchu instanceof Cat); // Prints false
}
}The Java instanceof operator allows you to test whether an object is an
instance of or a descendant of a particular class.
> Click or hit Control-Enter to run Example.main above
Liskov Substitution Principle
Substitutability is a principle in object-oriented programming stating that, in a computer program, if S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e. an object of type T may be substituted with any object of a subtype S) without altering any of the desirable properties of T (correctness, task performed, etc.).
Substitutability in Practice
public class Pet { }
public class Dog extends Pet {
public String toString() {
return "Dog";
}
}
public class Example {
public static void main(String[] unused) {
Dog chuchu = new Dog();
Pet xyz = new Pet();
Example.printAnything(chuchu);
Example.printAnything(xyz);
}
public static void printAnything(Object toPrint) {
System.out.println(toPrint.toString());
}
}Since everything is an Object, any Java object inherits all of the desirable
properties of Object: like toString.
Subtype Polymorphism
We can always use toString, but every class can implement it
differently.
Same Names, Different Behavior
Where else have we seen this before?
> Click or hit Control-Enter to run the code above
Polymorphism
Polymorphism: the provision of a single interface to entities of different types.
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Subtype polymorphism: a single method can act on all descendants of a given class
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Method overloading: a method can behave differently depending on its arguments
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Generics (discussed later)
Worked Examples
Please refer to last semester’s video for a walkthrough of how to complete one of the recent homework problems.
Questions About Objects?
Announcements
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Congrats to the Blue Team for finishing up MP1! Good luck to the Orange Team who have their deadline day today.
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MP2 will be released at 8PM tonight.
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We have a anonymous feedback form to the course website. Use it to give us feedback!