Polymorphism

Polymorphism
> Click or hit Control-Enter to run Example.main above

The Java type hierarchy is used when resolving the names of variables and methods:

Does the class have a variable or method with the given name? If so, use it.
If not, search the parent class—but limited by public and protected
Continue up the tree until the name is found or the search fails

> Click or hit Control-Enter to run Example.main above

Polymorphism: the provision of a single interface to entities of different types.

We’ll discuss interfaces in more detail when we talk about about packages. For now, let’s identify two kinds of Java polymorphism using examples.

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 Pet is also an Object
Each Dog is also a Pet and also an Object

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

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

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

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.).

Barbara Liskov was one of the first women in the US to earn a doctorate in computer science. She won the Turing Award, the highest honor in computer science, in 2008.

She’s given a bunch of great talks that you can find on YouTube, like this one.

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.

We can always use toString, but every class can implement it differently.

Where else have we seen this before?

> Click or hit Control-Enter to run the code above

Polymorphism: the provision of a single interface to entities of different types.

Subtype polymorphism: a single method can act on all descendants of a given class
Method overloading: a method can behave differently depending on its arguments
Generic types (discussed in lab next week)

MP3 is due today by 5PM. Good luck wrapping up! We have office hours all day downstairs.
MP4 will be out today and due in two weeks.
My office hours continue today at 11AM in the lounge outside of Siebel 0226.
The final exam will be Friday, May 11th 2018 @ 1:30PM in Foellinger Auditorium. It’s on the course calendar.
We will not have class the Friday before Spring Break: Friday March 16th. Enjoy Spring Break! (It’s also on the calendar.)

Hierarchical Name and Method Resolution

Polymorphism

Subtype Polymorphism

Object Conversion: Upcasting

But Instances Retain Their Types

Object Conversion: Downcasting

Liskov Substitution Principle

Barbara Liskov, Turing Award Winner

Substitutability in Practice

Subtype Polymorphism

Same Names, Different Behavior

Polymorphism

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