# 5.6 Polymorphism Each coding exercise has its own folder. Open its `.java` file and press **Run** to try it. **Run Tests** checks every graded exercise in this lesson at once. **Polymorphism** is a big word that you can break down into “poly” which means many and “morphism” which means form. So, it just means many forms. In Java it means that the method that gets called at **run-time** (when the code is run) depends on the *type* of the object at run-time. This is similar to a toddler toy that has pictures of animals and when a handle is pulled an arrow spins. When the arrow stops the toy plays the sound associated with that animal. ![A see n say toy](images/SeeNSay.jpg) *A see n say toy* If you were simulating this toy in software you could create an `Animal` class that had a `makeNoise` method. Each subclass of `Animal` would override the `makeNoise` method to make the correct noise for that type. This type of polymorphism is called **inheritance-based polymorphism**. You have a common parent class, but the behavior is specified in the child class. > In Java an object variable has both a **declared (compile-time) type** and an **actual (run-time) type**. The *declared (compile-time) type* of a variable is the type that is used in the declaration. The *actual (run-time) type* is the class that actually creates the object using new. The variable `nameList` declared below has a **declared type** of `List` and an **actual** or **run-time type** of `ArrayList`. The compiler will check if the declared type has the methods or inherits the methods being used in the code and give an error if it doesn’t find the method(s). The List interface does have an `add` method so this code will compile. At run-time the execution environment will first look for the `add` method in the `ArrayList` class since that is the actual or run-time type. If it doesn’t find it there it will look in the parent class and keep looking up the inheritance tree until it finds the method. It may go up all the way to the Object class. The method will be found, since otherwise the code would not have compiled. ```java List nameList = new ArrayList(); nameList.add("Hi"); ``` The variable `message` declared below has a **declared type** of `Object` and an **actual** or **run-time type** of `String`. Since the declared type of `message` is `Object` the code `message.indexOf("h");` will cause a compiler error since the `Object` class does not have an `indexOf` method. ```java Object message = new String("hi"); message.indexOf("h"); // ERROR!! Objects don't have indexOf! ``` At compile time, the compiler uses the declared type to check that the methods you are trying to use are available to an object of that type. The code won’t compile if the methods don’t exist in that class or some parent class of that class. At run-time, the actual method that is called depends on the actual type of the object. Remember that an object keeps a reference to the class that created it (an object of the class called `Class`). When a method is called at run-time the first place that is checked for that method is the class that created the object. If the method is found there it will be executed. If not, the parent of that class will be checked and so on until the method is found. In the last lesson on inheritance hierarchies, we were actually seeing polymorphic behavior at run-time in the following ways. 1. Polymorphic assignment statements such as `Shape s = new Rectangle();` 2. Polymorphic parameters such as `print(Shape)` being called with different subclass types. 3. Polymorphic array and `ArrayList` types such as `Shape[] shapeArray = { new Rectangle(), new Square() };` In all of these cases, there are no errors at compile-time because the compiler checks that the “subclass is-a superclass” relationship is true. But at run-time, the Java interpreter will use the object’s actual subclass type and call the subclass methods for any overridden methods. This is why they are polymorphic – the same code can have different results depending on the object’s actual type at run-time. You can step through this code using the Java Visualizer by clicking on the following link [Shape Example](https://cscircles.cemc.uwaterloo.ca/java_visualize/#code=public+class+Shape+%7B%0A+++public+void+what()+%7B+System.out.print(%22Shape+%22)%3B%7D%0A+++++++++%0A+++public+static+void+main(String%5B%5D+args)+%7B%0A+++++++++%0A++++++Shape%5B%5D+shapes+%3D+%7Bnew+Shape(),+new+Rectangle(),+new+Square(),+%0A++++++++++++++++++++++++++++++new+Circle()%7D%3B%0A++++++for+(Shape+s+%3A+shapes)%0A++++++%7B%0A+++++++++s.what()%3B%0A+++++++++System.out.print(%22+%22)%3B%0A++++++%7D%0A+++%7D%0A%0A%7D+%0A%0Aclass+Rectangle+extends+Shape+%7B%0A+++public+void+what()+%7B+System.out.print(%22Rectangle+%22)%3B+%7D%0A%7D%0A%0Aclass+Square+extends+Rectangle+%7B%0A%7D%0A++++++%0Aclass+Oval+extends+Shape+%7B%0A+++public+void+what()+%7B+System.out.print(%22Oval+%22)%3B+%7D%0A%7D%0A%0Aclass+Circle+extends+Oval+%7B%0A+++public+void+what()+%7B+System.out.print(%22Circle+%22)%3B%7D%0A%7D%0A%0A&mode=display&curInstr=38). You can step through this code using the Java Visualizer by clicking on the following link [Student Example](https://cscircles.cemc.uwaterloo.ca/java_visualize/#code=public+class+Student+%7B%0A+++%0A+++public+String+getFood()+%7B%0A++++++return+%22Pizza%22%3B%0A+++%7D%0A+++%0A+++public+String+getInfo()++%7B+%0A++++++return+this.getFood()%3B+%0A+++%7D%0A+++%0A+++public+static+void+main(String%5B%5D+args)%0A+++%7B%0A++++++Student+s1+%3D+new+GradStudent()%3B%0A++++++System.out.println(s1.getInfo())%3B%0A+++%7D%0A%7D%0A%0Aclass+GradStudent+extends+Student+%7B%0A+++%0A+++public+String+getFood()+%7B%0A++++++return+%22Taco%22%3B%0A+++%7D%0A++++++++%0A+++%0A%7D%0A&mode=display&curInstr=10). You can step through the code using the Java Visualizer by clicking on the following link: [Car Example](https://cscircles.cemc.uwaterloo.ca/java_visualize/#code=public+class+Car%0A%7B%0A+++private+int+fuel%3B%0A%0A+++public+Car()+%7B+fuel+%3D+0%3B+%7D+%0A+++public+Car(int+g)+%7B+fuel+%3D+g%3B+%7D%0A%0A+++public+void+addFuel()+%7B+fuel%2B%2B%3B+%7D%0A+++public+void+display()+%7B+System.out.print(fuel+%2B+%22+%22)%3B+%7D%0A+++%0A+++public+static+void+main(String%5B%5D+args)%0A+++%7B%0A++++++Car+car+%3D+new+Car(5)%3B%0A++++++Car+fastCar+%3D+new+RaceCar(5)%3B%0A++++++car.display()%3B%0A++++++car.addFuel()%3B%0A++++++car.display()%3B%0A++++++fastCar.display()%3B%0A++++++fastCar.addFuel()%3B%0A++++++fastCar.display()%3B%0A++++%7D%0A%7D%0A%0Aclass+RaceCar+extends+Car%0A%7B%0A+++public+RaceCar(int+g)+%7B+super(2*g)%3B+%7D%0A%7D+%0A&mode=display&curInstr=0). > Check your understanding: What is the output from running the main method in the Shape class? > > ```java > public class Shape > { > public void what() > { > System.out.print("Shape "); > } > > public static void main(String[] args) > { > > Shape[] shapes = > { > new Shape(), new Rectangle(), new Square(), new Circle() > }; > for (Shape s : shapes) > { > s.what(); > } > } > } > > class Rectangle extends Shape > { > public void what() > { > System.out.print("Rectangle "); > } > } > > class Square extends Rectangle {} > > class Oval extends Shape > { > public void what() > { > System.out.print("Oval "); > } > } > > class Circle extends Oval > { > public void what() > { > System.out.print("Circle "); > } > } > ``` > > Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-5-6-polymorphism.html#qoo_10 > Check your understanding: What is the output from running the `main` method in the `Student` class? > > ```java > public class Student > { > > public String getFood() > { > return "Pizza"; > } > > public String getInfo() > { > return this.getFood(); > } > > public static void main(String[] args) > { > Student s1 = new GradStudent(); > System.out.println(s1.getInfo()); > } > } > > class GradStudent extends Student > { > > public String getFood() > { > return "Taco"; > } > } > ``` > > Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-5-6-polymorphism.html#qoo_11 > Check your understanding: What is the output from running the `main` method in the `Car` class? > > ```java > public class Car > { > private int fuel; > > public Car() > { > fuel = 0; > } > > public Car(int g) > { > fuel = g; > } > > public void addFuel() > { > fuel++; > } > > public void display() > { > System.out.print(fuel + " "); > } > > public static void main(String[] args) > { > Car car = new Car(5); > Car fastCar = new RaceCar(5); > car.display(); > car.addFuel(); > car.display(); > fastCar.display(); > fastCar.addFuel(); > fastCar.display(); > } > } > > class RaceCar extends Car > { > public RaceCar(int g) > { > super(2 * g); > } > } > ``` > > Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-5-6-polymorphism.html#qoo_12 > Check your understanding: Given the following class definitions and a declaration of `Book b = new Dictionary()` which of the following will cause a compile-time error? > > ```java > public class Book > { > public String getISBN() > { > // implementation not shown > } > > // constructors, fields, and other methods not shown > } > > public class Dictionary extends Book > { > public String getDefinition() > { > // implementation not shown > } > } > ``` > > Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-5-6-polymorphism.html#qoo_13 ## Coding Challenge: Shopping Cart 2 ![Shopping](images/shoppingcart.png) In the last lesson, you created a class called `DiscountedItem` as part of a Shopping Cart application. Please copy your solutions from the [last lesson](/lessons/CSA-5.5-Inheritance-Hierarchies) into the exercise below (or in another IDE) before completing this challenge. The `ShoppingCart` contains a polymorphic `ArrayList` called `order` that you can use to add `Items` or `DiscountedItems` to the shopping cart. The `Item` class keeps track of the name and the price of each Item. The `DiscountedItem` class you wrote in the last lesson adds on a discount amount. In this challenge, you will write a method called `int countDiscountedItems()` in the `ShoppingCart` class. - This method will use a loop to traverse the `ArrayList` of `Items` called `order`. - In the loop, you will test if each `Item` is a `DiscountedItem` by using the `instanceof` keyword (`object instanceof Class` returns `true` or `false`) similar to its use in the `add(Item)` method. - If it is a `DiscountedItem`, then you will count it. - At the end of the loop, the method will return the count. - Make sure you print out the number of discounted items in the main method or in `printOrder()`, so that you can test your method. Add more items to the order to test it. ### Exercise 1 Copy in your code for `DiscountedItem` below and then write a method called `countDiscountedItems` which traverses the polymorphic `ArrayList`. Use `instanceof` to test each item to see if it is a `DiscountedItem`. > Open the `01-challenge-9-6-shopping2` folder and edit `Tester.java`. Press **Run** to try it; **Run Tests** checks it (4 tests). ## Summary - At compile time, methods in or inherited by the **declared type** determine the correctness of a non-static method call. - At run-time, the method in the **actual object type** is executed for a non-static method call. This is called **polymorphism**. - For static methods, *only* the declared type is used to determine what method to execute.