# 5.3 Overriding Methods 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. A subclass inherits all public methods from its superclass, and these methods remain public in the subclass. But, we also usually add more methods or instance variables to the subclass. Sometimes, we want to modify existing inherited methods. This is called **overriding** methods. **Overriding** an inherited method means providing a public method in a subclass with the same **method signature** (method name, parameter type list and return type) as a public method in the superclass. The method in the subclass will be called *instead of* the method in the superclass. One method that is frequently overridden is the `toString` method. The example below shows a similar method called `greet`. In the following example the `MeanGreeter` inherits the `greet()` method from `Greeter`, but then overrides it. Run the program to see. > To override an inherited method, the method in the child class must have the same name, parameter list, and return type (or a subclass of the return type) as the parent method. Any method that is called must be defined within its own class or its superclass. You may see the @Override annotation above a method. This is optional but it provides an extra compiler check that you have matched the method signature exactly. ```java @Override public String greet() { return "Go Away"; } ``` ## Exercise 1 Add another subclass called SpanishGreeter (or another language that you know) that extends Greeter and override the greet() method to return `Hola!` (or hi in another language) instead of `Hi!`. Create an object to test it out. > Open the `01-GreeterEx` folder and edit `Greeter.java`. Press **Run** to try it; **Run Tests** checks it (4 tests). Its tests check your code, not what it prints: “Code has been changed”, “Testing code for extends Greeter”, “Testing code for public String greet()”, “Testing code for .greet()”. ## Overloading Methods Don’t get **overriding** a method confused with **overloading** a method! **Overloading** a method is when several methods have the same name but the parameter types, order, or number are different. So with overriding, the method signatures look identical but they are in different classes, but in overloading, only the method names are identical and they have different parameters. ```java // overriding methods g2.greet(); // This could be calling an overridden greet method in g2's class g1.greet("Sam"); // This calls an overloaded greet method ``` In the example below the `greet(String who)` method overloads the `greet()` method of `Greeter`. Notice that `MeanGreeter` inherits this method and it isn’t overridden. ### Exercise 2 After running the code, try overriding the greet(String) method in the MeanGreeter class to return `Go away` + the who String. > Open the `02-GreeterOverride` folder and edit `Greeter.java`. Press **Run** to try it; **Run Tests** checks it (2 tests). > To overload a method the method must have the same name, but the parameter list must be different in some way. It can have a different number of parameters, different types of parameters, and/or a different order for the parameter types. The return type can also be different but you can’t have two methods that differ only in their return type. > Check your understanding: Which of the following declarations in `Student` would correctly *override* the `getFood` method in `Person`? > > ```java > public class Person > { > private String name = null; > > public Person(String theName) > { > name = theName; > } > > public String getFood() > { > return "Hamburger"; > } > } > > public class Student extends Person > { > private int id; > private static int nextId = 0; > > public Student(String theName) > { > super(theName); > id = nextId; > nextId++; > } > > public int getId() > { > return id; > } > > public void setId(int theId) > { > this.id = theId; > } > } > ``` > > Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-5-3-overriding.html#qoo_5 You can step through an example of this in the Java Visualizer by clicking on the following link [Override Example](https://cscircles.cemc.uwaterloo.ca/java_visualize/#code=public+class+Person+%0A%7B%0A+++private+String+name+%3D+null%3B%0A+++++++++%0A+++public+Person(String+theName)%0A+++%7B%0A++++++name+%3D+theName%3B%0A+++%7D%0A+++++++++%0A+++public+String+getFood()+%0A+++%7B%0A++++++return+%22Hamburger%22%3B%0A+++%7D%0A+++%0A+++public+static+void+main(String%5B%5D+args)%0A+++%7B%0A++++++%0A++++++Person+p+%3D+new+Student(%22Jamal%22)%3B%0A++++++System.out.println(p.getFood())%3B%0A+++%7D%0A%7D%0A++++++++%0Aclass+Student+extends+Person%0A%7B%0A+++private+int+id%3B%0A+++private+static+int+nextId+%3D+0%3B%0A+++++++++%0A+++public+Student(String+theName)%0A+++%7B%0A++++++super(theName)%3B%0A++++++id+%3D+nextId%3B%0A++++++nextId%2B%2B%3B%0A+++%7D%0A+++++++++%0A+++public+int+getId()+%7Breturn+id%3B%7D%0A+++++++++%0A+++public+void+setId+(int+theId)+%0A+++%7B%0A++++++this.id+%3D+theId%3B%0A+++%7D%0A+++%0A+++public+String+getFood()+%0A+++%7B%0A++++++return+%22Pizza%22%3B%0A+++%7D%0A%7D&mode=display&curInstr=19). > Check your understanding: Which of the following declarations in `Person` would correctly *overload* the `getFood` method in `Person`? > > ```java > public class Person > { > private String name = null; > > public Person(String theName) > { > name = theName; > } > > public String getFood() > { > return "Hamburger"; > } > } > > public class Student extends Person > { > private int id; > private static int nextId = 0; > > public Student(String theName) > { > super(theName); > id = nextId; > nextId++; > } > > public int getId() > { > return id; > } > > public void setId(int theId) > { > this.id = theId; > } > } > ``` > > Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-5-3-overriding.html#qoo_6 You can step through an example of this using the Java Visualizer by clicking on the following link [Overload Example](https://cscircles.cemc.uwaterloo.ca/java_visualize/#code=public+class+Person+%0A%7B%0A+++private+String+name+%3D+null%3B%0A+++++++++%0A+++public+Person(String+theName)%0A+++%7B%0A++++++name+%3D+theName%3B%0A+++%7D%0A+++++++++%0A+++public+String+getFood()+%0A+++%7B%0A++++++return+%22Hamburger%22%3B%0A+++%7D%0A+++%0A+++public+String+getFood(boolean+veggieOnly)%0A+++%7B%0A++++++if+(veggieOnly)%0A++++++%7B%0A+++++++++return+%22Grilled+Cheese%22%3B%0A++++++%7D%0A++++++return+getFood()%3B%0A++++++%0A+++%7D%0A+++%0A+++public+static+void+main(String%5B%5D+args)%0A+++%7B%0A++++++%0A++++++Person+p+%3D+new+Person(%22Jamal%22)%3B%0A++++++System.out.println(p.getFood(true))%3B%0A+++%7D%0A%7D%0A++++++++%0Aclass+Student+extends+Person%0A%7B%0A+++private+int+id%3B%0A+++private+static+int+nextId+%3D+0%3B%0A+++++++++%0A+++public+Student(String+theName)%0A+++%7B%0A++++++super(theName)%3B%0A++++++id+%3D+nextId%3B%0A++++++nextId%2B%2B%3B%0A+++%7D%0A+++++++++%0A+++public+int+getId()+%7Breturn+id%3B%7D%0A+++++++++%0A+++public+void+setId+(int+theId)+%0A+++%7B%0A++++++this.id+%3D+theId%3B%0A+++%7D%0A+++%0A+++public+String+getFood()+%0A+++%7B%0A++++++return+%22Pizza%22%3B%0A+++%7D%0A%7D&mode=display&curInstr=9). What happens if you change the main method in the Java Visualizer to create a new `Student` object instead of a `Person` object? Does it still print the same thing? ## Inherited Get/Set Methods Inheritance means that an object of the child class automatically includes the instance variables and methods defined in the parent class. But if the inherited instance variables are private, which they should be, the child class can not directly access the them using dot notation. The child class can use public **accessors** (also called getters or get methods) which are methods that get instance variable values and public **mutators** (also called modifier methods or setters or set methods) which set their values. For example, if a parent class has a private instance variable, `name`, then the parent will often provide a public `getName` method and a public `setName` method as shown below. ### Exercise 3 Demonstrated inherited get/set methods. > Open the `03-InheritedGetSet` folder and run `Employee.java`. It is an example, so it has no tests. > Check your understanding: Given the following class definitions which of the following would not compile if it was used in place of the missing code in the main method? > > ```java > class Item > { > private int x; > > public void setX(int theX) > { > x = theX; > } > // ... other methods not shown > } > > public class EnhancedItem extends Item > { > public int y; > > public void setY(int theY) > { > y = theY; > } > > // ... other methods not shown > > public static void main(String[] args) > { > EnhancedItem currItem = new EnhancedItem(); > // missing code > } > } > ``` > > Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-5-3-overriding.html#qoo_7 You can step through this code in the Java Visualizer by clicking on the following link [Private Fields Example](https://cscircles.cemc.uwaterloo.ca/java_visualize/#code=class+Item%0A%7B%0A+++private+int+x%3B%0A%0A+++public+void+setX(int+theX)%0A+++%7B%0A++++++x+%3D+theX%3B%0A+++%7D%0A+++//+...+other+methods+not+shown%0A%7D%0A%0Apublic+class+EnhancedItem+extends+Item%0A%7B%0A+++private+int+y%3B%0A%0A+++public+void+setY(int+theY)%0A+++%7B%0A++++++y+%3D+theY%3B%0A+++%7D%0A%0A+++//+...+other+methods+not+shown%0A+++%0A+++public+static+void+main(String%5B%5D+args)%0A+++%7B%0A++++++EnhancedItem+currItem+%3D+new+EnhancedItem()%3B%0A++++++currItem.setX(3)%3B%0A++++++//+currItem.setY(2)%3B%0A++++++//+currItem.x+%3D+3%3B%0A++++++//+currItem.y+%3D+2%3B%0A+++%7D%0A+%7D&mode=display&curInstr=10). ## Coding Challenge: Pet Sounds ![Pets](images/animalclinic.png) The following Pet class keeps track of a pet’s name and type and has a constructor, get method, and a method called speak() that prints an animal noise. 1. Write a subclass called Dog that inherits from Pet. 2. Write a Dog constructor that has one argument, the name, and calls the super constructor passing it the name and the animal type `dog`. 3. Override the method speak() in the Dog class to print out a barking sound like `Woof!`. (Do not override the get method. This superclass method should work for all subclasses). 4. Uncomment the Dog object in the main method to test it out. 5. Write a similar Cat class that inherits from Pet and has a similar constructor with type `cat` and overrides the method speak() with a `Meow!`. Test it out. ### Exercise 4 Complete the Dog and Cat classes below to inherit from Pet with a constructor and a method speak() that prints out `Woof!` or `Meow!`. > Open the `04-challenge-9-3-Pet-Sounds` folder and edit `Pet.java`. Press **Run** to try it; **Run Tests** checks it (4 tests). ## Summary - Method **overriding** occurs when a public method in a subclass has the same method signature as a public method in the superclass. - Any method that is called must be defined within its own class or its superclass. - A subclass is usually designed to have modified (overridden) or additional methods or instance variables. - A subclass will inherit all public methods from the superclass (for example all the set and get methods); these methods remain public in the subclass. - **Overloading** a method is when several methods have the same name but the parameter types, order, or number are different.