Interfaces and Abstract Classes

CS 240 – Advanced Programming Concepts

Polymorphism

Polymorphism

• Poly = “many”
• Morph = “change”
• Polymorphism = “many forms” (we don’t say many changes)
• Objects can take on many forms in an object-oriented program
• The form of the class in which they are declared
• The form of any parent class in the class’ inheritance hierarchy
• ‘Object’ is a parent class whether declared or not (always at the top of the inheritance hierarchy)
• The form is represented by the type of the reference that refers to the object
• Reference and object types may differ (but must be compatible according to inheritance—’is a’)
• Simple definition: A reference of one type referring to an object of a different type

Simple Polymorphism Examples

• Employee emp = new Employee();   OK
• Person emp = new Employee();        OK
• Object emp = new Employee();         OK
• Dog emp = new Employee();            NOT OK

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�Result in Memory

Person emp = new Employee();

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Although they still exist in memory, ‘hireDate’ and ‘salary’ cannot be accessed from the ‘emp’ reference

Reasons for Polymorphism

• Reduce dependencies on specific implementations
• Person emp = new Employee();
• List list = new ArrayList();
• Get the same result (reference of one type referring to an object of another type) when you use either of the following:
• Heterogeneous Collections
• Collections (such as arrays or ArrayLists) of a parent type that contain children of different types
• Polymorphic parameters
• Parameters in a method call that expect a parent reference or object but receive a child of the expected type

Citations

1. Diagrams created by course authors: Ken Rodham and Jerod Wilkerson. 

Polymorphism Example Part 1

The City Simulation Example

City Simulation Example

• Create a simulation of a city, using an inheritance hierarchy of vehicles
• Will have different kinds of vehicles in the city that can all ‘go’
• Will start the simulation by placing vehicles in an array and calling a ‘go()’ method

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• What type of polymorphism is this?

Vehicle Inheritance Hierarchy

Simulation Example 1

public class CitySimulation {

public void run() {

Vehicle [] vehicles = new Vehicle[6];

vehicles[0] = new Car();

vehicles[1] = new Car();

vehicles[2] = new Truck();

vehicles[3] = new Truck();

vehicles[4] = new Boat();

vehicles[5] = new Airplane();

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for(int i = 0; i < vehicles.length; i++) {

vehicles[i].go()

}

}

}

Simulation Example Problems

• No common code to inherit for ‘go()’ method (so what should we write?)
• Even if there were common code, how would creators of subclasses know they need to override the ‘go() method?
• Solution: Abstract Method (which requires an Abstract Class)

Citations (Polymorphism Example Part 1)​

1. Diagrams created by course authors: Ken Rodham and Jerod Wilkerson. 

Polymorphism Example Part 2

Abstract Classes

Polymorphism Example Problems

• No common code to inherit for ‘go()’ method (so what should we write?)
• Even if there were common code, how would creators of subclasses know they need to override the ‘go() method?
• Solution: Abstract Method (which requires an Abstract Class)

Abstract Vehicle Class

public abstract class Vehicle {

public abstract void go();

}

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Abstract Methods

• Require containing class to be abstract
• Must be overridden in child classes unless the child is abstract
• A way to say a class has a behavior that will be defined in the subclasses
• Allows polymorphic method invocations of methods declared but not defined by the reference type

Polymorphic Method Invocation

• AKA: Polymorphic Operation

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Vehicle v = new Car();

v.go();

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• Allowable because…
• Every class that has one or more abstract methods must be abstract
• You can’t create an instance of an abstract class

So,

• Vehicle references can only refer to vehicle’s non-abstract child classes, so anything a vehicle reference can refer to, will have a non-abstract go() method

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Vehicle v = new Vehicle(); Illegal!

Abstract Classes

• Cannot be instantiated
• Can be used as reference types (polymorphism)
• Can be used as array types (polymorphism)
• May have non-abstract methods
• Don’t have to have abstract methods
• Provide a guarantee:
• If you have a non-null reference of an abstract type, it refers to an object that is not abstract and therefore has non-abstract implementations for all methods

Simulation Example with Abstract Vehicle Class

public class CitySimulation {

public void run() {

Vehicle [] vehicles = new Vehicle[6];

vehicles[0] = new Vehicle(); // Illegal if abstract

vehicles[1] = new Car();

vehicles[2] = new Truck();

vehicles[3] = new Truck();

vehicles[4] = new Boat();

vehicles[5] = new Airplane();

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for(int i = 0; i < vehicles.length; i++) {

// Guaranteed to invoke a real (non-abstract) method

vehicles[i].go()

}

}

}

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Polymorphism Example Part 3

Interfaces

Updated Requirements

• Simulation must also contain people and dogs

How Do We Start the Simulation Now?

• Would like the people and dogs to start moving at the same time as the vehicles
• Can we put ‘Person’ and ‘Dog’ objects in our Vehicle array?
• Can we change the type of the array to ‘Object’ and then put them in?
• Need polymorphism (heterogeneous collection) without inheritance

Inheritance Hierarchy with Interfaces

Interfaces

• Cannot be instantiated
• Can be used as reference types (polymorphism)
• Can be used as collection (array) types (polymorphism)
• May NOT have non-abstract methods
• All methods are abstract (with three exceptions in recent Java versions)
• All methods are public (whether you declare them as public or not)
• With one exception in recent Java versions (9 and later)
• Provide same guarantee as abstract classes:
• If you have a non-null reference of an interface type, it refers to an object that implements the interface and is not abstract and therefore has non-abstract implementations for all behaviors (methods)

Interfaces (cont.)

• Can implement any number of interfaces and still subclass some other class
• Breaks inheritance barrier of polymorphism
• Provides a way to use polymorphism where an inheritance relationship does not exist
• Examples:
• Moveable m = new Car();
• Moveable m = new Person();
• Moveable m = new Dog();
• Moveable m = new Moveable(); Illegal!
• Moveable m = new Vehicle(); Illegal!

Interfaces (cont. 2)

• Can have constant variables
• All variables are public, static, and final
• In Java 8 and later:
• Can have instance methods with bodies (must be declared as default)
• default void myDefaultMethod() {…}
• Can have static methods (with bodies)
• In Java 9 and later:
• Can have private methods (not inherited so only useful as helper methods to default methods)

Simulation Example with Interface

public class CitySimulation {

public void run() {

Moveable[] moveables = new Moveable[6];

moveables[0] = new Car();

moveables[1] = new Car();

moveables[2] = new Truck();

moveables[3] = new Truck();

moveables[4] = new Person();

moveables[5] = new Dog();

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for(int i = 0; i < moveables.length; i++) {

moveables[i].go()

}

}

}

Citations (Polymorphism Example Part 3)

1. Diagrams created by course authors: Ken Rodham and Jerod Wilkerson. 

Creating an Interface

Creating an Interface

public interface Moveable {

    void go();

}

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Extending an Interface

public interface MyInterface extends Moveable, Comparable {

    void myMethod();

    void myOtherMethod();

}

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• No single inheritance limit with interfaces
• Implementing classes must implement all method of the interface and each parent interface or be declared abstract

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Implementing an Interface

Implementing an Interface

public class Person implements Moveable {

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public void go() {

// Code to make person go

}

}

Implementing an Interface with An Abstract Class in Java

public abstract class Vehicle implements Moveable {

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}

Extending a Class and Implementing Multiple Interfaces

public class Employee extends Person implements Moveable, Comparable{

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public void go() {

// Code to make person go

}

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public int compareTo(Object obj) {

// Code to compare two employees

}

}

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