Interfaces in java.. introduction, classes, objects

Abstract Classes & Interfaces
• Definitions
– abstract methods = Methods that are declared,
with no implementation
– abstract class = A class with abstract methods,
not meant to be instantiated
– interface = A named collection of method
definitions (without implementations)
Examples
– Food is an abstract class. Can you make an
instance of food? No, of course not. But you
can make an instance of an apple or a steak or a
peanut butter cup, which are types of food.
Food is the abstract concept; it shouldn’t exist.
– Skills are interfaces. Can you make an instance
of a student, an athlete or a chef? No, but you
can make an instance of a person, and have that
person take on all these skills. Deep down, it’s
still a person, but this person can also do other
things, like study, sprint and cook.

• Q: So what’s the difference between an
interface and an abstract class?
• A:
– An interface cannot implement any methods,
whereas an abstract class can
– A class can implement many interfaces but can
have only one superclass (abstract or not)
– An interface is not part of the class hierarchy.
Unrelated classes can implement the same
interface
• Syntax:
– abstract class:
public class Apple extends Food { … }
– interface:
public class Person implements
Student, Athlete, Chef { … }

• Q: Why are they useful?
• A: By leaving certain methods undefined,
these methods can be implemented by
several different classes, each in its own way.
Example: Chess Playing Program
– an abstract class called ChessPlayer can have an
abstract method makeMove(), extended
differently by different subclasses.
public abstract class ChessPlayer {
<variable declarations>
<method declarations>
public void makeMove(); }
– an interface called ChessInterface can have a
method called makeMove(), implemented
differently by different classes.
public interface ChessInterface {
public void makeMove(); }

• Q: What about inheritance?
• A: Follow these simple rules:
– An abstract class can’t inherit from more than one
other class.
– Interfaces can inherit from other interfaces, and a
single interface can inherit from multiple other
interfaces
Example:
interface Singer {
void sing();
void warmUpVoice();
}
interface Dancer {
void dance();
void stretchLegs();
}
interface Talented extends Singer, Dancer
{
// can sing and dance. Wowwee.
}

• Q: Where else can interfaces be used?
• A: You can pass an interface as a parameter
or assign a class to an interface variable, just
like you would to an abstract class.
Example:
Food myLunch = new Sandwich();
Food mySnack = new Apple();
Student steve = new Person();
//assuming that Person implements Student
– If Person has methods eat(Food f) and
teach(Student s), the following is possible:
Person bob = new Person();
steve.teach(bob);
steve.eat(myLunch);
System.out.println(“Yum.”);

Data Factories
• Q: I’m confused. Why are we using this?
• A:
• Which implementation requires the most line
changes if you made a design decision to use
a new kind of Robot called SuperRobot for
r1-r100?
Robot r1 = new RobotSE();
…
Robot r1 =
DataFactory.makeRobot();
Robot r2 =
Robot r3 =
…
Robot r100 =
public class DataFactory {
public RobotSE makeRobot()
{ return new RobotSE(); }
}

Interfaces in java.. introduction, classes, objects

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Interfaces in java.. introduction, classes, objects