20.4 Java interfaces and abstraction

Interfaces vs Abstract Classes
Abstraction vs Encapsulation
Interfaces and Abstraction
Software University
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SoftUni Team
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1. Abstraction
 Abstraction vs Encapsulation
2. Interfaces
 Default Methods
 Static Methods
3. Abstract Classes
4. Interfaces vs Abstract Classes
Table of Contents
2

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Working with Abstraction
Abstraction

 Latin origin
 Preserving information that is relevant in a context
 Forgetting information that is irrelevant in that
context
What is Abstraction?
5
Abs
(away from)
Trahere
(to draw)
Abstraction

 Abstraction means ignoring irrelevant features, properties, or
functions and emphasizing the relevant ones …
 ... relevant to the context of the project we develop
 Abstraction helps managing complexity
 Abstraction lets you focus on what the object does instead of
how it does it
Abstraction in OOP
6
"Relevant" to
what?

 There are 2 ways to achieve abstraction in Java
 Interfaces (100% abstraction)
 Abstract class (0% - 100% abstraction)
Achieving Abstraction
7
public interface Animal {}
public abstract class Mammal {}
public class Person extends Mammal implements Animal {}

Abstraction vs. Encapsulation
 Abstraction
 Process of hiding the
implementation details
and showing only
functionality to the user
 Achieved with interfaces
and abstract classes
 Encapsulation
 Used to hide the code
and data inside a single
unit to protect the data
from the outside world
 Achieved with access
modifiers (private,
protected, public)
8

Interfaces
Working with Interfaces

 Internal addition by compiler
Interface
10
public interface Printable {
int MIN = 5;
void print();
}
interface Printable {
public static final int MIN = 5;
public abstract void print();
}
compiler
"public abstract"
before methods
"public static final"
before fields
Public or default
modifier
Keyword
Name

 Relationship between classes and interfaces
 Multiple inheritance
implements vs extends
11
Interface
implementsextends extends
Interface
Interface
ClassClass
Class
Class
InterfaceInterface Interface Interface
Interface
extendsimplements

 Implementation of print() is provided in class Document
Interface Example
12
public interface Printable {
void print();
}
class Document implements Printable {
public void print() { System.out.println("Hello"); }
public static void main(String args[]) {
Printable doc = new Document();
doc.print(); // Hello
}
}
Polymorphism

Problem: Car Shop
13
Seat
-countryProduced: String
+toString(): String<<interface>>
<<Car>>
+TIRES: Integer
+getModel(): String
+getColor(): String
+getHorsePower(): Integer
Serializable
Check your solution here :https://judge.softuni.bg/Contests/1581/Interfaces-and-Abstraction-Lab

Solution: Car Shop (1)
14
public interface Car {
int TIRES = 4;
String getModel();
String getColor();
Integer getHorsePower();
String countryProduced();
}

Solution: Car Shop (2)
15
public class Seat implements Car, Serializable {
// TODO: Add fields, constructor and private methods
@Override
public String getModel() { return this.model; }
@Override
public String getColor() { return this.color; }
@Override
public Integer getHorsePower() { return this.horsePower; }
}

 Interface can extend another interface
Extend Interface
16
public interface Showable {
void show();
}
public interface Printable extends Showable {
void print();
}

 Class which implements child interface must provide
implementation for parent interface too
Extend Interface
17
class Circle implements Printable
public void print() {
System.out.println("Hello");
}
public void show() {
System.out.println("Welcome");
}

 Refactor your first problem code
 Add for rentable cars
 Add class Cinterface for sellable cars
 Add interface arImpl
 Add class Audi, which extends CarImpl and implements
rentable
 Refactor class Seat to extends CarImpl and implements rentable
Problem: Car Shop Extended
18

Solution: Car Shop Extended (1)
19
public interface Sellable extends Car {
Double getPrice();
}
public interface Rentable extends Car {
Integer getMinRentDay();
Double getPricePerDay();
}

Solution: Car Shop Extended (2)
20
public class Audi extends CarImpl implements Rentable {
public Integer getMinRentDay() {
return this.minDaysForRent; }
public Double getPricePerDay() {
return this.pricePerDay; }
// TODO: Add fields, toString() and Constructor
}

 Since Java 8 we can have method body in the interface
 If you need to override default method think about your design
Default Method
21
public interface Drawable {
void draw();
default void msg() {
System.out.println("default method:");
}
}

 Implementation is not needed for default methods
Default Method
22
class TestInterfaceDefault {
Drawable d = new Rectangle();
d.draw(); // drawing rectangle
d.msg(); // default method
}
}

 Since Java 11, we can have static method in interface
Static Method
23
public interface Drawable {
void draw();
static int cube(int x) { return x*x*x; }
}
Drawable d = new Rectangle();
d.draw();
System.out.println(Drawable.cube(3)); } // 27

 Design a project, which has
 Interface for Person
 3 implementations
for different nationalities
 Override where needed
Problem: Say Hello
24
<<Person>>
European
-name: String
<<interface>>
<<Person>>
+getName(): String
+sayHello():String
<<Person>>
Bulgarian
-name: String
+sayHello(): String
<<Person>>
Chinese
-name: String
+sayHello(): String

Solution: Say Hello (1)
25
public interface Person {
String getName();
default String sayHello() { return "Hello"; }
}
public class European implements Person {
private String name;
public European(String name) { this.name = name; }
public String getName() { return this.name; }
}

Solution: Say Hello (2)
26
public class Bulgarian implements Person {
public Bulgarian(String name) {
this.name = name;
}
public String getName() { return this.name; }
public String sayHello() { return "Здравей"; }
}
// TODO: implement class Chinese

Abstract Classes
Abstract Classes and Methods
27

 Cannot be instantiated
 May contain abstract methods
 Must provide implementation for all inherited
interface members
 Implementing an interface might map the interface
methods onto abstract methods
Abstract Class
28
public abstract class Animal {
}

Abstract Methods
 Declarations are only permitted in abstract classes
 Bodies must be empty (no curly braces)
 An abstract method declaration provides no actual
implementation:
29
public abstract void build();

Interfaces vs Abstract Classes
30

Interface vs Abstract Class (1)
 Interface
 A class may implement
several interfaces
 Cannot have access
modifiers, everything is
assumed as public
 Abstract Class (AC)
 May inherit only
one abstract class
 Provides implementation
and/or just the signature
that has to be overridden
 Can contain access
modifiers for the fields,
functions, properties
31

Interface vs Abstract Class (2)
 Interface
 If we add a new method
we must track down
all the implementations of
the interface and
define implementation
for the new method
 Abstract Class
 Fields and constants
can be defined
 If we add a new method
we have the option of
providing default
implementation
32

 Refactor the code from the last problem
 Add BasePerson abstract class
 In which move all code duplication from European, Bulgarian,
Chinese
Problem: Say Hello Extended
33
BasePerson
#BasePerson(name)
-name: String
-setName(): void

Solution: Say Hello Extended
34
public abstract class BasePerson implements Person {
protected BasePerson(String name) {
this.setName(name);
}
private void setName(String name) { this.name = name; }
@Override
public String getName() {
return this.name;
}
}

 …
 …
 …
Summary
35
 Abstraction – hiding implementation and
showing functionality
 Interfaces
 implements vs extends
 Default and Static methods
 Abstract classes
 Interfaces vs Abstract Classes

 https://softuni.bg/modules/59/java-advanced

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20.4 Java interfaces and abstraction

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