Java interfaces vs. abstract classes

.gitignore

*.class
target

readme.md

Java interfaces vs. abstract classes

Early on in Head First Design Patterns the authors say, "'Program to an interface' really means 'Program to a supertype'".

By replacing public interface FlyBehavior with public abstract class FlyBehavior from the book's strategy pattern example at http://www.headfirstlabs.com/books/hfdp/HeadFirstDesignPatterns_code102507.zip we can try this out:

$ javac *.java
$ java Run
I'm a real Mallard duck
I'm flying!!
I'm a rubber duckie
I can't fly
$ ./plantuml.sh
Creating abstract-classes.uml...
PlantUMLDoclet.createPlantUml() -  start
open outputfile: abstract-classes.uml
write interfaces/ abstract classes...
write content...
write Element MallardDuck...
write Element Duck...
write Element FlyWithWings...
write Element FlyNoWay...
write Element RubberDuck...
write Element FlyBehavior...
PlantUMLDoclet.createPlantUml() -  end
Done.
Converting abstract-classes.uml to png...
Done.

If you have Maven installed:

• mvn clean compile package exec:java --quiet
• java -jar target/supertype*jar

See also:

• http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html
• http://plantuml.sourceforge.net/doclet.html

abstract-classes.png

abstract-classes.uml

@startuml

' definitions
' -----------

' package ""
abstract Duck
abstract FlyBehavior
class FlyNoWay
class FlyWithWings
class MallardDuck
class RubberDuck
' end package

' content
' -------

' Duck:
Duck --> "1" FlyBehavior
Duck : void setFlyBehavior(FlyBehavior)
Duck : void performFly()
Duck : void swim()

' MallardDuck:
Duck <|-- MallardDuck
MallardDuck : void display()

' RubberDuck:
Duck <|-- RubberDuck
RubberDuck : void display()

' FlyWithWings:
FlyBehavior <|-- FlyWithWings

' FlyNoWay:
FlyBehavior <|-- FlyNoWay

' FlyBehavior:

@enduml

Duck.java

public abstract class Duck {

    FlyBehavior flyBehavior;

    public Duck() {
    }

    public void setFlyBehavior(FlyBehavior fb) {
        flyBehavior = fb;
    }

    abstract void display();

    public void performFly() {
        flyBehavior.fly();
    }

    public void swim() {
        System.out.println("All ducks float, even decoys!");
    }
}

FlyBehavior.java

//public interface FlyBehavior {
//    public void fly();
//}
public abstract class FlyBehavior {

    /**
     * Interfaces only allow public methods but by using an abstract class, we
     * can declare a protected method.
     */
    abstract protected void fly();
}

FlyNoWay.java

//public class FlyNoWay implements FlyBehavior {
public class FlyNoWay extends FlyBehavior {

    @Override
    //public void fly() {
    protected void fly() {
        System.out.println("I can't fly");
    }
}

FlyWithWings.java

//public class FlyWithWings implements FlyBehavior {
public class FlyWithWings extends FlyBehavior {

    @Override
    //public void fly() {
    protected void fly() {
        System.out.println("I'm flying!!");
    }
}

interfaces.png

interfaces.uml

@startuml

' definitions
' -----------

' package ""
abstract Duck
interface FlyBehavior
class FlyNoWay
class FlyWithWings
class MallardDuck
class RubberDuck
' end package

' content
' -------

' MallardDuck:
Duck <|-- MallardDuck
MallardDuck : void display()

' FlyWithWings:
FlyBehavior <|.. FlyWithWings
FlyWithWings : void fly()

' FlyBehavior:
FlyBehavior : void fly()

' RubberDuck:
Duck <|-- RubberDuck
RubberDuck : void display()

' FlyNoWay:
FlyBehavior <|.. FlyNoWay
FlyNoWay : void fly()

' Duck:
Duck --> "1" FlyBehavior
Duck : void setFlyBehavior(FlyBehavior)
Duck : void performFly()
Duck : void swim()

@enduml

MallardDuck.java

public class MallardDuck extends Duck {

    public MallardDuck() {
        flyBehavior = new FlyWithWings();
    }

    @Override
    public void display() {
        System.out.println("I'm a real Mallard duck");
    }
}

plantuml.sh

#!/bin/sh
UML='abstract-classes.uml'
echo "Creating $UML..."
javadoc \
-private \
-quiet \
-J-DdestinationFile=$UML \
-J-DcreatePackages=false \
-J-DshowPublicMethods=true \
-J-DshowPublicConstructors=false \
-J-DshowPublicFields=true \
-doclet de.mallox.doclet.PlantUMLDoclet -docletpath ~/bin/plantUmlDoclet.jar \
-sourcepath . Duck.java FlyBehavior.java FlyNoWay.java FlyWithWings.java MallardDuck.java RubberDuck.java
echo "Done."

TYPE='png'
echo "Converting $UML to $TYPE..."
java -jar ~/bin/plantuml.jar -graphvizdot ~/bin/dot -t $TYPE $UML
echo "Done."

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>.</groupId>
    <artifactId>supertype</artifactId>
    <packaging>jar</packaging>
    <version>1.0-SNAPSHOT</version>
    <name>supertype</name>
    <url>http://maven.apache.org</url>
    <build>
        <sourceDirectory>.</sourceDirectory>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.3.1</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>java</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <mainClass>Run</mainClass>
                    <arguments>
                        <argument>argument1</argument>
                    </arguments>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>2.3</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <transformers>
                                <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                                    <mainClass>Run</mainClass>
                                </transformer>
                            </transformers>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <configuration>
                    <skipTests>true</skipTests>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

RubberDuck.java

public class RubberDuck extends Duck {

    public RubberDuck() {
        flyBehavior = new FlyNoWay();
    }

    @Override
    public void display() {
        System.out.println("I'm a rubber duckie");
    }
}

Run.java

public class Run {

    public static void main(String[] args) {

        MallardDuck mallard = new MallardDuck();
        RubberDuck rubberDuckie = new RubberDuck();

        mallard.display();
        mallard.performFly();
        rubberDuckie.display();
        rubberDuckie.performFly();

    }
}

Thanks for sharing. Great job (plantuml diagram generation).