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Java interfaces vs. abstract classes
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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:

diagram of abstract classes

$ 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:

@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
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!");
}
}
//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();
}
//public class FlyNoWay implements FlyBehavior {
public class FlyNoWay extends FlyBehavior {
@Override
//public void fly() {
protected void fly() {
System.out.println("I can't fly");
}
}
//public class FlyWithWings implements FlyBehavior {
public class FlyWithWings extends FlyBehavior {
@Override
//public void fly() {
protected void fly() {
System.out.println("I'm flying!!");
}
}
@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
public class MallardDuck extends Duck {
public MallardDuck() {
flyBehavior = new FlyWithWings();
}
@Override
public void display() {
System.out.println("I'm a real Mallard duck");
}
}
#!/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."
<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>
public class RubberDuck extends Duck {
public RubberDuck() {
flyBehavior = new FlyNoWay();
}
@Override
public void display() {
System.out.println("I'm a rubber duckie");
}
}
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();
}
}
@jmbruel

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@jmbruel jmbruel commented Nov 23, 2014

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

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