View source for Chapter:Interfaces - Writing an interface - Exercises

=Exercises on writing your own interface=

Exercises on writing a small interface and some classes which implements that interface.

We should now write a MediaPlayer - not a very fancy one. To do this we start by writing a new interface for files that are, in some way, Playable. Since we started, in [[Chapter:Inheritance_-_Extending_your_own_classes|the previous chapter on inheritance]], to use <code>FBFile</code> etc we will stick to using that so let's call our interface <code>FBPlayable</code>.

What do we expect from a playable file? Well, since we call the interface FBPlayable we can expect it to be able to play itself ... so a method called <code>play()</code> sounds reasonable.

Before we start we want you to do any of the following:
* copy the files from the previous chapter exercises where you were using FBFile etc, to a new directory
** If you don't have the files, you can download them from [https://github.com/progund/inheritance/tree/master/it-is-possible-to-extend-your-own-classes here].
* continue editing the files in the same directory as you used in the previous section

Ok, we're ready to start. Let's do some basics first - the interface and some classes.

==Q1==
Create an interface <code>FBPlayable</code>. It can use the same package as FBFile. The interface should specify the method <code>play()</code> as:
* public
* return type shall be void
* have no parameters

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Expand using link to the right to see the full content.
<div class="mw-collapsible-content">
Suggested solution:

<source lang="Java">
package com.superpower.filebrowser;

public interface FBPlayable  {

    public void play();

}
</source>
</div></div>

==Q2== 
[[File:FBPlayable FBMp3 FBOgg.png|thumb|400px|right|FBOgg and FBMp3 implementing FBPlayable<br>Dotted arrows mean "implements"]]
Add a class <code>FBMp3</code>. This class should:

* extend <code>FBFile</code>, since we want it to be a file
* implement <code>FBPlayable</code>, since we want it to be able to "behave" as a Playable. The implementation of the <code>play</code> method can be done by simply outputting some words about the file being played.... it would be better to actually play the file but for now we're happy with this solution.
* have a constructor with one parameter (of type String)

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Expand using link to the right to see the full content.
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Suggested solution:

<source lang="Java">
package com.superpower.filebrowser;

public class FBMp3 extends FBFile implements FBPlayable {

    public FBMp3(String name) {
        super(name);
    }

    public String thumbnail() {
        return "[mp3 ]";
    }

    @Override // It is called overriding, also when you implement an interface!
    public void play() {
        System.out.println("faked playing of file: " + this);
    }
    
}
</source>
</div></div>

==Q3==
Add a class <code>FBOgg</code> in a similar way you did with the <code>FBMp3</code> class.

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Suggested solution:

<source lang="Java">
package com.superpower.filebrowser;

public class FBOgg extends FBFile implements FBPlayable {

    public FBOgg(String name) {
        super(name);
    }

    public String thumbnail() {
        return "[ogg ]";
    }

    public void play() {
        System.out.println("faked playing of file: " + this);
    }
    
}
</source>
</div></div>

==Q4==
In the class <code>FileList</code> you need to add code to handle <code>FBPlayable</code>. So, add the following methods to the class <code>FileList</code>:

<source lang="Java">
    public static List<FBPlayable> listPlayable() {
        return listPlayable(".");
    }
    
    public static List<FBPlayable> listPlayable(String dir) {
        return listPlayable(new File(dir));
    }
    
    public static List<FBPlayable> listPlayable(File dir) {
        List<FBFile>     files = list(dir);
        List<FBPlayable> list  = new ArrayList<FBPlayable>();

        for (FBFile f : files) {
            if (f instanceof FBPlayable ) {
                list.add((FBPlayable)f);
            }
        }
        
        return list;
    }
</source>

Change the following method in the class <code>FileList</code> to look like this.
<source lang="Java">
    public static List<FBFile> list(File dir) {

        if ( (dir==null) || dir.isFile() ) {
            return null;
        }
        
        // By returning a List we can chose any the subclasses (in
        // this case actually sub interfaces - more on that in the
        // coming chapter) and change it afterwards, since the users
        // of this method only uses the methods as "provided" by List
        List<FBFile> list = new ArrayList<FBFile>();
        
        // Note that we use File below. File is a class in Java.
        File[] files = dir.listFiles();
        if (files==null) {
            System.err.println("Woooops, list files returned null");
            System.err.println(" * dir: " + dir);
        }
        for (File file : files) {
            if (file.isFile()) {
                if (file.getPath().endsWith(".java")) {
                    list.add(new FBJavaFile(file.getPath()));
                } else if (file.getPath().endsWith(".mp3")) {
                    list.add(new FBMp3(file.getPath()));
                } else if (file.getPath().endsWith(".ogg")) {
                    list.add(new FBOgg(file.getPath()));
                } else {
                    list.add(new FBFile(file.getPath()));
                }
            } else if (file.isDirectory()) {
                list.add(new FBDir(file.getPath()));
            }
        }
        return list;
    }
</source>

''Note: we check the suffix of the files and create objects using that information.''

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Suggested solution:

We're listing the entire FileList class here to make it a bit easier
if you want to copy/paste.

<source lang="Java">
package com.superpower.filebrowser.utils;

import java.util.List;
import java.util.ArrayList;
import java.io.File;

import com.superpower.filebrowser.*;


public class FileList {

    public static List<FBFile> list() {
        return list(".");
    }

    public static List<FBFile> list(String dir) {
        return list(new File(dir));
    }

    public static List<FBPlayable> listPlayable() {
        return listPlayable(".");
    }
    
    public static List<FBPlayable> listPlayable(String dir) {
        return listPlayable(new File(dir));
    }
    
    public static List<FBPlayable> listPlayable(File dir) {
        List<FBFile>     files = list(dir);
        List<FBPlayable> list  = new ArrayList<FBPlayable>();

        for (FBFile f : files) {
            if (f instanceof FBPlayable ) {
                list.add((FBPlayable)f);
            }
        }
        
        return list;
    }
    
    public static List<FBFile> list(File dir) {

        if ( (dir==null) || dir.isFile() ) {
            return null;
        }
        
        // By returning a List we can chose any the subclasses (in
        // this case actually sub interfaces - more on that in the
        // coming chapter) and change it afterwards, since the users
        // of this method only uses the methods as "provided" by List
        List<FBFile> list = new ArrayList<FBFile>();
        
        // Note that we use File below. File is a class in Java.
        File[] files = dir.listFiles();
        if (files==null) {
            System.err.println("Woooops, list files returned null");
            System.err.println(" * dir: " + dir);
        }
        for (File file : files) {
            if (file.isFile()) {
                if (file.getPath().endsWith(".java")) {
                    list.add(new FBJavaFile(file.getPath()));
                } else if (file.getPath().endsWith(".mp3")) {
                    list.add(new FBMp3(file.getPath()));
                } else if (file.getPath().endsWith(".ogg")) {
                list.add(new FBOgg(file.getPath())); 
                } else {
                    list.add(new FBFile(file.getPath()));
                }
            } else if (file.isDirectory()) {
                list.add(new FBDir(file.getPath()));
            }
        }
        return list;
    }

}

</source>
</div></div>

==Q5==
Ok, let's make sure we can list files as in the previous section. But first let's make it possible to pass a directory to the <code>TextMain</code> class. Pass this directory to the <code>FileList</code>'s <code>list</code> method.

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Expand using link to the right to see the full content.
<div class="mw-collapsible-content">
Suggested solution:

<source lang="Java">
    public static void main(String[] args) {

        String dir = ".";
        if ( args.length!=0) {
            dir = args[0];
        }
        
        List<FBFile> files = FileList.list(dir);

        for (FBFile f : files) {
            System.out.println(f.thumbnail() + " " + f.name());
        }
        
    }
</source>
</div></div>

==Q6==
Let's write a new class, <code>MediaPlayer</code> in the same package as <code>TextMain</code>. This class should have a main method that:
* checks if the users supplied a directory (see previous exercise) and uses that when listing files
* asks <code>FileList</code> for a list of <code>FBPlayable</code> objects.
* loops through the objects and plays them one by one (using the <code>play</code> method).

<div class="mw-collapsible mw-collapsed">
Expand using link to the right to see the full content.
<div class="mw-collapsible-content">
Suggested solution:

<source lang="Java">
package com.superpower.filebrowser.main;

import java.util.List;

import com.superpower.filebrowser.FBPlayable;
import com.superpower.filebrowser.utils.FileList;


public class MediaPlayer {

    public static void main(String[] args) {

        String dir = ".";
        if ( args.length!=0) {
            dir = args[0];
        }
        
        System.out.println("Playing in " + dir + ":");
        for (FBPlayable p : FileList.listPlayable(dir)) {
            p.play();
        }
        
    }

}
</source>
</div></div>

==Q7 - bonus==
Read this, if you want to learn more about uses of interfaces.

For a solution which doesn't create a new class for each kind of file type, please see the following PDF presentation:
* [[:Media:Interfaces-VI-Writing your own Alternative version.pdf|An example with FBFile objects]] (18 sidor) (No video yet)
* Source code for the FBFile bonus example slide: [https://github.com/progund/interfaces/tree/master/interfaces-instead-of-class-explosion Github repository]

No exercise, but rather voluntary reading.

=Links=
==Source code (exercises/solutions)==
{{SolutionSource|dir=writing-your-own-interfaces|zip=https://github.com/progund/interfaces/archive/master.zip|vcs=git|vcs-link=https://github.com/progund/interfaces.git}}

==Further reading==
* [https://docs.oracle.com/javase/tutorial/java/concepts/interface.html Oracle tutorial - What is an interface?]
* [https://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html Oracle tutorial - Interfaces in Java]
* [https://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html Java online API docs - Comparable interface]
* [https://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html Java online API docs - Comparator interface]
* [https://docs.oracle.com/javase/tutorial/java/IandI/polymorphism.html Oracle tutorial - Polymorphism]
* [http://www.oracle.com/technetwork/java/oo-140949.html Oracle technetwork - Object-oriented programming]
* [https://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html Oracle tutorial on arrays (bonus reading!)]

==Where to go next==
Next page is: [[Chapter:Interfaces_-_Program_against_an_interface|Interfaces_-_Program_against_an_interface]]

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