Coding for Android on Windows - First projects
This page is the second chapter of the article Coding for Android on Windows.
Feel free to write comments and edit my work or add content.
Your first projects
The Android Developers page offers some tutorials that helps you to get started. As usual, start with a Hello World. It is not really complicated. The page shows you from scratch how to create a new Android project. This page also shows you again how to create a virtual device. Remember for your following projects, that it is always better to use .xml files for entering text that shall be displayed instead of actually writing it directly into your code. That helps to change the text afterwards and to convert your application to other languages. How to use .xml files is explained in the second part of the article mentioned above.
Another tutorial that helped me a lot is the Notepad Tutorial. To be honest, this tutorial is much more complicated than the other one and I haven't really understood everything when I read it, but it definitely helps you to understand, how an Android project works and how you can connect different parts of your program with each other or even with other applications.
Using the NDK
Now you know the basics of coding for an Android phone by just using Java. Integrating native code written in C or C++ is more complicated, but once you have understood how it works, you will get used to it very quick. I assume you have already installed the NDK. If not, follow the steps described here. The NDK uses the JNI (Java Native Interface). JNI cannot just be used for Android programming, but also for simple Java coding. I think the Wikipedia article is a good way to start understanding the JNI. Afterwards, read carefully through the NDK documentation and understand what the intent of the NDK is. Also read this page. On this page you also find all the tutorials you have just downloaded together with the NDK. Read the chapter Exploring the hello-jni Sample. I do not really like the instructions you find on this page and as I was struggling with them, I try to give you another explanation how to get this application running.
Create a new Android project just like explained on the mentioned page above and open the project hello-jni from the sample folder. I guess after reading the NDK documentation you are more or less familiar with the structure of this program. An Android app does not have a Main function, but an onCreate method. This method is called when the application is started. So we have to say inside this method how the app shall look like in the beginning. As we just want to display text, we have have to create a TextView:
TextView tv = new TextView(this);
We want to receive the content of this TextView from a JNI method:
tv.setText( stringFromJNI() );
The method stringFromJNI() must be declared inside the Java part of the program. It is very important to use the key word native. Otherwise you would become an error. So the declaration looks like this:
public native String stringFromJNI();
To tell your application where to look for the native methods, you have to include the library you will create later from your C code:
static {
System.loadLibrary("hello-jni");
}
The rest has to be done in C code. Open the file Android.mk and hello-jni.cc in the folder jni. Take a look on the Android.mk. This is not really the code you want to ride but more like an interface between the C and the Java code. You always have to have such a file that looks similar to this one when you are using the NDK. It is always called the same.
Your actual code has to be in the C file. Take a look on this file. The methods are always named in a special way: You take the package name, followed by the class name and the actual method name. These parts of the name are connected with underscores and also all the dots (for example in the package name) become underscores. So it's better/necessary to avoid underscores within your class or method names. Otherwise you will probably become an error. Even if you do not need any attributes, you always have two parameters belonging to your method. But you do not have to really care about them. As long as you are creating objects within your C code and work on these objects, you can do it like you are used to. But as you see in the code, you have to work with pointers when you want to work with values/objects received via the JNI or if you want to return results back. In our case, we are just creating a string s and return it to the Java part of the program.
Now we are finished with the code and can compile it. I do not really understand what they are doing in the instructions on the page I have just mentioned above. So here is the way I compile the C/C++ Code and create the library:
- Start Cygwin to use the Linux-like console.
- Navigate to the JNI-folder within your project. For example "cd /cygdrive/c/java_workspace/projectname/jni". Cygdrive is the command needed by cygwin to navigate to the correct folder and the c indicates that your project is on the C drive of your computer.
- Now you have to use the Android NDK to build the library. As explained above, I am using the modified version Crystax of the NDK.
- Type "/cygdrive/c/Android/Crystax/ndk-build" or something like that into cygwin, depending on where you have stored the Android NDK. Do NOT write "cd" in front of your command.
- If you do not get an error, your C-file has been compiled. Refresh your project in Eclipse and you will see a new folder obj that contains your library.
Now everything has been done and we can start the application. You can either use your Android phone or the virtual device you have created as explained above to test the app. Usually it is much faster to upload the app to your phone than using the virtual device.
Next Chapter: Image Processing - Java Code
Previous Chapter: Set up your Machine
