How To Decompile Android APK file(.apk) and Get Source Code

Sometimes you may be amazed to see beautiful android apps with great design and simplicity. You may want to look into the source code of that app and to design the same way they have done. Usually APK file is not readable because its in dex format (Dalvik excutable file) which can be read by dalvik compiler. So there is a trick I found on stackoverflow to decompile any apk file. Just follow the below steps to get source code of any android app, if you have got .apk file

Step 1:

1. Create a new folder in your pc and copy over the .apk file that you want to decode.
2. Now rename the extension of this .apk file to .zip (e.g. rename from filename.apk to filename.zip) and save it. 
3. Now you can access the classes.dex files, etc. At this stage you are able to see drawables but not xml and java files, so continue.

Step 2:

1. Now extract this .zip file in the same folder (or NEW FOLDER).
2. Download dex2jar and extract it to the same folder (or NEW FOLDER).
3. Move the classes.dex file into the dex2jar folder.
4. Now open command prompt and change directory to that folder (or NEW FOLDER). Then write d2j-dex2jar classes.dex (for mac terminal or ubuntu write ./d2j-dex2jar.sh classes.dex) and press enter. You now have the classes.dex.dex2jar file in the same folder. 
5. Download java decompiler, double click on jd-gui, click on open file, and open classes.dex.dex2jar file from that folder: now you get class files.
6. Save all of these class files (In jd-gui, click File -> Save All Sources) by src name. At this stage you get the java source but the .xml files are still unreadable, so continue.

Step 3:

Now open another new folder

1. Put in the .apk file which you want to decode
2. Download the latest version of apktool AND apktool install window (both can be downloaded from the same link) and place them in the same folder.
3. Open a command window.
4. apktool d myApp.apk (where myApp.apk denotes the filename that you want to decode)

Now you get a file folder in that folder and can easily read the apk's xml files.

Step 4:

It's not any step just copy contents of both folder(in this case both new folder)to the single one
and enjoy the source code...
Source: stackoverflow.com