Answer may be:

The build has three steps:

• In the first step, the compilation of the resource folder will take place using AAPT(Android Asset Packaging Tool). It compiled into a single file known as R.java that carries only constant. 
• In the second step, the source code of the java file needs to compile using javac which is converted to Dalvik bytecode using the dx tool, and the final output comes in classes.ex.
• In the third step, the Android apkbuilder is necessary to take all the inputs and build the android packaging key(APK) file.