This codes source tree is extracted from Android's project: https://android.googlesource.com/platform/development/+/05523fb0b48280a5364908b00768ec71edb847a2/samples Since Android upgraded its building tools to gradle, Google team discarded those samples maintenance work, but I found there are still some samples very useful. So I'll change some good samples's building tool framework from "make" to gradle for these samples.