After I upgraded my android studio from 0.86 and 1.2. I tried to build my project, I got a lot of errors like appcompat-v7:21.0.0': No resource found that matches the given name: attr: android :xxxx"
Actually, that is because appcompat-v7:21.0.0' required API 21. So, I need to install API 21 Platform SDK, Extra> Google Support Repository and Google Support Library.
The most important part, the part I missed, in the build.gradle set complie sdk to 21

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