NewProject 项目下载下来，不能直接运行，需要build.gradle 文件中粘贴以下内容

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript { repositories { jcenter() } dependencies { classpath 'com.android.tools.build:gradle:2.3.1'

    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files
}

}

allprojects { repositories { jcenter() } }

task clean(type: Delete) { delete rootProject.buildDir }

因为首次在下载项目时 build.gradle 文件是空的。

注：“build.gradle 该文件打开，显示的也就是项目文件”