出现一个如下错误：
Error:Execution failed for task ‘:app:preDebugAndroidTestBuild’.
Conflict with dependency ‘com.android.support:support-annotations’ in project ‘:app’. Resolved versions for app (26.1.0) and test app (27.1.1) differ. See https://d.android.com/r/tools/test-apk-dependency-conflicts.html for details.
通过“Conflict with dependency ‘com.android.support:support-annotations’ in project ‘:app’. Resolved versions for app (26.1.0) and test app (27.1.1) differ”可知是jar包冲突，打开External Libraries一看，果然：
解决方法：
在build.gradle文件中添加如下代码，强制使二者统一：
androidTestImplementation('com.android.support:support-annotations:26.1.0') {
force = true
}
即：
好了~
