Android Studio顯示錯誤訊息"Cannot resolve XXX"

狀況:
變數名稱顯示紅色,有錯誤訊息"Cannot resolve XXX"

解法:
1_在功能列>Build>Rebuild Project

2_當解法1無效時,可更動(如輸入空白再刪除等)build.gradle(Project)內容,以觸發Sync功能

3_亦可嘗試清除快取