无法从SQLite数据库（Android）检索特定数据

Hi guys i'm kinda new in Java and SQLite programming, and I have a problem to fetch and retrieve specific data from SQLite database,, I want to create a login program, and user will input username and password (LoginScreen.java), it will check the existing data in database (database.java), and if exists, it will return true to LoginScreen.java.Hi guys i'm kinda new in Java and SQLite progra