Mechanics of testing with a mock database connection

I have this application with a database on its back-end, and I'm having a lot of trouble wrapping my head around how to test this thing. (It's an Android app, but I think that the testing concepts are similar. In my application under test, I have a database adapter:

public class MyDatabaseAdapter() {
    Cursor returnCursorFromQuery(SQLQuery query) {
         // execute an SQL query and wrap the resul开发者_Go百科t in a Cursor object
    }
}

I have a method, and I'm trying to test that it gives the right output when my database SELECT query returns no rows:

MyDatabaseAdapter adapter;

public int methodUnderTest() {
    this.adapter = new MyDatabaseAdapter();
    return populate();
}

private int populate() {
    SQLQuery query = new SQLQuery("SELECT * FROM my_table");
    Cursor aCursor = this.adapter.returnCursorFromQuery(query);

    // populate the UI

    return aCursor.getCount();
}

I have a mock cursor object that returns zero rows against all queries in my testing framework, what I don't understand is how I get my private populate() method to run its query against the mock cursor object rather than the cursor connected to my actual database. Or if I write a mock database adapter object, how to I get the methodUnderTest() to use the mock adapter instead of the one that it's programmed to use?

Any direction would be really helpful. Thanks.

---

You can make MyDatabaseAdapter implement an IDatabaseAdapter interface, and then create a mock MockDatabaseAdapter that returns what you want to test. Then instead of setting this.adapter = new MyDatabaseAdapter(); in MethodUnderTest set this.adapter in the constructor of the class, from a passed-in parameter of type IDatabaseAdapter:

public MyClass(IDatabaseAdapter adapter)
{
     this.adapter = adapter;
}

Then you can pass in new MyDatabaseAdapter() in the production code and an instance of the mock class in the unit tests.