Checking if a final instance variable has been initialized

Question

I am still learning Java but I am not sure why this is causing compile time error:

public class AnswerRepository implements IAnswerRepository
{
    private final static SQLiteDatabase database;

    public AnswerRepository(Activity activity)
    {
        if(database != null)
        {
            database = DbOpenHelper.getInstance(activity);
        }
    }
}

I am just trying to check if a final variable has been assigned first before assigning a value to it. But it seems compile time checking doesn't like it. Why is that?

Answer 1

final variables can only be initialized once. Normally, they must be initialized in the constructor, but if they are static, then they need to be initialized when they are defined, like this:

private final static SQLiteDatabase database = new SQLiteDatabase(...);

or, you can initialize it later:

private static SQLiteDatabase database;

static variables will be initialized before object constructors are called. So in this case, database will always be null and since it is null, re-initializing it in the object constructor will cause a compile time error.