android - How to get latest 7 days and order it if date format is yyyyMdd?

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I try to describe my code more clearly.

So here is my issue:

I'm stuck with my database , i want to search latest 7 days , but i store my date format is yyyymdd like 2017/9/30

If i want to search all of my records and order it , my succeed code like this:

public List<Contact> sortingDate() SUBSTR(SUBSTR(Date, 6), INSTR(SUBSTR(Date, 6), '/') + 1)n" +
 " ELSE SUBSTR(SUBSTR(Date, 6), INSTR(SUBSTR(Date, 6), '/') + 1) END DESC; ",null);
 //looping through all rows and adding to list
 if (cursor.moveToFirst()) 
 do 
 Contact contact = new Contact();
 contact.setID(Integer.parseInt(cursor.getString(0)));
 contact.setDate(cursor.getString(1));
 contact.setBeforeMorning(cursor.getString(2));
 contact.setAfterMorning(cursor.getString(3));
 contact.setBeforeNoon(cursor.getString(4));
 contact.setAfterNoon(cursor.getString(5));
 contact.setBeforeNight(cursor.getString(6));
 contact.setAfterNight(cursor.getString(7));
 System.out.println("The result is :" + cursor.getString(1));
 //Adding contact to list
 contactList.add(contact);
 while (cursor.moveToNext());
 

 return contactList;
 

It will print:

And then i try to add search latest days code to my function, i set a default date 2017/09/30

// order latest days
 public List<Contact> latestDays() SUBSTR(Date, 6, 1)n" +
 " ELSE SUBSTR(Date, 6, 2) END DESC,n" +
 " CASE WHEN LENGTH(SUBSTR(SUBSTR(Date, 6), INSTR(SUBSTR(Date, 6), '/') + 1)) = 1n" +
 " THEN '0' 

I call my function:

private DatabaseHandler db;
contactList = db.latestDays();
System.out.println(contactList.size());

It shows System.out: 0

So what code i miss it if i want to get latest 7 days like 2017/9/24 2017/9/29 2017/9/30 in my database ?

Any help would be appreciated. Thanks in advance.

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