Question

I am using nested TreeMap
[UserMap [LibraryMap [BookMap]]

When I used BookMap.clear() instead of new , it clears all data and I remain with last 2 entered data in BookMap. Do I need to create new object ? I expect after adding the first BookMap and doing clear that won’t affect the LibraryMap but it did.

TreeMap<Integer, Integer> BookMap = new TreeMap<Integer, Integer>();
    TreeMap<Integer, TreeMap<Integer, Integer>> LibraryMap = new TreeMap<Integer, TreeMap<Integer, Integer>>();
    TreeMap<Integer, TreeMap<Integer, TreeMap<Integer, Integer>>> UserMap = new TreeMap<Integer, TreeMap<Integer, TreeMap<Integer, Integer>>>();


    // Adding data to a tree map
    BookMap.put(1, 2000);
    BookMap.put(2, 2000);
    BookMap.put(3, 2003);

    LibraryMap.put(1,BookMap);
    //BookMap.clear(); 
    BookMap = new TreeMap<Integer, Integer>(); 
    BookMap.put(4, 2006);
    BookMap.put(5, 2007);

    LibraryMap.put(2,BookMap);

    BookMap= new TreeMap<Integer, Integer>();
    BookMap.put(6,2009);
    BookMap.put(7, 2012);

    LibraryMap.put(3,BookMap);

    UserMap.put(1,LibraryMap);

Answer 1

If you want to associate a new BookMap to a new key in LibraryMap, you need to create a new one.

If you use clear, your variable BookMap is still a reference to the instance you associated with key 1 in LibraryMap. In other words:

LibraryMap.put(1,BookMap);
BookMap.clear(); //still the same instance as 1-line above
BookMap.put(4, 2006); // still the same
BookMap.put(5, 2007); // ...
LibraryMap.put(2,BookMap); // LibraryMap.get(1) and .get(2) will return the same instance

By the way, unrelated, but using a capital letter for a variable name is wrong: the convention is capital letter for Classes and lowcase for variables (bookMap).