why does my treemap is not sorted in java?

Question

I have a treemap:

private Map<String, Integer> dataMap = new TreeMap<String, Integer>();

and later in the code I have the following part:

for (String item : data) {
    JSONObject itemJson = new JSONObject(item);
    System.out.println("-- before " + itemJson.getString("dataLabel") + " " + itemJson.getInt("dataValue"));
    dataMap.put(itemJson.getString("dataLabel"), itemJson.getInt("dataValue"));
}

for(String data : dataMap.keySet()) {
    System.out.println("-- after " + data);
}

The first loop displays sorted elements, such as:

-- before January 1
-- before February 2
-- before March 3
-- before April 4
.
.
.

but the second loop mixes the order:

-- after April
-- after February
-- after January
-- after March
.
.
.

Why so? and how can I get sorted results on the second loop? And by sorted I mean the same order as I was adding them during the first loop? Thanks

Answer 1

TreeMap sorts the entries by key, not by value. From the docs (emphasis mine):

A Red-Black tree based NavigableMap implementation. The map is sorted according to the natural ordering of its keys (this is your case), or by a Comparator provided at map creation time, depending on which constructor is used.

The output is correct because the strings are sorted alphabetically.

how can I get sorted results on the second loop? And by sorted I mean the same order as I was adding them during the first loop?

Use a LinkedHashMap instead, because stores the entries according to the order they were inserted. From the docs (emphasis mine):

Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map.