CODEWITHSUNDEEP
java
ankushb
ankushb

Reputation: 198

TreeMap putall issue while using new comparator

TreeMap<String, Integer> map1 = new TreeMap<String, Integer>();
map1.put("A", 1); map1.put("B", 2); map1.put("C", 3);
TreeMap<String, Integer> map2 = new TreeMap<>((str1, str2) -> map1.get(str1) - map1.get(str2) > 0 ? -1 : 1);
map2.putAll(map1);
Iterator<String> iterator = map2.keySet().iterator();
    while(iterator.hasNext()) {
        String key = iterator.next();
        System.out.println(key + "  " + map2.get(key) + " " + map1.get(key));
}

Output of this is 

C  null 3

B  null 2

A  null 1

Please explain why I am getting null values from map2, even after doing map2.putAll(map1)

Strangely when I iterate through entry iterator is giving the right output

    Iterator<Entry<String, Integer>> entryIterator = map2.entrySet().iterator();
    while(entryIterator.hasNext()) {
        Entry<String, Integer> entry = entryIterator.next();
        System.out.println(entry.getKey() + " " + entry.getValue());
    }

EDIT As answered issue was with comparator. It is working with

    TreeMap<String, Integer> map2 = new TreeMap<>((str1, str2) -> str1.equals(str2) ? 0 : map1.get(str1) - map1.get(str2) > 0 ? -1 : 1);

Upvotes: 4

Views: 663

Answers (2)

andy
andy

Reputation: 1356

Check the TreeMap source code

final Entry<K,V> getEntryUsingComparator(Object key) {
    @SuppressWarnings("unchecked")
        K k = (K) key;
    Comparator<? super K> cpr = comparator;
    if (cpr != null) {
        Entry<K,V> p = root;
        while (p != null) {
            int cmp = cpr.compare(k, p.key);
            if (cmp < 0)
                p = p.left;
            else if (cmp > 0)
                p = p.right;
            else
                return p;
        }
    }
    return null;
}

it using comparator to get the entry, like binary search. your comparator return 0, then TreeMap never can find the entry, so return null.

Upvotes: 0

M A
M A

Reputation: 72884

You missed returning zero in the comparator when the map values are equal:

TreeMap<String, Integer> map2 = new TreeMap<>(
    new Comparator<String>() {

        @Override
        public int compare(String str1, String str2) {
            if(map1.get(str1).equals(map1.get(str2))) {
                return 0;
            }
            return map1.get(str1) - map1.get(str2) > 0 ? -1 : 1;
        }
    });

From the documentation of TreeMap:

Note that the ordering maintained by a tree map, like any sorted map, and whether or not an explicit comparator is provided, must be consistent with equals if this sorted map is to correctly implement the Map interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Map interface is defined in terms of the equals operation, but a sorted map performs all key comparisons using its compareTo (or compare) method, so two keys that are deemed equal by this method are, from the standpoint of the sorted map, equal. The behavior of a sorted map is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Map interface.

Upvotes: 2

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