TreeMap java implementation - putting 1st element

public V put(K key, V value) {
    Entry<K,V> t = root;
    if (t == null) {
        compare(key, key); // type (and possibly null) check
        root = new Entry<>(key, value, null);
        size = 1;
        modCount++;
        return null;
    }
    int cmp;
    ...
}

final int compare(Object k1, Object k2) {
    return comparator==null ? ((Comparable<? super K>)k1).compareTo((K)k2)
        : comparator.compare((K)k1, (K)k2);
}

After facing some bug in my application, I had to debug TreeMaps put method. My issue was in comparing objects that were put in the map. What is odd, is that when I put FIRST element to the Map, it key gets compared with itself. I can't understand why would it work like that. Any insights (besides the commented "type (and possibly null) check")? Why wouldn't they just check if key was null? What kind of "type" check is made out there and what for?

Solution

As mentioned in the comment, https://bugs.openjdk.java.net/browse/JDK-5045147 is the issue where this was introduced. From the discussion in that issue, the original fix was the following:

BT2:SUGGESTED FIX

Doug Lea writes:

"Thanks! I have a strong sense of deja vu that I've added this before(!) but Treemap.put should have the following trap added."
public V put(K key, V value) {
     Entry<K,V> t = root;

    if (t == null) {
  + if (key == null) {
  + if (comparator == null)
  + throw new NullPointerException();
  + comparator.compare(key, key);
  + }
         incrementSize();
         root = new Entry<K,V>(key, value, null);
         return null;
     }

The intention seems to throw a NPE in case the comparator of the TreeMap is null, or the comparator does not accept null keys (which conforms to the API specification). It seems the fix was shortened to one line:

compare(key, key);

which is defined as:

@SuppressWarnings("unchecked")
final int compare(Object k1, Object k2) {
    return comparator==null ? ((Comparable<? super K>)k1).compareTo((K)k2)
        : comparator.compare((K)k1, (K)k2);
}

Hence this test will do both the null check and the type check, namely the cast to Comparable.