Why NULL values are not allowed in TreeMap?

Why NULL values are not allowed in TreeMap?

Adding null values to a tree set If you try to compare any object with a null value using one of these methods, a NullPointerException will be thrown. Therefore, if you try to add null values to a TreeSet it generates a NullPointerException at the run time.

Why HashMap can have null and Concurrenthashmap is not null?

The main reason that nulls aren’t allowed in ConcurrentMaps (ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that may be just barely tolerable in non-concurrent maps can’t be accommodated. In a non-concurrent map, you can check this via map.

Can map have NULL values?

Values entered in a map can be null . However with multiple null keys and values it will only take a null key value pair once.

What is the difference between HashMap and tree map?

HashMap allows a single null key and multiple null values. TreeMap does not allow null keys but can have multiple null values. HashMap allows heterogeneous elements because it does not perform sorting on keys. TreeMap allows homogeneous values as a key because of sorting.

Why HashTable does not allow null values in Java?

Geek now you must be wondering why HashTable doesn’t allow null and HashMap do? The answer is simple s in order to successfully store and retrieve objects from a HashTable, the objects used as keys must implement the hashCode method and the equals method. Since null is not an object, it can’t implement these methods.

Does tree set allow null values?

TreeSet can not contain null values and are slower than HashSet. TreeSet contains only unique values and elements are sorted in ascending order. The following example shows how to create a TreeSet and add elements to it.

Does Hashtable allow null values?

Hashtable does not allow null keys but HashMap allows one null key and any number of null values.

Why Hashtable not allow null but HashMap allow?

equals when call contains/remove . Hashtable does not allow null keys but HashMap allows one null key and any number of null values. There is a simple explanation behind that. put() method in hashmap does not call hashcode() when null is passed as key and null Key is handled as a special case.

Can we have null key in TreeMap?

TreeMap sorts elements in natural order and doesn’t allow null keys because compareTo() method throws NullPointerException if compared with null.

What is difference between HashMap and Hashtable?

HashMap is non-synchronized. It is not thread-safe and can’t be shared between many threads without proper synchronization code whereas Hashtable is synchronized. HashMap allows one null key and multiple null values whereas Hashtable doesn’t allow any null key or value.

Does Hashtable allow duplicate values?

Hashtable Features It does not accept null key or value. It does not accept duplicate keys.

Why doesn’t hashtable support null values in treetreemap?

TreeMap and TreeSet are the only 2 collections which does not allow Heterogeneous elements to add to it. HashTable is a lot older, and so no one really thought of implementing null values at the time. If you look at their put method, it computes a hash code for both the key and the value — if the value is null, it throws a null pointer exception.

Why does hashhashtable not allow null values?

HashTable – Does not allow null keys This is because, in put(K key, V value) method, we have key.hashcode() which throws null pointer exception. HashMap allows null values as it doesn’t have any checks like HashTable, while it allows only one null key.

What is nullnullpointerexception in treemap?

NullPointerException – if the specified key is null and this map uses natural ordering, or its comparator does not permit null keys The easiest way to allow null values is to create the TreeMap with a comparator like Comparator.nullsFirst (Comparator.naturalOrder ()) or Comparator.nullsLast (Comparator.naturalOrder ())

Can HashMap tolerate null values?

So HashMap can tolerate null values. However for Hashtable and ConcurrentHashMap, the expectations are clear that multiple threads are going to act on the data. Hence they cannot afford to allow null values and give out incorrect answer. Same logic goes for keys.