Quick Answer: Does List Allow Null Values?

Can HashMap have duplicate values?

HashMap does not allow duplicate keys however it allows to have duplicate values.

HashSet permits to have a single null value..

Does linked list allow null values?

LinkedList allow any number of null values while LinkedHashSet also allows maximum one null element.

Does ConcurrentHashMap allow null values?

The JavaDoc of ConcurrentHashMap says this: Like Hashtable but unlike HashMap , this class does not allow null to be used as a key or value.

Which is better ArrayList or HashMap?

The ArrayList has O(n) performance for every search, so for n searches its performance is O(n^2). The HashMap has O(1) performance for every search (on average), so for n searches its performance will be O(n). While the HashMap will be slower at first and take more memory, it will be faster for large values of n.

What is difference between list set and map?

The main difference between List and Set interface in Java is that List allows duplicates while Set doesn’t allow duplicates. All implementation of Set honor this contract. While a Map holds two objects per Entry e.g. a key and a value and It may contain duplicate values but keys are always unique.

Can Hashtable have NULL values?

HashMap , allows storing one null key and multiple null values. … On the other hand Hashtable does not allow null keys. This is because the objects used as keys in a Hashtable implements the hashCode() and equals() methods for their storage and retrieval. Since null is not an object it cannot implement the methods.

Can a list have NULL values?

There is absolutely no use for a null value in a List. If an ArrayList (and any other List implementation) has less than 4534235 elements, then list. get(4534234) will throw an IndexOutOfBoundsException. It will not return null.

Which collection allows NULL values?

HashSetNull values in HashSet − The HashSet object allows null values but, you can add only one null element to it.

Does LinkedHashSet allow null values?

HashSet allows maximum one null element. LinkedHashSet also allows maximum one null element. TreeSet doesn’t allow even a single null element. If you try to insert null element into TreeSet, it throws NullPointerException.

Does TreeSet allow null values?

TreeSet allows null element but like HashSet it doesn’t allow. Like most of the other collection classes this class is also not synchronized, however it can be synchronized explicitly like this: SortedSet s = Collections.

How do you handle null values on a map?

Approach:Get the map with null values and the default value to be replaced with.Get the set view of the Map using Map. … Convert the obtained set view into stream using stream() method.Now map the null values to default value with the help of map() method.Collect the modified stream into Map using collect() method.More items…•

Can we add null in list Java?

3 Answers. Yes, you can always use null instead of an object. Just be careful because some methods might throw error.

Can ArrayList hold null values?

5) Nulls: ArrayList can have any number of null elements. HashMap allows one null key and any number of null values.

Can map contain null values?

A Map cannot contain duplicate keys and each key can map to at most one value. HashMap and LinkedHashMap allow null key and null value but TreeMap doesn’t allow null key and null value. … HashMap: HashMap implements all of the Map operations and allows null values and one null key.

How do you check if a value is null in an ArrayList?

size(); i++) { if (array. get(i) != null) { emptyArray = false; break; } } if (emptyArray == true) { return true; } return false; There has to be a more efficient way of doing this — but what is it?