Quick Answer: Can HashMap Have Duplicate Keys?

Does linked list allow duplicates?

1) Both ArrayList and LinkedList are an implementation of List interface, which means you can pass either ArrayList or LinkedList if a method accepts the java.

4) ArrayList and LinkedList also allow duplicates and null, unlike any other List implementation e.g.


Does map allow duplicate keys C++?

STL map does not allow same Keys to be used. You may want to go for multi-map for that. a map will not throw any compile/run time error while inserting value using duplicate key.

What happens if we trying to add duplicate values in set?

If we insert duplicate values to the Set, we don’t get any compile time or run time errors. It doesn’t add duplicate values in the set. Below is the add() method of the set interface in java collection that returns Boolean value either TRUE or FALSE when the object is already present in the set.

Which is better HashMap or TreeMap?

HashMap, being a hashtable-based implementation, internally uses an array-based data structure to organize its elements according to the hash function. HashMap provides expected constant-time performance O(1) for most operations like add(), remove() and contains(). Therefore, it’s significantly faster than a TreeMap.

Can a TreeMap contain duplicate keys?

A TreeMap cannot contain duplicate keys. TreeMap cannot contain the null key. However, It can have null values.

Can you store a null key in Java HashMap?

In Hashmap one null key is allowed and multiple null values are allowed. Since hashcode of null is 0 so null key actually stored at index 0 in hashmap.

Can map have duplicate keys C++?

Multimap is similar to map with an addition that multiple elements can have same keys. Also, it is NOT required that the key value and mapped value pair has to be unique in this case.

Can we insert null key in TreeMap?

A TreeMap contains values based on the key. It implements the NavigableMap interface and extends AbstractMap class. … It cannot have null key but can have multiple null values. It is same as HashMap instead maintains ascending order(Sorted using the natural order of its key).

Which map can have duplicate keys?

Duplicate keys are not allowed in a map. Basically Map Interface has two implementation classes HashMap and TreeMap the main difference is TreeMap maintains order of the objects but HashMap will not. HashMap allows null values and null keys.

Can ArrayList have duplicates?

4) Duplicates: ArrayList allows duplicate elements but HashMap doesn’t allow duplicate keys (It does allow duplicate values). 5) Nulls: ArrayList can have any number of null elements. … In HashMap the elements is being fetched by specifying the corresponding key.

Why duplicate keys are not allowed in HashMap?

HashMap doesn’t allow duplicate keys but allows duplicate values. … HashMap allows null key also but only once and multiple null values. This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

What is the difference between length () and size () of ArrayList?

Array has length property which provides the length of the Array or Array object. The java ArrayList has size() method for ArrayList which provides the total number of objects available in the collection. …

How do you avoid duplicates in ArrayList?

The easiest way to remove repeated elements is to add the contents to a Set (which will not allow duplicates) and then add the Set back to the ArrayList : Set set = new HashSet<>(yourList); yourList. clear(); yourList. addAll(set);

Can a Java map contains duplicate keys?

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.

Can we override key in HashMap?

If you want to make a mutable object as key in hashmap, then you have to make sure that state change for key object does not change the hash code of object. This can be done by overriding the hashCode() method. But, you must make sure you are honoring the contract with equals() also.

How HashMap hold multiple values in one key?

To store this data in a lookup table / map , we need to create a HashMap with key as string and then associate multiple values with same key. We can do this by storing a List of Integers i.e. List objects as values in HashMap.

How do I create a duplicate map key?

As we can see, if we try to insert two values for the same key, the second value will be stored, while the first one will be dropped. It will also be returned (by every proper implementation of the put(K key, V value) method): Map map = new HashMap<>(); assertThat(map.

Can HashMap have null key?

HashMap allows only one null key but multiple null values. The code to test a HashMap for null values is this.

What happens if we put a key object in HashMap which exists?

What happens if we put a key object in a HashMap which exists? Explanation: HashMap always contains unique keys. If same key is inserted again, the new object replaces the previous object.

How do you remove duplicates in ArrayList without using set?

Remove duplicates from arraylist without using collectionspackage arrayListRemoveduplicateElements;import java.util.ArrayList;public class RemoveDuplicates {public static void main(String[] args){ArrayList al = new ArrayList();al.add(“java”);al.add(‘a’);al.add(‘b’);More items…•

Does list allow duplicates in Java?

2) List allows duplicates while Set doesn’t allow duplicate elements. All the elements of a Set should be unique if you try to insert the duplicate element in Set it would replace the existing value.