When Should I Use HashMap?

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..

Which is faster 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.

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.

Why HashMap is not part of collection?

Map can not have such method because it need key-value pair. There are other reasons also such as Map supports keySet, valueSet etc. Collection classes does not have such views. Due to such big differences, Collection interface was not used in Map interface, and it was build in separate hierarchy.

Why HashMap is fail fast?

That aside, essentially, “fail-fast” in this sense means that an Iterator over a HashMap will throw an exception if it detects that another thread has modified the targeted HashMap – if you look in the source for HashMap, you will see this is done by simply checking a counter for the number of expected modifications.

What is difference between HashMap and Hashtable?

2) HashMap allows one null key and any number of null values. Hashtable doesn’t allow null keys and null values. 3) HashMap implementation LinkedHashMap maintains the insertion order and TreeMap sorts the mappings based on the ascending order of keys. Hashtable doesn’t guarantee any kind of order.

Should I use HashMap or Hashtable?

HashMap should be preferred over Hashtable for the non-threaded applications. In simple words , use HashMap in unsynchronized or single threaded applications . We should avoid using Hashtable, as the class is now obsolete in latest Jdk 1.8 . Oracle has provided a better replacement of Hashtable named ConcurrentHashMap.

Does HashMap maintain order?

HashMap itself doesn’t maintain insertion order – but LinkedHashMap does, so use that instead. … HashMap : 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.

Where is HashMap used in real time?

Originally Answered: Where we use hashmap in Java in real life? Wherever you are putting a list of objects in an array or list and then retrieving the value based on some attributes of an object, you can use hashmap. This is used extensively as in-memory cache for static/near static values.

Can HashMap have duplicate keys?

HashMap does not allow duplicate keys however it allows to have duplicate values. HashSet permits to have a single null value.

Does ArrayList guarantee order?

Both ArrayList and LinkedList are implementation of List interface. They both maintain the elements insertion order which means while displaying ArrayList and LinkedList elements the result set would be having the same order in which the elements got inserted into the List.

Which is faster ArrayList or linked list?

ArrayList is faster than LinkedList if I randomly access its elements. … ArrayList has direct references to every element in the list, so it can get the n-th element in constant time. LinkedList has to traverse the list from the beginning to get to the n-th element. LinkedList is faster than ArrayList for deletion.

Which collection is faster in Java?

If you need fast access to elements using index, ArrayList should be choice. If you need fast access to elements using a key, use HashMap. If you need fast add and removal of elements, use LinkedList (but it has a very poor seeking performance).

Is HashMap thread safe?

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. It is thread-safe and can be shared with many threads.

Which is faster HashMap or LinkedHashMap?

Yes, there will be the same performance difference as you get in all iterations over HashMap versus LinkedHashMap : HashMap will take time proportional to the number of entries plus the size of the hash table, and LinkedHashMap will just take time proportional to the number of entries.

How does a HashMap work?

HashMap in Java works on hashing principles. It is a data structure which allows us to store object and retrieve it in constant time O(1) provided we know the key. In hashing, hash functions are used to link key and value in HashMap.

What are the real time problems with HashMap?

The Order in which, We add the values to HashMap is not guaranteed, hence the Insertion order is not Preserved. Searching the Object is fast since the Java HashMap has the Keys. HashMap is not good for Multi-Threading because it does not support Synchronization.

Does ArrayList accept 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.