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FAQ: When to use hashmap?

Where is HashMap used in real time?

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. All your system properties, static business data – country codes, zip codes, cities, etc.

What is a HashMap used for?

Basically, a HashMap allows you to store items with identifiers. They are stored in a table format with the identifier being hashed using a hashing algorithm. Typically they are more efficient to retrieve items than search trees etc.

Which is better HashMap or ArrayList?

While the HashMap will be slower at first and take more memory, it will be faster for large values of n. The reason the ArrayList has O(n) performance is that every item must be checked for every insertion to make sure it is not already in the list.

Which is better HashMap or TreeMap?

HashMap is a general purpose Map implementation. It provides a performance of O(1), while TreeMap provides a performance of O(log(n)) to add, search, and remove items. Hence, HashMap is usually faster. Use a TreeMap if you need to keep all entries in natural order.

What are the real time problems with HashMap?

Java HashMap cannot have duplicate keys but it can have duplicates as values. Hash Map can have only one NULL key and multiple NULL Values. 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.

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How does a HashMap work?

A HashMap is a map used to store mappings of key-value pairs. 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 happens when HashMap is full?

This means that get won’t block but put, remove etc. might block at some point. An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased.

What is difference between HashMap and HashSet?

HashMap Stores elements in form of key-value pair i.e each element has its corresponding key which is required for its retrieval during iteration. HashSet stores only objects no such key value pairs maintained. Put method of hash map is used to add element in hashmap.

Why HashMap is called HashMap?

To access a value one must know its key. HashMap is known as HashMap because it uses a technique called Hashing. Hashing is a technique of converting a large String to small String that represents the same String. A shorter value helps in indexing and faster searches.

Can we convert HashMap to ArrayList?

Another way to convert a HashMap into an ArrayList is to use the Stream API to convert map keys and values to respective lists. // stream of keys in a ArrayList. Note: The Collectors. toCollection(ArrayList::new) passed to the collect() method to collect as new ArrayList.

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Can ArrayList have duplicates?

ArrayList allows duplicate values while HashSet doesn’t allow duplicates values. Ordering: ArrayList maintains the order of the object in which they are inserted while HashSet is an unordered collection and doesn’t maintain any order.

Does ArrayList guarantee order?

Yes. ArrayList is a sequential list. So, insertion and retrieval order is the same. If you add elements during retrieval, the order will not remain the same.

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. HashMap allows one null key and multiple null values whereas Hashtable doesn’t allow any null key or value.

Will TreeMap allow duplicates?

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

Does HashMap maintain order?

HashMap itself doesn’t maintain insertion order – but LinkedHashMap does, so use that instead. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries.

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