Can multiple threads access HashMap?

— Hashmap can solve performance issue by giving parallel access to multiple threads reading hashmap simultaneously. But Hashmap is not thread safe, so what will happen if one thread tries to put data and requires Rehashing and at same time other thread tries to read data from Hashmap, It will go in infinite loop.

What happens if multiple threads access HashMap?

If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally.

Can a HashMap store multiple values for same key?

HashMap can be used to store key-value pairs. But sometimes you may want to store multiple values for the same key. For example: For Key A, you want to store – Apple, Aeroplane.

How do I store multiple values on a map?

You could:

1. Use a map that has a list as the value. Map> .
2. Create a new wrapper class and place instances of this wrapper in the map. Map .
3. Use a tuple like class (saves creating lots of wrappers). Map> .
4. Use mulitple maps side-by-side.

What is HashMap and ConcurrentHashMap?

HashMap is a powerful data structure in Java used to store the key-pair values. Whereas, ConcurrentHashMap is introduced as an alternative to the HashMap. The ConcurrentHashMap is a synchronized collection class. The HashMap is non-thread-safe and can not be used in a Concurrent multi-threaded environment.

How do you make a HashMap concurrent?

If you have very highly concurrent application in which you may want to modify or read key value in different threads then it’s ideal to use Concurrent Hashmap. Best example is Producer Consumer which handles concurrent read/write. So what does the thread-safe Map means?

Why HashMap is not recommended in a multi threaded environment?

It is a problem if multiple threads are adding to the same HashMap instance without it being synchronized . Even if just 1 thread is modifying a HashMap and other threads are reading from that same map without synchronization, you will run into problems.

Can we have two same key in HashMap?

If you try to insert the duplicate key, it will replace the element of the corresponding key. HashMap is similar to HashTable, but it is unsynchronized. It allows to store the null keys as well, but there should be only one null key object and there can be any number of null values.

Does HashMap allow duplicate keys?

HashMap stores key, value pairs and it does not allow duplicate keys. If the key is duplicate then the old key is replaced with the new value.

Can we have same key in HashMap?

Is HashMap concurrent?

HashMap is explicitly unsafe for concurrent use. Use ConcurrentHashMap instead. The importance of synchronising or using ConcurrentHashMap can not be understated.

Can a hashmap have multiple values per key in Java?

A standard Java HashMap cannot store multiple values per key, any new entry you add will overwrite the previous one.

How do I share a hashmap across an application?

If you need to share the same instance of a HashMap across your application, you need to create a singleton. Using a singleton guarantees that the same instance of the HashMap will always be referenced by anything trying to access it. For example for a HashMap :

Why is HashMap unsynchronized in Java?

As it is told that HashMap is unsynchronized i.e. multiple threads can access it simultaneously. If multiple threads access this class simultaneously and at least one thread manipulates it structurally then it is necessary to make it synchronized externally.

What is the difference between Hashtable and HashMap in Java?

One object is used as a key (index) to another object (value). If you try to insert the duplicate key, it will replace the element of the corresponding key. HashMap is similar to HashTable, but it is unsynchronized. It allows to store the null keys as well, but there should be only one null key object and there can be any number of null values.