What does compute if absent do? – Quick

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computeIfPresent. If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value. If the function returns null , the mapping is removed.

Is ConcurrentHashMap computeIfAbsent thread safe?

ConcurrentHashMap is, indeed, threadsafe. But it only means that all read/write operations on such map are internally synchronized. A good practice will be to use one of the atomic methods implemented by ConcurrentHashMap , i.e: computeIfAbsent(..) , putIfAbsent(..) , etc.

How do you use computeIfPresent?

The computeIfPresent() method takes 2 parameters:

The computeIfAbsent(Key, Function) method of HashMap class is used to compute value for a given key using the given mapping function, if key is not already associated with a value (or is mapped to null) and enter that computed value in Hashmap else null.

What does computeIfPresent return?

The computeIfPresent method returns new value associated with the specified key, or null if none.

How does HashMap putAll work?

putAll() is an inbuilt method of HashMap class that is used for the copy operation. The method copies all of the elements i.e., the mappings, from one map into another. Return Value: The method does not return any values. Exception: The method throws NullPointerException if the map we want to copy from is NULL.

HashMap is non synchronized. It is not-thread safe and can’t be shared between many threads without proper synchronization code.

Why to use a hashtable or HashMap?

Unlike the new collection implementations, Hashtable is synchronized. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use ConcurrentHashMap in place of Hashtable.

How do hashmaps work?

HashMap works on principle of hashing. Hashing is a way to assigning a unique code for any variable/object after applying any formula/algorithm on its properties. Each object in java has it’s hash code in such a way that two equal objects must produce the same hash code consistently.

Hashing techniques in java. What is hashing: Hashing is a way to assign a unique code for any variable/object after applying any function/algorithm on its properties.