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ConcurrentHashMap putIfAbsent() Method in Java

Last Updated : 17 Sep, 2018
The java.util.concurrent.ConcurrentHashMap.putIfAbsent() is an in-built function in Java which accepts a key and a value as parameters and maps them if the specified key is not mapped to any value. Syntax: 
chm.putIfAbsent(key_elem, val_elem)
Parameters: The function accepts two parameters which are described below:
  • key_elem: This parameter specifies the key to which the specified val_elem is to be mapped if key_elem is not associated with any value.
  • val_elem: This parameter specifies the value to be mapped to the specified key_elem.
Return Value: The function returns the existing value mapped to the key and returns null if no value is previously mapped to the key. Exception: The function throws NullPointerException when the specified parameters are null. Below programs illustrate the ConcurrentHashMap.putIfAbsent() method : Program 1: An existing key is passed as parameter to the function.
Output: 
Initial Mappings are: {100=Geeks, 101=for, 102=Geeks, 103=Gfg, 104=GFG}
Returned value is: null
New mappings are: {100=Geeks, 101=for, 102=Geeks, 103=Gfg, 104=GFG, 108=All}
Program 2: A non-existing key is passed as parameter to the function.
Output: 
Initial Mappings are: {100=Geeks, 101=for, 102=Geeks, 103=Gfg, 104=GFG}
Returned value is: Geeks
New mappings are: {100=Geeks, 101=for, 102=Geeks, 103=Gfg, 104=GFG}
Reference: https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentHashMap.html#putIfAbsent()
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