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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.

Java

// Java Program Demonstrate putIfAbsent()
// method of ConcurrentHashMap 

import java.util.concurrent.*;

class ConcurrentHashMapDemo {
    public static void main(String[] args)
    {
        ConcurrentHashMap<Integer, String> chm = 
                     new ConcurrentHashMap<Integer, String>();

        chm.put(100, "Geeks");
        chm.put(101, "for");
        chm.put(102, "Geeks");
        chm.put(103, "Gfg");
        chm.put(104, "GFG");

        // Displaying the HashMap
        System.out.println("Initial Mappings are: "
                           + chm);

        // Inserting non-existing key along with value
        String returned_value = (String)chm.putIfAbsent(108, "All");

        // Verifying the returned value
        System.out.println("Returned value is: "
                           + returned_value);

        // Displayin the new map
        System.out.println("New mappings are: "
                           + chm);
    }
}

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.

Java

// Java Program Demonstrate putIfAbsent()
// method of ConcurrentHashMap 

import java.util.concurrent.*;
class ConcurrentHashMapDemo {
    public static void main(String[] args)
    {
        ConcurrentHashMap<Integer, String> chm = 
                          new ConcurrentHashMap<Integer, String>();

        chm.put(100, "Geeks");
        chm.put(101, "for");
        chm.put(102, "Geeks");
        chm.put(103, "Gfg");
        chm.put(104, "GFG");

        // Displaying the HashMap
        System.out.println("Initial Mappings are: "
                           + chm);

        // Inserting existing key along with value
        String returned_value = (String)chm.putIfAbsent(100, "All");

        // Verifying the returned value
        System.out.println("Returned value is: "
                           + returned_value);

        // Displayin the new map
        System.out.println("New mappings are: "
                           + chm);
    }
}

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()

ConcurrentHashMap putIfAbsent() Method in Java

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