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 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);    }} |
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 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);    }} |
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()
