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()