The putIfAbsent(Key, value) method of Hashtable class which allows to map a value to a given key if given key is not associated with a value or mapped to null. A null value is returned if such key-value set is already present in the HashMap.
Syntax:
public V putIfAbsent(K key, V value)
Parameters: This method accepts two parameters:
- key: specifies the key to which the specified value is to be mapped if key is not associated with any value.
- value: specifies the value to be mapped to the specified key.
Returns: This method returns the existing value mapped to the key and returns null if no value is previously mapped to the key.
Exception: This method throws:
- NullPointerException: when the specified parameters are null.
Below programs illustrate the putIfAbsent(Key, value) method:
Program 1:
// Java program to demonstrate// putIfAbsent(key, value) method. import java.util.*; public class GFG { // Main method public static void main(String[] args) { // create a table and add some values Map<String, Integer> table = new Hashtable<>(); table.put("Pen", 10); table.put("Book", 500); table.put("Clothes", 400); table.put("Mobile", 5000); // print map details System.out.println("hashTable: " + table.toString()); // Inserting non-existing key with value // using putIfAbsent method String retValue = String.valueOf(table .putIfAbsent("Booklet", 2500)); // Print the returned value System.out.println("Returned value " + "for Key 'Booklet' is: " + retValue); // print new mapping System.out.println("hashTable: " + table); // Inserting existing key with value // using putIfAbsent method retValue = String.valueOf(table .putIfAbsent("Book", 4500)); // Print the returned value System.out.println("Returned value" + " for key 'Book' is: " + retValue); // print new mapping System.out.println("hashTable: " + table); }} |
Output:
hashTable: {Book=500, Mobile=5000, Pen=10, Clothes=400}
Returned value for Key 'Booklet' is: null
hashTable: {Book=500, Mobile=5000, Pen=10, Clothes=400, Booklet=2500}
Returned value for key 'Book' is: 500
hashTable: {Book=500, Mobile=5000, Pen=10, Clothes=400, Booklet=2500}
Program 2: To Show NullPointerException
// Java program to demonstrate// putIfAbsent(key, value) method. import java.util.*; public class GFG { // Main method public static void main(String[] args) { // create a table and add some values Map<Integer, String> table = new Hashtable<>(); table.put(1, "100RS"); table.put(2, "500RS"); table.put(3, "1000RS"); // print map details System.out.println("hashTable: " + table.toString()); try { table.putIfAbsent(null, "8"); } catch (NullPointerException e) { System.out.println("Exception: " + e); } }} |
Output:
hashTable: {3=1000RS, 2=500RS, 1=100RS}
Exception: java.lang.NullPointerException
References: https://docs.oracle.com/javase/8/docs/api/java/util/Hashtable.html#putIfAbsent-K-V-
