HashTable putIfAbsent() method in Java with Examples
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-
