The computeIfAbsent(Key, Function) method of Hashtable class which allows you to compute value of a mapping for specified key if key is not already associated with a value (or is mapped to null).
- If mapping function of this method returns null, then no mapping is recorded.
- If the remapping function throws an exception, the exception is rethrown, and the no mapping is recorded.
- During computation, modification this map using this method is not allowed.
- This method will throw a ConcurrentModificationException if the remapping function modified this map during computation.
Syntax:
public V computeIfAbsent(K key, Function<? super K, ? extends V> remappingFunction)
Parameters: This method accepts two parameters:
- key: key with which the value is to be associated.
- remappingFunction: function to do the operation on value.
Returns: This method returns current (existing or computed) value associated with the specified key, or null if mapping returns null.
Exception: This method throws:
- ConcurrentModificationException: if it is detected that the remapping function modified this map.
Below programs illustrate the computeIfAbsent(Key, Function) method:
Program 1:
// Java program to demonstrate // computeIfAbsent(Key, Function) 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());
// provide value for new key which is absent
// using computeIfAbsent method
table.computeIfAbsent( "newPen" , k -> 600 );
table.computeIfAbsent( "newBook" , k -> 800 );
// print new mapping
System.out.println( "new hashTable: "
+ table);
}
} |
Output:
hashTable: {Book=500, Mobile=5000, Pen=10, Clothes=400} new hashTable: {newPen=600, Book=500, newBook=800, Mobile=5000, Pen=10, Clothes=400}
Program 2:
// Java program to demonstrate // computeIfAbsent(Key, Function) 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());
// provide value for new key which is absent
// using computeIfAbsent method
table.computeIfAbsent( 4 , k -> "600RS" );
// this will not effect anything
// because key 1 is present
table.computeIfAbsent( 1 , k -> "800RS" );
// print new mapping
System.out.println( "new hashTable: "
+ table);
}
} |
Output:
hashTable: {3=1000RS, 2=500RS, 1=100RS} new hashTable: {4=600RS, 3=1000RS, 2=500RS, 1=100RS}