HashTable compute() method in Java with Examples

The compute(Key, BiFunction) method of Hashtable class allows to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping is found).

If the remapping function passed in compute() of Hashtable returns null as a return value then the mapping is removed from Hashtable(or remains absent if initially absent).
If the remapping function throws an exception, the exception is rethrown, and the current mapping is left unchanged.
During computation, modification this map using this method is not allowed.
The compute() method can be used to update an existing value inside Hashtable.
For example, This mapping append string value of mapping:
```
Hashtable.compute(key, (k, v) -> v.append("strValue"))
```
This method will throw a ConcurrentModificationException if the remapping function modified this map during computation.

Syntax:

public V 
       compute(K key,
             BiFunction<? super K, ? super V, ? 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 new value associated with the specified key, or null if none.

Exception: This method throws:

ConcurrentModificationException: if it is detected that the remapping function modified this map.

Below programs illustrate the compute(Key, BiFunction) method:

Program 1:

// Java program to demonstrate 
// compute(Key, BiFunction) 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()); 
  
        // remap the values of hashTable 
        // using compute method 
        table.compute("Pen", (key, val) 
                                 -> val + 15); 
        table.compute("Clothes", (key, val) 
                                     -> val - 120); 
  
        // print new mapping 
        System.out.println("new hashTable: " + table); 
    } 
} 

Output:

hashTable: {Book=500, Mobile=5000, Pen=10, Clothes=400}
new hashTable: {Book=500, Mobile=5000, Pen=25, Clothes=280}

Output:

hashTable: {Book=500, Mobile=5000, Pen=10, Clothes=400}
new hashTable: {Book=500, Mobile=5000, Pen=25, Clothes=280}

Program 2:

// Java program to demonstrate 
// compute(Key, BiFunction) 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()); 
  
        // remap the values of hashTable 
        // using compute method 
        table.compute(3, (key, val) 
                             -> val.substring(0, 4) + "00RS"); 
        table.compute(2, (key, val) 
                             -> val.substring(0, 2) + "$"); 
  
        // print new mapping 
        System.out.println("new hashTable: " + table); 
    } 
} 

Output:

hashTable: {3=1000RS, 2=500RS, 1=100RS}
new hashTable: {3=100000RS, 2=50$, 1=100RS}

References: https://docs.oracle.com/javase/10/docs/api/java/util/Hashtable.html#compute(K, java.util.function.BiFunction)

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