EnumMap computeIfPresent(K, BiFunction) in Java

computeIfPresent(K, BiFunction): This method is available in java.util.EnumMap class of Java.

Syntax:

String java.util.Map.computeIfPresent(K key, BiFunction<? super Colour, ? super String, ? extends String> remappingFunction)

This method takes two arguments. If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

Note: If the remapping function returns null, the mapping is removed.

Parameters: Two parameters are required for this method.

key: key with which the specified value is to be associated.

remappingFunction: the remapping function to compute a value.

Returns: the new value associated with the specified key, or null if none.

Throws:

1. NullPointerException - if the specified key is null and this map does not support null keys, or the remappingFunction is null.

2. UnsupportedOperationException - if the put operation is not supported by this map.

3. ClassCastException - if the class of the specified key or value prevents it from being stored in this map.

4. IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map.

Approach 1: When no exception

Java

import java.util.EnumMap;
import java.util.function.BiFunction;

public class EnumMapcomputeIfPresent {

    public enum Colour {
        RED, GREEN, YELLOW, ORANGE
    };

    public static void main(String[] args) {

        EnumMap<Colour, String> enumMap = 
              new EnumMap<Colour, String>(Colour.class);

        BiFunction<? super Colour, ? super
                  String, ? extends String> add = 
                                      (a, b) -> a + b;
        enumMap.put(Colour.RED, " Hello");
        System.out.println(enumMap.computeIfPresent(Colour.RED, 
                                                     add));

    }
}

Output:

RED Hello

Approach 2: NullPointerException

Java

import java.util.EnumMap;

public class EnumMapcomputeIfPresent {

    public enum Colour {
        RED, GREEN, YELLOW, ORANGE
    };

    public static void main(String[] args) {

        EnumMap<Colour, String> enumMap =  
                new EnumMap<Colour, String>(Colour.class);

        enumMap.put(Colour.RED, " Hello");
        System.out.println(enumMap.computeIfPresent(Colour.RED, 
                                         null));

    }
}

Output:

Exception in thread "main" java.lang.NullPointerException
	at java.base/java.util.Objects.requireNonNull(Objects.java:208)
	at java.base/java.util.Map.computeIfPresent(Map.java:1075)