WeakHashMap merge(K, V, BiFunction) in Java

merge(K, V, BiFunction): This method is available in java.util.WeakHashMap class of Java.

Syntax:

V java.util.Map.merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)

This method takes three arguments. If the specified key is not already associated with a value or is associated with null, associate it with the given non-null value.

Parameters: Three parameters are required for this method.

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

value: the non-null value to be merged with the existing value associated with the key or if no existing value or a null value is associated with the key, to be associated with the key.

remappingFunction: the remapping function to recompute a value if present.

Returns: the new value associated with the specified key, or null if no value is associated with the key.

Throws:

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

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

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

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

Approach 1: When no exception

Java

import java.util.WeakHashMap;
import java.util.function.BiFunction;

public class WeakHashMapmerge {
    public static void main(String[] args) {

        WeakHashMap<String, Integer> weakHashMap = 
         new WeakHashMap<String, Integer>();

        weakHashMap.put("Java", 1);
        weakHashMap.put("Hello", 2);

        String key = "Java";
        Integer value = 1;

        BiFunction<? super Integer, ? super 
        Integer, ? extends Integer> remappingFunction = 
                             (k, v) -> k + v;

        System.out.println(weakHashMap.merge(key,
                         value, remappingFunction));

    }
}

Output:

2

Approach 2: NullPointerException 

Java

import java.util.WeakHashMap;
import java.util.function.BiFunction;

public class WeakHashMapmerge {
    public static void main(String[] args) {

        WeakHashMap<String, Integer> weakHashMap = 
            new WeakHashMap<String, Integer>();

        weakHashMap.put("Java", 1);
        weakHashMap.put("Hello", 2);

        String key = "Java";
        Integer value = 1;

        BiFunction<? super Integer, ? super Integer,
       ? extends Integer> remappingFunction = null;
        System.out.println(weakHashMap.merge(key, 
                         value, remappingFunction));

    }
}

Output:

Exception in thread "main" java.lang.NullPointerException
	at java.base/java.util.Objects.requireNonNull(Objects.java:208)
	at java.base/java.util.Map.merge(Map.java:1265)