WeakHashMap computeIfAbsent(K, Function) in Java

computeIfAbsent(K, Function): This method is available in java.util.WeakHashMap class of Java.

Syntax:

Integer java.util.Map.computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)

This method takes two arguments. If the specified key is not already associated with a value (or is mapped to null), attempt to compute its value using the given mapping function and enter it into this map unless null.

Parameters: Two parameters are required for this method.

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

mappingFunction: the mapping function to compute a value.

Returns: the current (existing or computed) value associated with the specified key, or null if the computed value is null.

Throws:

1. NullPointerException - if the specified key is null and this map does not support null keys or the mappingFunction 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.WeakHashMap;
import java.util.function.Function;

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

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

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

        String key = "Hello";
        Function<? super String, ? extends 
        Integer> mappingFunction = k -> Integer.parseInt(k);

        System.out.println(weakHashMap.computeIfAbsent(key,
                                  mappingFunction));
    }
}

Output:

2

Approach 2: NullPointerException 

Java

import java.util.WeakHashMap;
import java.util.function.Function;

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

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

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

        String key = "Hello";
        Function<? super String, ? extends 
                      Integer> mappingFunction = null;

        System.out.println(weakHashMap.computeIfAbsent(key,
                                           mappingFunction));
    }
}

Output:

Exception in thread "main" java.lang.NullPointerException
	at java.base/java.util.Objects.requireNonNull(Objects.java:208)
	at java.base/java.util.Map.computeIfAbsent(Map.java:998)