Java HashMap null values or keys

Question

Is it possible that I can put values directly to the key has null values of HashMap ?

What I want is to check if the value of one key in HashMap is null then i'm going to fill it.

Answer 1

If map.get(key) returns null, you know that either the key is not present in the Map or that it's present and has a null value.

To distinguish between the two :

if (map.containsKey(key)) {
    if (map.get(key) == null) {
        // key is present and has null value
    }
} else {
    // key is not present in the Map
}

Answer 2

In addition to @Eran answer I should note that having null values in Map is discouraged. First, it's not supported by every Map implementation. For example, ConcurrentHashMap does not support null values, so if later this Map will be shared across several threads and you decide to move to ConcurrentHashMap you will discover that you cannot do this without getting rid of nulls. Second, most of the new Java-8 API methods getOrDefault , merge , computeIfAbsent do not behave friendly with null values: usually they assume that null value is the same as the absence of the value. Finally handling null values is somewhat slower as you need to check against the same key twice (first via containsKey , second via get ).

Thus in general if it's possible you should avoid null values in maps. Sometimes it's possible to introduce a special constant which designates an absence of the value. That depends on particular task.

Answer 3

Here is the simplest (can be improved/code reviewed further) complete program. It will find the key and if the corresponding value is null it will replace that null with yourNewValue.

import java.util.HashMap;
import java.util.Map;

public class SimpleMapKeyValue {

    public static void main(final String[] args) throws Exception {
        final Map<String, Integer> map = new HashMap<>();
        map.put("A", 5);
        map.put("B", 10);
        map.put("D", null);
        map.put("E", 23);

        int yourNewValue = 100;

        for(Map.Entry<String, Integer> entry : map.entrySet()){
            if (entry.getValue() == null) {
                map.put(entry.getKey(),yourNewValue);
            }
            System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
        }
    }
}