NullPointerException with ConcurrentHashMap

Why This Happens

ConcurrentHashMap prohibits null keys and null values because null is ambiguous in concurrent context: get() returning null could mean the key is absent or the value is null. HashMap allows null but ConcurrentHashMap throws NullPointerException.

The Problem

ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
map.put("key", null); // NullPointerException: null value not allowed

The Fix

ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
String value = getValue();
if (value != null) {
    map.put("key", value);
}
// Or use Optional:
map.put("key", Optional.ofNullable(getValue()).orElse("default"));

Step-by-Step Fix

1. 1

   Identify the null insertion

   Find the put() or putIfAbsent() call that is inserting a null key or value.

2. 2

   Determine if null is valid

   Decide if null is a valid value in your use case or indicates an error.

3. 3

   Filter nulls or use a sentinel

   Check for null before inserting, use a sentinel value like empty string, or switch to HashMap if concurrency is not needed.