How and when check whether key is null in a hashtable

Question

As we know, null is not allowed in Hashtable . But when I checked the source code of Hashtable (jdk 1.8). I only saw the check of value and couldn't find the key check. Here is the source code below of the put method:

public synchronized V put(K key, V value) {
    // Make sure the value is not null
    if (value == null) {
        throw new NullPointerException();
    }

    // Makes sure the key is not already in the hashtable.
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry<K,V> entry = (Entry<K,V>)tab[index];
    for(; entry != null ; entry = entry.next) {
        if ((entry.hash == hash) && entry.key.equals(key)) {
            V old = entry.value;
            entry.value = value;
            return old;
        }
    }

    addEntry(hash, key, value, index);
    return null;
}

Answer 1

The key check is here:

int hash = key.hashCode();

This will throw a NullPointerException if the key is null.