Java - Reasons for Allowing null as a HashMap Key?

Question

Why Java decided to allow \'null\' as a key in HashMap? As we know hashcode() can\'t be calculated on null value. Is there any design consideration for this decision to allow nu

Answer 1

From the JDK 1.2 Java Collections API Change Summary (not sure where to find the official version on Oracle's website):

Added null-key support to HashMap. This was done for consistency with TreeMap and the late, unlamented ArrayMap, and because customers requested it. Now all of our general-purpose collection implementations accept null keys, values and elements.

Joshua Bloch and Doug Lea disagreed on this, and this caused problems for concurrent hash maps.