Mutating a hash map in Java while another thread reads it won't cause bad things like:

(a) memory you don't have access to to be read;

(b) other threads to crash;

(c) the VM state to be corrupted;

(d) the GC to crash;

and so on. The Java world is still in a consistent state. In a security-oriented world, this is important.

To the reply: It's a bit pedantic for me to say this, I know, but that's still "a consistent state". Basically, a Java program can't do anything it wouldn't otherwise be able to do by modifying a hashmap concurrently. Java programs can go into infinite loops. They can't access uninitialized memory.