Why is creating a Thread said to be expensive?

Question

The Java tutorials say that creating a Thread is expensive. But why exactly is it expensive? What exactly is happening when a Java Thread is created that makes its creation

Answer 1

Creating Threads requires allocating a fair amount of memory since it has to make not one, but two new stacks (one for java code, one for native code). Use of Executors/Thread Pools can avoid the overhead, by reusing threads for multiple tasks for Executor.