The program you tested just uses map as an ordinary object lock. The meaning of the book is that it is not possible to lock the map and then attempt to perform a series of operations on the map to achieve the effect of atomic operations.

It must be the same lock

Since ConcurrentHashMap cannot be locked to perform exclusive access, we cannot use client-side locking to create new atomic operations, such as adding atomic operations "if not added" to Vector in Section 4.4.1.

I was really confused when I first understood this sentence. Finally, I turned the book back to the "4.4.1" example, and I understood it after reading it.

Client locking and external locking must be the same lock.

//内部加锁机制为分段锁，锁不在map实例上
private static ConcurrentHashMap<String, String> map = new ConcurrentHashMap<String, String>();
//外部加锁是在map 实例上的
sychronized (concurrentHashMap) {
    //
}

The ConcurrentHashMap synchronization mechanism has an internal segment lock, and the external lock is not the same lock as it.

Therefore, when accessed by multiple threads, the atomicity of a thread performing a composite operation of "add if not added" on the map cannot be guaranteed, because another thread may perform an "add" operation on the map.

My personal understanding of the above is that if I have misunderstood something wrong, I welcome your guidance.