Java, should reading collections or variables be Thread Safe?

Question

Java

I heard about Collections.synchronizedList, ConcurrentHashMap, CopyOnWriteArrayList, etc.
But I have a question, why can't I read a normal collection (without changing it) from different Thread. Or the same variable if it is final.
I would be very happy and grateful if someone could explain clearly.

Answer 1

why can't I read a regular collection (without modifying it) from different Threads

You can if no one changes it. Otherwise, you may not see any changes.

Or the same variable if it is final

The more you can.

I would be very happy and grateful if someone could explain clearly.

The topic is complex, do not look for answers on the Internet, here they can deceive. Better read " Java Concurrency in Practice ".