The question mark in extends declaration

In java extends declaration syntax we often see something like this:

The question mark also show up on other place, inside an angle bracket. It means any class, its a wildcard. Its the Generic of Java. It called bounded wildcard.

What is immutable object in Java?

An immutable object in Java is the object that can not be changed after it is initialized ... . For example String object is immutable because you can not change String object's content ... another object "World" and assign the reference to str variable. Being immutable is a design choice ... immutable collection. [Why use immutable object?] >They are thread safe, because their state
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