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.
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