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Q21.What is a transient variable?
A transient variable is a variable that may not be serialized.

Q22.Which containers use a border Layout as their default layout?
The window, Frame and Dialog classes use a border layout as their default layout.

Q23.Why do threads block on I/O?
Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.

Q24.How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.