This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Cat is one kind of Animal,here Cat inherits all the properties of animal and it can have it's own properties.So if you are extending one class from super class you are deriving one class specific to that super class.
So if you are extending Thread you should be able to create specific kind Thread.That is why it is not good OO design.