Multiple inheritance can be a serious mess and has been a point of contention in the C++ world, therefore Java has removed the notion of multiple inheritance. However, you can implement multiple interfaces.
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Do you mean multiple inheritance? If so, preventing multiple inheritance avoids many of the associated problems such as where D extends B and C both of which extend A. If you call a method on D which is implemented in B and C, which implementation do you choose? Multi level inheritance (e.g. C extends B extends A) is allowed.