I am having trouble understanding the following article: http://www.ibm.com/developerworks/java/library/j-jtp01255.html Under, Generics are not covariant the author states, Because ln is a List, adding a Float to it seems perfectly legal. But if ln were aliased with li, then it would break the type-safety promise implicit in the definition of li -- that it is a list of integers, which is why generic types cannot be covariant. I can't understand the part where it says "if ln were aliased with li". What does the author means by alias?(reference?). The code snippet above the quoted line seems to illustrate WHAT is illegal in java and not WHY. It would be very helpful to me if somebody could explain with an example. Thanks in advance.