java generics covariance

by 
List<Integer> li = new ArrayList<Integer>();
List<Number> ln = li; // illegal
ln.add(new Float(3.1415));

In Java, Integer inherits from Number(java.lang.Number), so intuitively, anything that is an Integer(java.lang.Integer) is also a number, but what that article points out is that with generics it does not work that way, because considering that example, you could end up putting a float (which is a Number) into a List<Integer>, which is illegal because a float is not an integer.

Conclusion: Generics are not covariant.

Note: I recommend you read Effective Java (2nd Edition) Chapter 5: Generics.

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