Question

Can someone please explain why the following code is not allowed:

List<Number> l = new ArrayList<Integer>();

But I can do this:

Number[] a = new Integer[10];

I am learning generics and don’t understand this. Why does the type on the left be exactly the type from the right and child types are not allowed?

TIA!

Answer 1

This is a common problem for people trying to understand Generics in Java. For a detailed explanation, read the wiki article about covariance and contravariance. As your code sample demonstrates, Java’s generic classes are neither covariant nor contravariant.

Here’s a simpler, more intuitive explanation. Let’s say

List<Number> l = new ArrayList<Integer>();

was a valid declaration. Then, through polymorphism, you could give l to a function declared as

public void foo(List<Number> list) {
    l.add(new Double(42.0));
}

According to polymorphism, foo(l) should be a perfectly valid call (Double is a Number, after all), but you’d be adding a Double to an ArrayList<Integer>, which understandable violates type safety.