Why don't you get a compiler error when assign raw types to generic types?

Question

I have been looking at OJCPA code snippets and I am confused why the compiler does not throw an error at the following code.

List l = new ArrayList();
l.add("a");
l.add("b");
l.add(1);

List<String> strList = new ArrayList<>();
strList = l; //#1 - Why does the assignment compile?

for(String s: strList) {
      System.out.println(s); //#2 - It makes sense that it then throws a ClassCastException
}

I thought that the compiler would see List l as a raw type and because generics are invariant it would produce a compiler error, as it is not of type List< String >.

Thanks for you help.

Answer 1

It is allowed for backwards compatibility.

Suppose that you are calling a legacy method that returns a List of Strings, but it was written before generics were added to Java, so it returns a raw List.

You'd want this line to pass compilation :

List<String> strList = someOldMethodThatReturnsRawList();

Otherwise you'll have to keep using the raw List type in your new code in order to call that method.