I have the following code working and I'm not sure why it doesn't throw an exception:
List[] x=new ArrayList[2];
x[0]=new ArrayList<Integer>();
x[1]=new ArrayList<String>();
x[0].add("test");
x[1].add(5);
System.out.println(x[0]);
System.out.println(x[1]);
This prints:
[test]
[5]
x[0] is array of integer and it add String without any issue. x[1] is array of String and it add integer without any issue.
How does that work?
First of all, this has nothing to do with arrays. The following exhibits the same behaviour:
List x = new ArrayList();
x = new ArrayList<Integer>();
x.add("test");
System.out.println(x);
The key reason this behaves the way it does is that x is declared to have the raw List type. For a comprehensive discussion, see What is a raw type and why shouldn't we use it?
You have declared an array of raw typed lists. Generic values do not exist at runtime, after you have compiled your code. So basically your lists are of type List<Object>.
"test" is a String, which is basically an Object5 gets autoboxed to an Integer which is also an ObjectSo your compiler will not throw any error. If you are using an IDE it might be warn you:
Unchecked call to 'add(E)' as a member of raw type `java.util.List'
(From IntelliJ)