Question

ArrayList<Integer> arrI = new ArrayList<Integer>();
ArrayList arrO = arrI; // Warning
/*  It is ok to add a String as it is an ArrayList of Objects
    but the JVM will know the real type, arrO is an arrayList of
    Integer... 
*/
arrO.add("Hello"); 
/*  How I can get a String in an ArrayList<Integer> ?? 
    Even if the compiler told me that I will get an Integer!
*/
System.out.println(arrI.get(0));

Anybody can explain what’s happening here?

Answer 1

The JVM doesn’t know the real type, because generics are implemented via type erasure.

In terms of bytecode (and therefore runtime behaviour), your code is equivalent to:

ArrayList arrI = new ArrayList();
ArrayList arrO = arrI;
arrO.add("Hello"); 
System.out.println(arrI.get(0));