Java 11 not throwing ClassCastException when casting list of HashMap to list of String

Question

I was expecting ClassCastExpection when running this code in Java 11 but it executes without an issue is there an explanation for this case?

List<?> list = List.of(new HashMap<>());
List<String> listOfStrings = (List<String>) list;
System.out.println("List size: " + listOfStrings.size());

Try `System.out.println("List element[0]: " + listOfStrings.get(0));` — Elliott Frisch, Jun 19 '23 at 13:07
This is normal. There's a reason you get a compiler warning for casts like that, and it's because strange behavior can happen, as you've observed. — Louis Wasserman, Jun 19 '23 at 13:11
Related: [Java generics type erasure: when and what happens?](https://stackoverflow.com/questions/339699/java-generics-type-erasure-when-and-what-happens) — Federico klez Culloca, Jun 19 '23 at 13:15
This problem is called [heap pollution](https://en.wikipedia.org/wiki/Heap_pollution). — Jesper, Jun 19 '23 at 14:30

score 7 · Answer 1 · answered Jun 19 '23 at 13:01

7

Java generics are erased at runtime, so that cast cannot be checked. You'll get your ClassCastException once you access a not-String from the resulting list. This is why this code generates an unchecked cast warning.

answered Jun 19 '23 at 13:01

Jorn

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Java 11 not throwing ClassCastException when casting list of HashMap to list of String

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