Type Erasure in Java

Type erasure in Java is a concept that has always eluded me. I will read a StackOverflow answer on the topic, and I'll be more lost than Chuck Noland.

I decided to research the issue deeper when I stumbled upon an IDE warning in IntelliJ: Unchecked assignment: 'java.lang.Class' to 'java.lang.Class<T>', and this will usually happen when I do something like this, for example:

List<String> userNames = new ArrayList();

WELL, DUH! I could have used the diamond operator on new ArrayList and invoked Java's type inference, like so:

List<String> userNames = new ArrayList<>();

But for now, I can't. I explain at the bottom. So deal with it.

The above solution will allow the compiler to infer the type arguments from the surrounding context (i.e. the List<String> in this example). The reason the IDE throws out a warning is due to a runtime check cast, so let's examine the problem a little further.

Without using Generics, we're instantiating an empty ArrayList of Objects. Therefore, at runtime, there's the possibility that we may query our non-typed ArrayList, and cast the return value to something other than a String object or, as an example, cast an Integer to a String. There's no way of finding out that <T> = String for the ArrayList object.

This possbility occurs due to type erasure.

So, what exactly is type erasure?

From Baeldung, "type erasure can be explained as the process of enforcing type constraints only at compile time and discarding the element type information at runtime."

At compile-time, the compiler has full type information availiable, but the compiler will intentionally drop the information when the byte code is generated. Usually, the Java compiler will replace all type parameters with Object, unless the type parameter is bounded. The produced byte code contains only ordinary classes, interfaces, and methods.

Thus, due to type erasure, our list will be compiled into:

List userNames = new ArrayList();

Beforehand, the IDE throws the warning, since at runtime, the compiler has dropped the type information, and there's no way of ensuring type safety. The compiler has to enfore type casts to preserve type safety. The compiler will transform our non-Generic list, when added to and queried from, into:

List userNames = new ArrayList();
String mySuperCoolUserName = (String) userNames.get(0);

If the ArrayList happens to hold something other than a String, a ClassCastException would occur. For example:

List myList = new ArrayList();
myList.add("My list will be the best list!");

Object myFavString = (String) myList.get(1);

You can read more here:

If I'm still confused, please contact me so I can fix my reasoning!


The issue is caused due to a slippery monster dubbed Beanshell. I don't think it's website has been updated since 2005. To take you back, The Chronicles of Narnia: The Lion, the Witch and the Wardrobe was the popular film. In short, Beanshell is a scripting language for Java often used inside XML. Well, the parser has a hard time detecting and translating the diamond operator so Beanshell scripts will usually throw an exception as seen below:

... ParseException: Encountered > at line....
© 2021, Johan Oakes