Type erasure

Definition

Type erasure means generic type arguments exist only at compile time. At run time List<String> and List<Integer> are both just List, so you cannot ask a list what type it holds.

Example code

Two different lists, the same class at run time

import java.util.ArrayList;
import java.util.List;

class Demo {
    public static void main(String[] args) {
        List<String> strings = new ArrayList<>();
        List<Integer> integers = new ArrayList<>();
        System.out.println(strings.getClass() == integers.getClass());   // true: the type arguments are gone

        // Because of erasure, a raw type can sneak the wrong type in...
        @SuppressWarnings({"unchecked", "rawtypes"})
        List<String> sneaky = (List) integers;
        integers.add(42);
        try {
            String s = sneaky.get(0);                                      // ...and the cast fails here, at run time
        } catch (ClassCastException e) {
            System.out.println("ClassCastException: " + e.getMessage().substring(0, 30) + "...");
        }
    }
}

// Output:
// true
// ClassCastException: class java.lang.Integer cannot...