import java.util.*; public class CovariantLists { public static void main(String[] args) { { // Invariant List<String> ss = new ArrayList<>(); ss.add("XYZ"); // OK List<Object> os = ss; // compile error } { // Covariant List<String> ss = new ArrayList<>(); List<? extends Object> os = ss; // OK os.add(42); // compile error } } }
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