Why cannot a List<Integer> be passed to a method accepting List<Number>?

Allowing that substitution would let the method insert a Double into a list promised to contain only Integer values. The rejection protects the caller’s collection. For a reader that sums numeric input, accept List<? extends Number> and read each value as Number. If the method must insert integers into a destination, consider List<? super Integer> instead. Choose the signature from the direction of data flow, not from a cast that happens to suppress a compiler message. Compile a caller using Integer and another using Double for the read-only version. Confirm the producer version accepts the intended destination. Wildcards do not make a collection immutable, and forcing a raw List merely postpones type errors until later operations.