PECS (Producer Extends Consumer Super) - Java generic wildcards

PECS.java

public class PECS {

    public static void main(String[] args) {
//    PECS (Producer Extends Consumer Super)
//    Consumer
        List<? super Number> consumerNumbers = new ArrayList<>();
        consumerNumbers.add(Byte.valueOf((byte) 1));
        consumerNumbers.add(Short.valueOf((short) 1));
        consumerNumbers.add(Integer.valueOf((int) 1));
        consumerNumbers.add(Long.valueOf((long) 1));
        consumerNumbers.add(Float.valueOf((float) 1));
        consumerNumbers.add(Double.valueOf((double) 1));

        Object firstNumber = consumerNumbers.get(0);
//    but it doesn't work below
//      Number firstNumber = consumerNumbers.get(0);
//    Incompatible types.
//    Required: java.lang.Number
//    Found: capture<? super java.lang.Number>

//    Producer
        List<? extends Number> producerNumbers = Arrays.asList(1, 2, 3);
        Number number = producerNumbers.get(0);
//    but it doesn't work below
//      producerNumbers.add(Byte.valueOf((byte) 1));
//    add (capture<? extends java.lang.Number>) in List cannot be applied
//    to (java.lang.Byte)
    }

}