A brief example of PECS in Java generics - Producer extends, Consumer super

PECS.text

//numberList is a producer of subtypes of numbers. Hence is generified with ? extends Number. Producer->Extends
		final List<? extends Number> numberList = DoubleStream.of(-99, 1, 2, 3, 4, 5, 6)
				.boxed().collect(Collectors.toList());
		//numberList.add(new Integer(1)); //Cannot add integer to this list, even though integer is a subtype of number.
		numberList.add(null);//Cannot add anything to numberList other than null
		final Number number = numberList.get(0);//Can only get values as Number type
		System.out.println(number.doubleValue());

		//integerList is a consumer of supertypes of integers. Hence is generified with ? super Integer. Consumer->Super
		final List<Object> doubleList = IntStream.of(99, 1, 2, 3, 4, 5, 6).boxed()
				.collect(Collectors.toList());
		final List<? super Integer> integerList = doubleList; //List<Object> can be assigned
		//integerList.add(new Object()); //Cannot add Object to the list, even though Object is a super type of Integer
		integerList.add(1);//Integers can be added
		//final Integer integer = integerList.get(0); //Cannot get values as Integer or Number
		final Object integer = integerList.get(0); //Can only get value in an  Object reference
		System.out.println(integer.toString());