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PECS (producer extends, consumer super) is an easy way to remember how to use the keywords "extends" and "super" in generics.

First of all, <? extends Number> means that Number and Integer are valid types, but not Object. <? super Number> means that Number and Object are valid types, but not Integer.

The rule is that <? extends Number> should be used for producers, whereas <? super Number> should be used for consumers.

A producer is used to insert values, whereas a consumer is used to read values.

Some examples will make it clear.

• A producer like <? extends Number> can contain any objects of type Number or subclass, and can be used to read any of them.
  

  	List<? extends Number> list = new ArrayList<>(
  		Arrays.asList(new Integer[]{1,2,3}));
  	Number num = list.get(1);
  

  However I cannot insert any value:

  	list.add(1); // Compilation error
  

  Null is the only value that can be inserted:

  	list.add(null);	// Ok!
  

• A consumer like <? super Number> can contain any objects of type Number or superclass, and can be used to insert any subclasses.

  	List<? super Number> list = new ArrayList<>());
  	list.add(1);
  	list.add(4.0f);	
  

  However the following instruction generates a compilation error:

  	Number num = list.get(1); // Compilation error
  

  The only way to read a producer is by using Object:

  	Object obj = list.get(1); // Ok!
  

Consumers and producers can be used as parameters:

	public void myMethod(List<? super Number> list) {

You should use:

• <? extends Number> when it is a producer
• <? super Number> when it is a consumer
• <Number> when it is both a consumer and a producer