Understanding wildcards in Java generics

Question

I\'m not sure why the last statement in the following code is illegal. Integer should be a subtype of ?, so why can\'t I assign it to b

Answer 1

Here is a short summary of what you can and cannot do with generics:

    List listOfAnyNumbers = null;
    List listOfNumbers = null;
    List listOfIntegers = null;
    listOfIntegers = listOfNumbers;     // Error - because listOfNumbers may contain non-integers
    listOfNumbers = listOfIntegers;     // Error - because to a listOfNumbers you can add any Number, while to listOfIntegers you cannot.
    listOfIntegers = listOfAnyNumbers;  // Error - because listOfAnyNumbers may contain non-integers  
    listOfAnyNumbers = listOfIntegers;  // OK    - because listOfIntegers is a list of ?, where ? extends Number.
    listOfNumbers = listOfAnyNumbers;   // Error - because listOfAnyNumbers may actually be List, to which you cannot add any Number. 
    listOfAnyNumbers = listOfNumbers;   // OK    - because listOfNumbers is a list of ?, where ? extends Number.