What's the difference between <?> and <? extends Object> in Java Generics?

Solution 1:

<?> and <? extends Object> are synonymous, as you'd expect.

There are a few cases with generics where extends Object is not actually redundant. For example, <T extends Object & Foo> will cause T to become Object under erasure, whereas with <T extends Foo> it will become Foo under erasure. (This can matter if you're trying to retain compatibility with a pre-generics API that used Object.)

Source: http://download.oracle.com/javase/tutorial/extra/generics/convert.html; it explains why the JDK's java.util.Collections class has a method with this signature:

public static <T extends Object & Comparable<? super T>> T max(
    Collection<? extends T> coll
)

Solution 2:

Although <?> is supposed to be a shortcut for <? extend object>, there is a tiny difference between the two. <?> is reifiable while <? extend object> is not. The reason they did this is to make it easier to distinguish reifiable type. Anything that looks like <? extends something>,<T>,<Integer> are nonreifiable.

For example, this code would work

List aList = new ArrayList<>();
boolean instanceTest = aList instanceof List<?>;

but this gives an error

List aList = new ArrayList<>();
boolean instancetest = aList instanceof List<? extends Object>;

for more info read Java generics and collections by Maurice Naftalin