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I've seen the wildcard used before to mean any object - but recently saw a use of:
<? extends Object> Since all objects extend Object, are these two usages synonymous?
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The extends keyword extends a class (indicates that a class is inherited from another class). In Java, it is possible to inherit attributes and methods from one class to another. We group the "inheritance concept" into two categories: subclass (child) - the class that inherits from another class.
super is a lower bound, and extends is an upper bound.
super E> , it means "something in the super direction" as opposed to something in the extends direction.
Producer – If you want to only retrieve the elements from a generic collection, use extends . Consumer – If you want to only put elements into a generic collection, use super . If you do both retrieve and put operations with the same collection, you shouldn't use either extends or super .
<?> 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 ) If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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