generics - Unbounded wildcards in Java

Question

Is there ever a difference between an unbounded wildcard e.g. <?> and a bounded wildcard whose bound is Object, e.g. <? extends Object>?

I recall reading somewhere that there was a difference in the early drafts of generics, but cannot find that source anymore.

Answer 1

From a practical point to most people, <? extends Object> is the same as <?>, like everyone have suggested here.

However, they differ in two very minor and subtle points:

1. The JVMS (Java Virtual Machine Specification) has a special specification for the unbounded wildcards, as ClassFileFormat-Java5 specifies that unbounded wildcard gets encoded as *, while encodes a Object-bounded wildcard as +Ljava/lang/Object;. Such change would leak through any library that analyzes the bytecode. Compiler writers would need to deal with this issue too. From revisions to "The class File Format"

2. From reifiablity standpoint, those are different. JLS 4.6 and 4.7 codify List<?> as a reifiable type, but List<? extends Object> as a erasured type. Any library writer adding .isReifiable() (e.g. mjc lib) needs to account for that, to adhere to the JLS terminology. From JLS 4.6 and 4.7.