What is the difference between ? and Object in Java generics?

An instance of HashMap<String, String> matches Map<String, ?> but not Map<String, Object>. Say you want to write a method that accepts maps from Strings to anything: If you would write

public void foobar(Map<String, Object> ms) {
    ...
}

you can't supply a HashMap<String, String>. If you write

public void foobar(Map<String, ?> ms) {
    ...
}

it works!

A thing sometimes misunderstood in Java's generics is that List<String> is not a subtype of List<Object>. (But String[] is in fact a subtype of Object[], that's one of the reasons why generics and arrays don't mix well. (arrays in Java are covariant, generics are not, they are invariant)).

Sample: If you'd like to write a method that accepts Lists of InputStreams and subtypes of InputStream, you'd write

public void foobar(List<? extends InputStream> ms) {
    ...
}

By the way: Joshua Bloch's Effective Java is an excellent resource when you'd like to understand the not so simple things in Java. (Your question above is also covered very well in the book.)

Another way to think about this problem is that

HashMap<String, ?> hash1;

is equivalent to

HashMap<String, ? extends Object> hash1;

Couple this knowledge with the "Get and Put Principle" in section (2.4) from Java Generics and Collections:

The Get and Put Principle: use an extends wildcard when you only get values out of a structure, use super wildcard when you only put values into a structure, and don't use a wildcard when you both get and put.

and the wild card may start making more sense, hopefully.

It's easy to understand if you remember that Collection<Object> is just a generic collection that contains objects of type Object, but Collection<?> is a super type of all types of collections.

The answers above covariance cover most cases but miss one thing:

"?" is inclusive of "Object" in the class hierarchy. You could say that String is a type of Object and Object is a type of ?. Not everything matches Object, but everything matches ?.

int test1(List<?> l) {
  return l.size();
}

int test2(List<Object> l) {
  return l.size();
}

List<?> l1 = Lists.newArrayList();
List<Object> l2 = Lists.newArrayList();
test1(l1);  // compiles because any list will work
test1(l2);  // compiles because any list will work
test2(l1);  // fails because a ? might not be an Object
test2(l2);  // compiled because Object matches Object

You can't safely put anything into Map<String, ?>, because you don't know what type the values are supposed to be.

You can put any object into a Map<String, Object>, because the value is known to be an Object.