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    <p>The hard thing about the wildcard is to realize <code>? extends Foo</code> does not mean "anything that extends Foo", but instead it means "some specific type that extends Foo". And since you are outside that definition, you have no way to know <em>which</em> specific sub-type of <code>Foo</code> it is.</p> <p>Update:</p> <p>As I said, it's complicated. Here are some comments on your code.</p> <pre><code>// a list of some specific type, and you don't know what type that is. // it's a sub-type ob Object, yes, which means that you can do // Object foo = a.get(0); , but the compiler has no way of knowing // whether it's a String so you can't pass in a String List &lt;?&gt; a = new LinkedList&lt;Object&gt;(); // same here. '?' and '? extends Object' are equivalent List &lt;? extends Object&gt; b = new LinkedList&lt;Object&gt;(); // this is a list of Objects and superclasses thereof. // since there are no superclasses of Object, this is equivalent to // List&lt;Object&gt;. And through inheritance, a String is an Object, so // you can pass it in. List &lt;? super Object&gt; c = new LinkedList&lt;Object&gt;(); </code></pre> <p>Update 2:</p> <p>The problem here is that you are dealing with fixed, but unresolveable variables.</p> <pre><code>// you can pass in a List&lt;String&gt; and a String, // but if you pass in a List&lt;?&gt;, the compiler has no idea what // '?' is and just can't substitute 'String'. // 'T' doesn't help you here, because 'T' can't match both // '?' and 'String'. static &lt;T&gt; void abc(List&lt;T&gt; a, T b) {} // this works because 'T' substitutes '?' and doesn't have to worry // about a 2nd parameter static &lt;T&gt; void def(List&lt;T&gt; a) {} </code></pre> <p>Read this question, it might shed some light on the problem:</p> <p><a href="https://stackoverflow.com/questions/2723397/java-generics-what-is-pecs">What is PECS (Producer Extends Consumer Super)?</a></p>
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    1. POJava Generics: How does method inference work when wildcard is being used in the method parameters?
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    1. COThat's right. But irregardless, that unknown subtype which extends Foo will always be a subtype of a cosmic supertype which is Object.
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    2. CO@yapkm01 well yes, but potentially a different type from the one you are passing in (String)
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    3. COMy point is, when 2 parameters being used is totally different from one another (say number and string), the ultimate inference climbing back upwards is always Object. The way i look at it is as long as you don't supply an actual type parameter, using inference irregardless of any parameter types (which could be different from one another), it always goes back to Object. Object is the main holder for everything. So it should work right?
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