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POJava Generics: How does method inference work when wildcard is being used in the method parameters?
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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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    copied!<p>Supposedy i have the following: </p> <pre><code>class x { public static void main(String [] args) { List &lt;?&gt; a = new LinkedList&lt;Object&gt;(); List &lt;? extends Object&gt; b = new LinkedList&lt;Object&gt;(); List &lt;? super Object&gt; c = new LinkedList&lt;Object&gt;(); abc(a, "Hello"); // (1) Error abc(b, "Hello"); // (2) Error abc(c, "Hello"); // (3) ok def(b); // (4) ok // Showing inference at work Integer[] a = {10, 20, 30}; // (5) T is inferred to be ? extends Object Method signature: ppp(? extends Object, ? extends Object[]) Method call signature: ppp(String, Integer[]); ppp("Hello", a); // ok } static &lt;T&gt; void abc(List&lt;T&gt; a, T b) {} static &lt;T&gt; void def(List&lt;T&gt; a) {} static &lt;T&gt; void ppp(T t1, T[] t2){} } </code></pre> <p>To begin with, look at clause 5 showing inference at work. Now clause 5 section is a working section.</p> <p>If that is what it is, then why does clause (1) &amp; (2) have errors? </p> <p>From my view, all these 3 methods calling have the same inference generated since no actual type parameters is used on the abc method call.</p> <p>method parameter &#060;T> abc (List &#060;T> a, T b>)<br> inferred &#060;Object> abc (List &#060;Object>, Object) // (4) </p> <p>Please bear in mind, method abc() and def() is my method. Compiler doesn't know what i want to do with the List in this method. I might just print the list size or might not even do anything at all as shown above. So there is no get or set involved here.</p> <p>CONTINUATION --> This is getting very confusing for me.</p> <pre><code>class y { public static void main(String [] args) { List &lt;Integer&gt; a = new LinkedList&lt;Integer&gt;(); List &lt;Object&gt; b = new LinkedList&lt;Object&gt;(); ppp("Hello", new Integer(1)); // (20) ok qqq("Hello", a); // (21) error qqq("Hello", b); // (22) ok } static &lt;T&gt; void ppp(T t1, T t2) {} static &lt;T&gt; void qqq(T t1, List &lt;T&gt; t2) {} } </code></pre> <p>Note that clause 21 is the same as clause 20 except 2nd parameter is being made to be a List instead of Integer.</p> <p>Clause 20 is ok cos' T is inferred to be Object.<br> Clause 22 is ok. Same reason as clause 20.<br> Clause 21 failed? T could also be inferred to be Object too - would work too?</p>
 

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