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POJava generics infers Object for nested method call instead of T if nested method argument is not typesafe
 

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  1. POJava generics infers Object for nested method call instead of T if nested method argument is not typesafe
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    <p>today I have ran into strange <code>javac</code> behavior regarding generic type inference. Here's example class to ilustrate this strange behavior:</p> <pre><code>import java.util.Map; import java.util.Collections; import java.util.HashMap; public class Test { protected &lt;T&gt; T strange(T t, Map&lt;String, String&gt; map) { return t; } protected void ok(Map&lt;String, String&gt; map) {} protected &lt;T&gt; T test(T t) { T res = strange(t , new HashMap&lt;String, String&gt;()); //Doesn't work //res = strange(t, new &lt;String, String&gt;HashMap()); ok(new &lt;String, String&gt;HashMap()); res = strange(t, Collections.&lt;String, String&gt;emptyMap()); //Doesn't work //res = strange(t, Collections.EMPTY_MAP); res = strange(t, (Map&lt;String, String&gt;) Collections.EMPTY_MAP); ok(Collections.EMPTY_MAP); return res; } } </code></pre> <p>Notice <code>//Doesn't work</code> comments. If you uncomment this code you will get strange compiler error:</p> <pre><code>Test.java:18: error: incompatible types res = strange(t, Collections.EMPTY_MAP); ^ required: T found: Object where T is a type-variable: T extends Object declared in method &lt;T&gt;test(T) </code></pre> <p>Strange thing is that error complains about <code>strange</code> method return type is Object and not T, but when second unsafe parameter is cast to correct type then T is inferred correctly. </p> <p>Can someone explain if this is correct behavior? Because it looks strange to me. Can it be a compiler bug? </p> <p>Why this line works</p> <pre><code>T res = strange(t , new HashMap&lt;String, String&gt;()); </code></pre> <p>and this one is doesn't?</p> <pre><code>T res = strange(t, new &lt;String, String&gt;HashMap()); </code></pre> <p>I have tested it with java 7 and 6.</p>
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    1. USAntanas
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    1. COJava has syntax rules. This `new HashMap<String, String>());` is completely different than `new <String, String>HashMap());`.
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      1. USSotirios Delimanolis
    2. COWhat is the difference?
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    3. COOh I see now. It is described in your [comment](http://stackoverflow.com/questions/18688882/what-is-the-purpose-of-type-arguments-in-constructor-call-following-new/18688969#18688969). But this doesn't answer my original question about problem that Object is infered instead of T
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      1. USAntanas
 

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