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POThe type T is not generic; it cannot be parameterized with arguments <?> error in a generic function
 

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  1. POThe type T is not generic; it cannot be parameterized with arguments <?> error in a generic function
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    <p>I want to create a generic function that takes any Map &amp; a String key, if the key is not present in the map, then it should create a new instance of the Value Type (which is passed) &amp; put it in the map &amp; then return it.</p> <p>Here is my implementation</p> <pre><code>public &lt;T&gt; T getValueFromMap(Map&lt;String, T&gt; map, String key, Class&lt;T&gt; valueClass){ T value = map.get(key); if (value == null){ try { value = valueClass.newInstance(); } catch (InstantiationException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace(); } map.put(key, value); } return value; } </code></pre> <p>It works if I use it with a normal (not generic) List as the value type, </p> <pre><code>Map&lt;String,List&gt; myMap; List value = getValueFromMap(myMap, "aKey", List.class) //works </code></pre> <p>but not with generic type lists</p> <pre><code>Map&lt;String,List&lt;String&gt;&gt; myMap; List&lt;String&gt; value = getValueFromMap(myMap, "aKey", List.class) //does not work </code></pre> <p>Also if I try to make <code>Map&lt;String, T&gt;</code> map parameter generic by changing it to <code>Map&lt;String, T&lt;?&gt;&gt;</code> it complains that <code>the type T is not generic; it cannot be parameterized with arguments &lt;?&gt;</code></p> <p>Can the generic parameters be themselves made generic?</p> <p>Is there any way to create function with above mentioned requirements?</p> <p>~Update</p> <p>Thanks for all the insightful answers everyone. </p> <p>I have verified that the accepted solution works for any value type with this code</p> <pre><code> Map&lt;String, Map&lt;String, List&lt;String&gt;&gt;&gt; outer = new HashMap&lt;String, Map&lt;String,List&lt;String&gt;&gt;&gt;(); Map&lt;String, List&lt;String&gt;&gt; inner = getValueFromMap(outer, "b", (Class&lt;Map&lt;String, List&lt;String&gt;&gt;&gt;)(Class&lt;?&gt;)HashMap.class); List&lt;String&gt; list = getValueFromMap(inner, "a", (Class&lt;List&lt;String&gt;&gt;)(Class&lt;?&gt;)ArrayList.class); </code></pre>
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    1. USSumit Jain
    1. USSumit Jain
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      1. POThe type T is not generic; it cannot be parameterized with arguments <?> error in a generic function
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      1. VTUpMod
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    1. COI think your problem is that when using the Class.newInstance() method no generic parameters are known.
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      1. USKevin Bowersox
    2. COI think we don't need to know the generic type, because those are removed at runtime, anyway. Please point it, if I am wrong.
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      1. USSumit Jain
 

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