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    <p>The first example workds becase you are using raw type that is not recomended. </p> <p>To solve your compilation error you might want to not limit the class into T but allow somethi g super the T</p> <p><code>public &lt;T&gt; T getValueFromMap(Map&lt;String, T&gt; map, String key, Class&lt;? super T&gt; valueClass)</code></p> <p>But then you will have not save convertion and you will need to cast result of new instance into (T). And you can not use List as the class because is an iterface and you can not create a instace of it. </p> <pre><code>public &lt;K,V&gt; V getValueFromMap(Map&lt;K, V&gt; map, K key, Class&lt;? super V&gt; valueClass){ if(map.containsKey(key) == false) { try { map.put(key, (V) valueClass.newInstance()); } catch (InstantiationException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } } return map.get(key); } </code></pre> <hr> <pre><code>Map&lt;String,ArrayList&lt;String&gt;&gt; myMap = new HashMap&lt;String, ArrayList&lt;String&gt;&gt;(); List&lt;String&gt; value = getValueFromMap(myMap, "aKey", ArrayList.class); //will work </code></pre> <p><strong>EDIT</strong></p> <p>But what about the base form <code>Map&lt;String,List&lt;String&gt; myMap</code>, that OP expect ?</p> <p>As we should know <code>ArrayList</code> is super type of <code>List</code>, but <code>ArrayList&lt;String&gt;</code> is not super type of <code>List&lt;String&gt;</code> but it is assignable.</p> <p>So lets try it out: </p> <p>Example 1: <code>getValueFromMap(myMap, "aKey", ArrayList.class); //Compilation error</code> </p> <p>Example 2: <code>getValueFromMap(myMap, "aKey", List.class); //Runtime error</code></p> <p>Solution for this might be some assumption that if class is List then create ArrayList. </p> <p>This force us to create some <em>not decent code</em>. </p> <pre><code>public static &lt;V&gt; V createInstace(Class&lt;V&gt; valueClass) throws InstantiationException, IllegalAccessException { if(List.class.isAssignableFrom(valueClass)) { return (V) new ArrayList&lt;V&gt;(); } return valueClass.newInstance(); } </code></pre> <p>This code do not really have anything common with generics but is working. </p> <p>At the end we finish with </p> <pre><code>public static void main(String[] args) { Map&lt;String,List&lt;String&gt;&gt; myMap = new HashMap&lt;String, List&lt;String&gt;&gt;(); List&lt;String&gt; value = getValueFromMap(myMap, "aKey", List.class); //does not work value.add("Success"); System.out.println(value); } public static &lt;K,V&gt; V getValueFromMap(Map&lt;K, V&gt; map, K key, Class&lt;? super V&gt; valueClass){ if(map.containsKey(key) == false) { try { map.put(key, (V) createInstace(valueClass)); } catch (InstantiationException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } } return map.get(key); } public static &lt;V&gt; V createInstace(Class&lt;V&gt; valueClass) throws InstantiationException, IllegalAccessException { if(List.class.isAssignableFrom(valueClass)) { return (V) new ArrayList&lt;V&gt;(); } return valueClass.newInstance(); } </code></pre> <p>And the result if this is <code>[Sucess]</code>.</p>
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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. USDamian Leszczyński - Vash
    1. USDamian Leszczyński - Vash
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    1. COYes, but for this to work we have to keep the value type of the map to be the concrete type..we can not use Map<String,List<String>> myMap here according to my compiler, but I still don't understand the reason for it, why can't Class<? super T> take ArrayList.class when T is List<String> if it can take it when T is ArrayList<String>, as List is supertype of ArrayList).
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      1. USSumit Jain
    2. COThe reason of that is you can not create instace of List, you could have a type initialized that in case of List creates ArrayList for Set HashSet for Map a HashMap. But this might corrupte expected part of code where a LinkedList is required.
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      1. USDamian Leszczyński - Vash
    3. COno, you didn't get my question, I know we have to pass ArrayList in the function parameter, but I am asking you about the map declaration, why can't Map<String,List<String>> work here?
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      1. USSumit Jain
 

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