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    <p>The big difference between</p> <pre><code>public &lt;T extends Animal&gt; void takeThing(ArrayList&lt;T&gt; list) </code></pre> <p>and</p> <pre><code>public void takeThing(ArrayList&lt;? extends Animal&gt; list) </code></pre> <p>is that in the former method you can refer to "T" within the method as the concrete class that was given. In the second method you cannot do this.</p> <p>Here a more complex example to illustrate this:</p> <pre><code>// here i can return the concrete type that was passed in public &lt;T extends Animal&gt; Map&lt;T, String&gt; getNamesMap(ArrayList&lt;T&gt; list) { Map&lt;T, String&gt; names = new HashMap&lt;T, String&gt;(); for (T animal : list) { names.put(animal, animal.getName()); // i assume there is a getName method } return names; } // here i have to use general Animal public Map&lt;Animal, String&gt; getNamesMap(ArrayList&lt;? extends Animal&gt; list) { Map&lt;Animal, String&gt; names = new HashMap&lt;Animal, String&gt;(); for (Animal animal : list) { names.put(animal, animal.getName()); // i assume there is a getName method } return names; } </code></pre> <p>With the first method if you pass in an List of Cats you get a Map with Cat as key. The second method would always return a Map with general Animal key.</p> <p>By the way this is not valid java syntax:</p> <pre><code>public &lt;? extends Animal&gt; void takeThing(ArrayList&lt;?&gt; list) </code></pre> <p>Using this form of generic method declaration you have to use a valid java identifier and not "?".</p> <p>Edit:</p> <p>The form "? extends Type" only applies to variable or parameter type declaration. Within a generic method declration it has to be "Identifier extends Type" as you are able to refer to the "Identifier" from within your method.</p>
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    1. POWhen to use wildcards in Java Generics?
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    1. COThe big difference is not that you can not refer to ? as a type. If that were the only thing ? is good for, then it would be completely useless.
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      1. USbennidi
    2. CO@bennidi: What big differences do you see else in the case of a method declaration? You can give the same parameters in both cases. The generic method declaration gives you the benefit of refering to the type which enables higher type safty (which is definitly not useless).
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    3. COType safety is what you get when you use an identifier and refer to that within you method signature. I didn't say that this was useless. I said that "?" would be useless if its only capabilities were just to not be referable in the rest of the method. The big difference is that you can't add anything to List<? extends Animal>. Please, See me answer below.
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