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copied!<p>There are certain places, where wildcards, and type parameters do the same thing. But there are also certain places, where you have to use type parameters. </p> <ol> <li>If you want to enforce some relationship on the different types of method arguments, you can't do that with wildcards, you have to use type parameters. </li> </ol> <p>Taking your method as example, suppose you want to ensure that the <code>src</code> and <code>dest</code> list passed to <code>copy()</code> method should be of same parameterized type, you can do it with type parameters like so:</p> <pre><code>public static <T extends Number> void copy(List<T> dest, List<T> src) </code></pre> <p>Here, you are ensured that both <code>dest</code> and <code>src</code> have same parameterized type for <code>List</code>. So, it's safe to copy elements from <code>src</code> to <code>dest</code>.</p> <p>But, if you go on to change the method to use wildcard:</p> <pre><code>public static void copy(List<? extends Number> dest, List<? extends Number> src) </code></pre> <p>it won't work as expected. In 2nd case, you can pass <code>List<Integer></code> and <code>List<Float></code> as <code>dest</code> and <code>src</code>. So, moving elements from <code>src</code> to <code>dest</code> wouldn't be type safe anymore. If you don't need such kind of relation, then you are free not to use type parameters at all.</p> <p>Some other difference between using wildcards and type parameters are:</p> <ul> <li>If you have only one parameterized type argument, then you can use wildcard, although type parameter will also work.</li> <li>Type parameters support multiple bounds, wildcards don't.</li> <li><p>Wildcards support both upper and lower bounds, type parameters just support upper bounds. So, if you want to define a method that takes a <code>List</code> of type <code>Integer</code> or it's super class, you can do:</p> <pre><code>public void print(List<? super Integer> list) // OK </code></pre> <p>but you can't use type parameter:</p> <pre><code> public <T super Integer> void print(List<T> list) // Won't compile </code></pre></li> </ul> <hr /> <p><strong>References:</strong></p> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeArguments.html#FAQ203" rel="noreferrer">Angelika Langer's Java Generics FAQs</a></li> </ul>

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