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POIssue with generic types of ArrayList and interface inheritance
 

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    <p>I'm having trouble understanding why the following doesn't work and I'm sure the answer is related to something basic I am not understanding and hope someone can help.</p> <p>I understand about using interfaces in an <code>ArrayList</code> such that if I have:</p> <pre><code>public interface Weapon { ... } public class Gun implements Weapon { ...} public class Knife implements Weapon { ... } </code></pre> <p>you can then insert anything that implements Weapon into the an array of weapons:</p> <pre><code>ArrayList&lt;Weapon&gt; weapons = new ArrayList&lt;Weapon&gt;(); weapons.add(new Gun()); weapons.add(new Knife(); </code></pre> <p>I get that, but what is confusing me is the understanding of why <code>ArrayList&lt;Gun&gt;</code> isn't compatible with <code>ArrayList&lt;Weapon&gt;</code> in other ways. To illustrate, the following is legal:</p> <pre><code>public void interfaceIsTheArgument(Weapon w) { ... } ... interfaceIsTheArgument(new Gun()); interfaceIsTheArgument(new Knife()); </code></pre> <p>but the following is not:</p> <pre><code>public void interfaceIsTheArgument(ArrayList&lt;Weapon&gt; w) { ... } ... interfaceIsTheArgument(new ArrayList&lt;Gun&gt;()); interfaceIsTheArgument(new ArrayList&lt;Knife&gt;()); </code></pre> <p>because the last function call reports that the method isn't applicable for its arguments. </p> <p>My question is why if the method knows it tasks an <code>ArrayList</code> with an interface as the generic type, why isn't it okay to pass in an array list of knives in that last statement?</p>
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    1. USDaniel Daranas
    1. USJerry Brady
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    1. COI don't know the exact answer, but what you can do for your second example is use `public void interfaceIsTheArgument(ArrayList<? extends Weapon>)`. That will work.
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