StackOverflow2013

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PO
primarykey
Id
8621258
data
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0
AnswerCount
0
ClosedDate
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CreationDate
2011-12-23T22:41:23.070
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2012-01-02T23:51:37.927
LastEditDate
2017-05-23T12:17:44.467
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256196
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8621226
PostTypeId
2
Score
13
ViewCount
0
LastEditorDisplayName
text
Body
To "fix" the code, you need to use a generic bound: <pre><code>public void interfaceIsTheArgument(List<? extends Weapon> w) { ... } ... interfaceIsTheArgument(new ArrayList<? extends Weapon> ()); interfaceIsTheArgument(new ArrayList<Knife>()); </code></pre> The key reason is <code>List<Gun></code> is not a subclass of <code>List<Weapon></code>. The reason for this fact can be illustrated by this code: <pre><code>List<Gun> guns = new ArrayList<Gun>(); // If List<Weapon> was a super type of List<Gun>, this next line would be allowed List<Weapon> weapons = guns; // Won't compile, but let's assume it did weapons.add(new Knife()); // Compiles, because Knife is a Weapon Gun gun = guns.get(0); // Oops! guns.get(0) is a Knife, not a Gun! </code></pre> By using the bound <code><? extends Weapon></code>, we are saying we'll accept any generic type that is a subclass of <code>Weapon</code>. Using bounds can be very powerful. This kind of bound is an upper bound - we are specifying the top-level class as being <code>Weapon</code>. There's also a lower bound, that uses this syntax: <pre><code>List<? super Weapon> // accept any type that is a Weapon or higher in the class hierarchy </code></pre> So, when to use each one? Remember this word <code>PECS</code>: "Producer extends, consumer super". This means on the producer side of the code (where the objects are created) use <code>extends</code>, and on the consumer side of the code (where the objects are used) us <code>super</code>. Once you try it a few times, you'll understand through experience why it works well. <a href="https://stackoverflow.com/questions/2723397/java-generics-what-is-pecs">This SO question/answer</a> covers it well.
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1. POIssue with generic types of ArrayList and interface inheritance
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