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    <p><em>Multi-level wildcards</em> can be a bit tricky at times, when not dealt with properly. You should first learn how to read a multi-level wildcards. Then you would need to learn to interpret the meaning of <code>extends</code> and <code>super</code> bounds in multi-level wildcards. Those are important concepts that you must first learn before starting to use them, else you might very soon go mad.</p> <p><strong>Interpreting a multi-level wildcard:</strong></p> <p>**Multi-level wildcards* should be read top-down. First read the outermost type. If that is yet again a paramaterized type, go deep inside the type of that parameterized type. The understanding of the meaning of <em>concrete parameterized type</em> and <em>wildcard parameterized type</em> plays a key role in understand how to use them. For example:</p> <pre><code>List&lt;? extends Number&gt; list; // this is wildcard parameterized type List&lt;Number&gt; list2; // this is concrete parameterized type of non-generic type List&lt;List&lt;? extends Number&gt;&gt; list3; // this is *concrete paramterized type* of a *wildcard parameterized type*. List&lt;? extends List&lt;Number&gt;&gt; list4; // this is *wildcard parameterized type* </code></pre> <p>First 2 are pretty clear. </p> <p>Take a look at the 3rd one. How would you interpret that declaration? Just think, what type of elements can go inside that list. All the elements that are capture-convertible to <code>List&lt;? extends Number&gt;</code>, can go inside the outer list:</p> <ul> <li><code>List&lt;Number&gt;</code> - Yes</li> <li><code>List&lt;Integer&gt;</code> - Yes</li> <li><code>List&lt;Double&gt;</code> - Yes</li> <li><code>List&lt;String&gt;</code> - <strong>NO</strong></li> </ul> <p><strong>References:</strong></p> <ul> <li><a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1.10" rel="noreferrer">JLS §5.1.10 - Capture Conversion</a></li> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/TechnicalDetails.html" rel="noreferrer">Java Generics FAQs - Angelika Langer</a> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/TechnicalDetails.html#Wildcard%20Capture" rel="noreferrer">Wildcard Capture</a></li> </ul></li> <li><a href="http://www.ibm.com/developerworks/java/library/j-jtp04298/index.html" rel="noreferrer">IBM Developer Works Article - Understanding Wildcard Captures</a></li> </ul> <p>Given that the 3<sup>rd</sup> instantiation of list can hold the above mentioned type of element, it would be wrong to assign the reference to a list like this:</p> <pre><code>List&lt;List&lt;? extends Number&gt;&gt; list = new ArrayList&lt;List&lt;Integer&gt;&gt;(); // Wrong </code></pre> <p>The above assignment should not work, else you might then do something like this:</p> <pre><code>list.add(new ArrayList&lt;Float&gt;()); // You can add an `ArrayList&lt;Float&gt;` right? </code></pre> <p>So, what happened? You just added an <code>ArrayList&lt;Float&gt;</code> to a collection, which was supposed to hold a <code>List&lt;Integer&gt;</code> only. That will certainly give you trouble at runtime. That is why it's not allowed, and compiler prevents this at compile time only.</p> <p>However, consider the 4<sup>th</sup> instantiation of multi-level wildcard. That list represents a family of all instantiation of <code>List</code> with type parameters that are subclass of <code>List&lt;Number&gt;</code>. So, following assignments are valid for such lists:</p> <pre><code>list4 = new ArrayList&lt;Integer&gt;(); list4 = new ArrayList&lt;Double&gt;(); </code></pre> <p><strong>References:</strong></p> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeArguments.html#What%20do%20multilevel%20wildcards%20mean?" rel="noreferrer">What do multi-level wildcards mean?</a></li> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#What%20is%20the%20difference%20between%20various%20multi-level%20instantiations?" rel="noreferrer">Difference between a <code>Collection&lt;Pair&lt;String,Object&gt;&gt;</code>, a <code>Collection&lt;Pair&lt;String,?&gt;&gt;</code> and a <code>Collection&lt;? extends Pair&lt;String,?&gt;&gt;</code>?</a></li> </ul> <hr /> <p><strong>Relating to single-level wildcard:</strong></p> <p>Now this might be making a clear picture in your mind, which relates back to the invariance of generics. A <code>List&lt;Number&gt;</code> is not a <code>List&lt;Double&gt;</code>, although <code>Number</code> is superclass of <code>Double</code>. Similarly, a <code>List&lt;List&lt;? extends Number&gt;&gt;</code> is not a <code>List&lt;List&lt;Integer&gt;&gt;</code> even though the <code>List&lt;? extends Number&gt;</code> is a superclass of <code>List&lt;Integer&gt;</code>.</p> <p><strong>Coming to the concrete problem:</strong></p> <p>You have declared your map as:</p> <pre><code>HashMap&lt;String, Hashmap&lt;String, HashSet&lt;? extends AttackCard&gt;&gt;&gt; superMap; </code></pre> <p><sub>Note that there is <strong>3-level of nesting</strong> in that declaration. <strong>Be careful</strong>. It's similar to <code>List&lt;List&lt;List&lt;? extends Number&gt;&gt;&gt;</code>, which is different from <code>List&lt;List&lt;? extends Number&gt;&gt;</code>.</sub></p> <p>Now what all element type you can add to the <code>superMap</code>? Surely, you can't add a <code>HashMap&lt;String, HashSet&lt;Assassin&gt;&gt;</code> into the <code>superMap</code>. Why? Because we can't do something like this:</p> <pre><code>HashMap&lt;String, HashSet&lt;? extends AttackCard&gt;&gt; map = new HashMap&lt;String, HashSet&lt;Assassin&gt;&gt;(); // This isn't valid </code></pre> <p>You can only assign a <code>HashMap&lt;String, HashSet&lt;? extends AttackCard&gt;&gt;</code> to <code>map</code> and thus only put that type of map as value in <code>superMap</code>. </p> <p><strong>Option 1:</strong></p> <p>So, one option is to modify your last part of the code in <code>Assassin</code> class(I guess it is) to:</p> <pre><code>private HashMap&lt;String, HashSet&lt;? extends AttackCard&gt;&gt; pool = new HashMap&lt;&gt;(); public HashMap&lt;String, HashSet&lt;? extends AttackCard&gt;&gt; get() { return pool; } </code></pre> <p>... and all will work fine.</p> <p><strong>Option 2:</strong></p> <p>Another option is to change the declaration of <code>superMap</code> to:</p> <pre><code>private HashMap&lt;String, HashMap&lt;String, ? extends HashSet&lt;? extends AttackCard&gt;&gt;&gt; superMap = new HashMap&lt;&gt;(); </code></pre> <p>Now, you would be able to put a <code>HashMap&lt;String, HashSet&lt;Assassin&gt;&gt;</code> to the <code>superMap</code>. How? Think of it. <code>HashMap&lt;String, HashSet&lt;Assassin&gt;&gt;</code> is capture-convertible to <code>HashMap&lt;String, ? extends HashSet&lt;? extends AttackCard&gt;&gt;</code>. Right? So the following assignment for the inner map is valid:</p> <pre><code>HashMap&lt;String, ? extends HashSet&lt;? extends AttackCard&gt;&gt; map = new HashMap&lt;String, HashSet&lt;Assassin&gt;&gt;(); </code></pre> <p>And hence you can put a <code>HashMap&lt;String, HashSet&lt;Assassin&gt;&gt;</code> in the above declared <code>superMap</code>. And then your original method in <code>Assassin</code> class would work fine.</p> <hr /> <p><strong>Bonus Point:</strong></p> <p>After solving the current issue, you should also consider to change all the concrete class type reference to their respective super interfaces. You should change the declaration of <code>superMap</code> to:</p> <pre><code>Map&lt;String, Map&lt;String, ? extends Set&lt;? extends AttackCard&gt;&gt;&gt; superMap; </code></pre> <p>So that you can assign either <code>HashMap</code> or <code>TreeMap</code> or <code>LinkedHashMap</code>, anytype to the <code>superMap</code>. Also, you would be able to add a <code>HashMap</code> or <code>TreeMap</code> as values of the <code>superMap</code>. It's really important to understand the usage of <a href="http://en.wikipedia.org/wiki/Liskov_substitution_principle" rel="noreferrer">Liskov Substitution Principle</a>.</p>
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    1. USRohit Jain
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    1. COI would like to comment that I'm initializing the superMap to be just "new Hashset<>()". The actual HashMaps contained in superMap come from other classes. like: superMap.put("key1", new OtherClass.getHashMap())
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    2. COAlso in the end the solution is a bit disappointing. Because the problem arose in the first place, because I wanted a strong type check in the respective subclasses. For instance AssassinPool should contain HashSet ONLY of Assassin
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    3. COWhile my field in AssassinPool is HashMap<String, HashSet<Assassin>> I can't return it as HashMap<String, HashMap<? extends AttackCard>> :( I am depressingly confused
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