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POneed for casting to a generic type in java
 

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  1. POneed for casting to a generic type in java
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    <p>while implementing a priority queue using a linkedlist(actually an inner class <code>Node</code>),I coded the <code>insert()</code> and <code>max()</code> methods as below.It uses the lazy approach of keeping the items unordered and then seaching through them for the maximum element only when a <code>max()</code> or <code>deleteMax()</code> call occurs.</p> <pre><code>public class LinkedListMaxPQ&lt;Item extends Comparable&lt;Item&gt;&gt;{ private int N; private Node head; public void insert(Item item) { Node old = head; head = new Node(); head.item = item; head.next = old; N++; } public Item max() { Item maxitem = (Item) this.head.item; for(Node t=head.next;t!=null;t=t.next){ if(gt(t.item,maxitem)){ maxitem = (Item) t.item; } } return maxitem; } private boolean gt(Comparable x,Comparable y){ return x.compareTo(y) &gt; 0; } private class Node&lt;Item extends Comparable&lt;Item&gt;&gt;{ Item item; Node next; } } </code></pre> <p>I am wondering why I need the cast in <code>Item maxitem = (Item) this.head.item</code> ? Since the class uses a generic type <code>Item which extends Comparable</code> and the inner class also uses Item extends Comparable ,one would think that such a cast is unnecessary.</p> <p>If I omit the cast </p> <pre><code>Item maxitem = this.head.item; </code></pre> <p>compiler will complain that there is a type mismatch</p> <blockquote> <p>Type mismatch: cannot convert from Comparable to Item</p> </blockquote> <p>Can someone explain why this occurs?</p>
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    1. USdamon
    1. USdamon
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    1. CObecause isn't a Item is something that you can compare with Item, isn't the same
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      1. USFelquir
    2. COpossible duplicate of [Type erasure, overriding and generics](http://stackoverflow.com/questions/502614/type-erasure-overriding-and-generics) Using `for(Node<Item> t=head.next;t!=null;t=t.next){` will remove the type mismatch error.
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      1. USJonathan Drapeau
 

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