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PODifference between List, List<?>, List<T>, List<E>, and List<Object>
 

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    <p>What are the differences between <code>List</code>, <code>List&lt;?&gt;</code>, <code>List&lt;T&gt;</code>, <code>List&lt;E&gt;</code>, and <code>List&lt;Object&gt;</code>? </p> <p>Now I do not blindly ask this question, so please don't close this thread. Let me first introduce the base code:</p> <pre><code>private static List&lt;String&gt; names = new ArrayList&lt;String&gt;(); static { names.add("Tom"); names.add("Peter"); names.add("Michael"); names.add("Johnson"); names.add("Vlissides"); } public static void test(List&lt;String&gt; set){ System.out.println(set); } public static void main(String args[]){ test(names); } </code></pre> <p>I do understand that:</p> <p>1.<code>List</code>: is a raw type, therefore not <code>typesafe</code>. It will only generate a runtime error when the casting is bad. We want a compile time error when the cast is bad. Not recommended to use.</p> <p>2.<code>List&lt;?&gt;</code>: is an unbounded wildcard. But not sure what this is for? So if I change the <code>test</code> method to</p> <pre><code>public static void test(List&lt;?&gt; set){ System.out.println(set); } </code></pre> <p>it still works good. <strong>If you can explain the usage of this, I would greatly appreciate it.</strong></p> <p><strong>EDIT</strong>: If I do this:</p> <pre><code>public static void test(List&lt;?&gt; set){ set.add(new Long(2)); //--&gt; Error set.add("2"); //--&gt; Error System.out.println(set); } </code></pre> <p>but if I change <code>test</code> to this:</p> <pre><code>public static void test(List&lt;String&gt; set){ set.add(new Long(2)); //--&gt; Error set.add("2"); //--&gt; Work System.out.println(set); } </code></pre> <p>3.<code>List&lt;T&gt;</code>: </p> <pre><code>public static void test(List&lt;T&gt; set){ //T cannot be resolved System.out.println(set); } </code></pre> <p>I guess I don't understand this syntax. I saw something like this, and it works:</p> <pre><code>public &lt;T&gt; T[] toArray(T[] a){ return a; } </code></pre> <p>Please explain this for me please? Sometimes, <strong>I see <code>&lt;T&gt;</code>, or <code>&lt;E&gt;</code>, or <code>&lt;U&gt;</code>, <code>&lt;T,E&gt;</code>. Are they all the same or do they represent something different?</strong></p> <p>4.<code>List&lt;Object&gt;</code></p> <pre><code>public static void test(List&lt;Object&gt; set){ System.out.println(set); } </code></pre> <p>Then I got the error <code>The method test(List&lt;Object&gt;) is not application for the argument List&lt;String&gt;</code> for the below code. I am confused. I thought <code>String</code> was a subset of <code>Object</code>?</p> <pre><code>public static void main(String args[]){ test(names); } </code></pre> <p><strong>EDIT:</strong> If I try this </p> <pre><code>test((List&lt;Object&gt;)names); </code></pre> <p>then I got <code>Cannot cast from List&lt;String&gt; to List&lt;Object&gt;</code></p>
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    1. CONo offense, but I think you need to spend some time learning about generics in Java.
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    2. CO@Bernard: None taken
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    3. CO@Harry: if you can refer a book or online resource that you've read and you do not understand the explanation there about your points, then i would answer your question. Asking what is the differenc between List<T> and List<E> is the same as asking what is the difference between `int a;` and `int b;`
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