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POJava how to: Generic array creation from class with generic?
 

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  1. POJava how to: Generic array creation from class with generic?
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    <p>I have a class <code>FirstClass&lt;O&gt;</code> and a class <code>SecondClass&lt;O&gt;</code>, and I want to make an O[] in <code>SecondClass&lt;O&gt;</code> inside a routine which is called from <code>FirstClass&lt;O&gt;</code>, where O is a generic class parameter. I fail to find how to do this.</p> <p>I need an O[] specifically (and not <code>ArrayList&lt;O&gt;</code> or similar) because I need to get elements from it very often inside the body of a loop, and it matters for the execution time of my algorithm.</p> <p>So I would want something along these lines.</p> <pre><code>public class FirstClass&lt;O&gt; { void someRoutine(n and other params) { //Do some stuff SecondClass&lt;O&gt; = new SecondClass(n, possibly_other_params); //Do some stuff } } </code></pre> <p>and</p> <pre><code>public class SecondClass&lt;O&gt; { O[] myArray; SecondClass&lt;O&gt;(int n, possibly_other_params) { //Here the constructor that creates an O[n] } } </code></pre> <p>Some methods I found on the web, but do not work for my case:</p> <ul> <li>Use <code>O[] array = (O[]) new Object[n];</code> but that doesn't compile.</li> <li>Use <code>Object[] array = new Object[n];</code> and do an (O) cast every time I request something from the array, but this is way too slow</li> <li>Use <code>Array.newInstance(Class&lt;O&gt; type, int n);</code>, with <code>O o;</code> and <code>type=o.class</code> but it complains that <code>type</code> is now of type <code>Class&lt;CAP#1&gt;</code> instead of type <code>Class&lt;O&gt;</code>, whatever CAP#1 means...</li> </ul> <p>How should I do this properly in Java, with optimal execution speed in mind?</p>
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    1. COHave you actually profiled this? Is the casting actually what is causing your performance concerns? Given that generics in Java are erased at runtime, I don't see an easy work-around here to get it working as you're expecting. I think you'll be better off trying to re-visit your expected API, then trying to get Java's generics to work they way you *want*.
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    2. COJust use `ArrayList`, sheesh.
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    3. COThe problem with ArrayList is that I constantly need to change one element in the array, and this basically all that this inner loop does and hence is the most expensive part of the algorithm. Repeatedly doing `a[i]=a[j]` to obtain some permutation is quite a bit cheaper than doing `list.set(i,list.get(j))` all the time. Putting a fixed array type makes it indeed faster, but then I need to make copy-paste the code for each of the 25 possible types O can be, and that's not really good style. :/ Well, if it's not possible then it's not possible of course. Thanks for the input anyway.
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