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POcasting ArrayList.toArray() with ArrayList of Generic Arrays
 

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  1. POcasting ArrayList.toArray() with ArrayList of Generic Arrays
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    <p>A difficult question which I'm close to giving up all hope on. I'm trying to make a function, but am having problems getting <code>ArrayList.toArray()</code> to return the type I want. </p> <p>Here's the minimal example that demonstrates my problem:</p> <pre><code>public static &lt;T&gt; T[] test(T one, T two) { java.util.List&lt;T&gt; list = new ArrayList&lt;T&gt;(); list.add(one); list.add(two); return (T[]) list.toArray(); } </code></pre> <p>Normally I'd be able to use the form <code>(T[]) list.toArray(new T[0])</code> but there are two added difficulties: </p> <ol> <li>Because of the covariance rules I cannot typecast arrays, <code>(T[]) myObjectArray</code> gives a <code>ClassCastException</code></li> <li>You cannot create a new instance of a generic type, meaning I cannot instance <code>new T[]</code>. Nor can I use clone on one of the elements of <code>ArrayList</code> or try to get its class.</li> </ol> <p>I've been trying to get some information using the following call:</p> <pre><code>public static void main(String[] args) { System.out.println(test("onestring", "twostrings")); } </code></pre> <p>the result is of the form <code>[Ljava.lang.Object;@3e25a5</code> suggesting that the typecast on the return is not effective. strangely the following:</p> <pre><code>public static void main(String[] args) { System.out.println(test("onestring", "twostrings").getClass()); } </code></pre> <p>comes back with: </p> <pre><code>Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String; at patchLinker.Utilities.main(Utilities.java:286) </code></pre> <p>So my best guess is that it think's it's a String array label wise, but internally is an Object array, and any attempts to access brings that inconsistancy out. </p> <p>If anyone can find any way of working around this (since the two normal workarounds are denied to me) I'd be very greatful. </p> <p>K.Barad JDK1.6</p> <hr> <p>Edit</p> <hr> <p>Thanks to Peter Lawrey for solving the initial problem. Now the issue is how to make the solution apply to my less trivial example:</p> <pre><code>public static &lt;T&gt; T[] unTreeArray(T[][] input) { java.util.List&lt;T&gt; outList = new ArrayList&lt;T&gt;(); java.util.List&lt;T&gt; tempList; T[] typeVar; for (T[] subArray : input) { tempList = java.util.Arrays.asList(subArray); outList.addAll(tempList); } if (input.length &gt; 0) { typeVar = input[0]; } else { return null; } return (T[]) outList.toArray((T[]) java.lang.reflect.Array.newInstance(typeVar.getClass(),outList.size())); } public static void main(String[] args) { String[][] lines = { { "111", "122", "133" }, { "211", "222", "233" } }; unTreeArray(lines); } </code></pre> <p>The result at the moment is</p> <pre><code>Exception in thread "main" java.lang.ArrayStoreException at java.lang.System.arraycopy(Native Method) at java.util.ArrayList.toArray(Unknown Source) at patchLinker.Utilities.unTreeArray(Utilities.java:159) at patchLinker.Utilities.main(Utilities.java:301) </code></pre> <p>I'm still doing some tests to see if I can get any more useful information than this, but at the moment I'm not sure where to start. I'll add any more information I get as I get it.</p> <p>K.Barad</p> <hr> <p>edit 2</p> <hr> <p>Some more information now. I've tried to build the array manually and transfer the items in elementwise, so I'd be typecasting as T, rather than T[]. I found an interesting result:</p> <pre><code>public static &lt;T&gt; T[] unTreeArray(T[][] input) { java.util.List&lt;T&gt; outList = new ArrayList&lt;T&gt;(); java.util.List&lt;T&gt; tempList; T[] outArray; for (T[] subArray : input) { tempList = java.util.Arrays.asList(subArray); outList.addAll(tempList); } if (outList.size() == 0) { return null; } else { outArray = input[0];// use as a class sampler outArray = (T[]) java.lang.reflect.Array.newInstance(outArray.getClass(), outList.size()); for (int i = 0; i &lt; outList.size(); i++) { System.out.println(i + " " + outArray.length + " " + outList.size() + " " + outList.get(i)); outArray[i] = (T) outList.get(i); } System.out.println(outArray.length); return outArray; } } </code></pre> <p>I still get the following output:</p> <pre><code>0 6 6 111 Exception in thread "main" java.lang.ArrayStoreException: java.lang.String at patchLinker.Utilities.unTreeArray(Utilities.java:291) at patchLinker.Utilities.main(Utilities.java:307) </code></pre> <p>Does this mean you can't write to a <code>T[]</code> even if you manage to create one indirectly? </p>
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    1. COI suggest sticking with collections and leaving behind old school reference arrays.
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    2. COWould love to, but there are times when you have no choice in the matter. Backward compatability always implies that something is backward.
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