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POHow to avoid unsafe cast warnings with Java Generics
 

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    <p>I'm quite new to Java Generics. But I've read a lot of summary links and examples. But I cannot get the simplest method right. I hope somenoe can help:</p> <p>I want to create a HashMap which maps one undefined Object to another one. Both objects may one of String or Integer.</p> <p>So I wrote this:</p> <pre><code>private final HashMap&lt;L, R&gt; left2Right = new HashMap&lt;L, R&gt;(); Extractor&lt;?&gt; extLeft = Extractor.getInstance(0); Extractor&lt;?&gt; extRight = Extractor.getInstance(1); L leftVal = extLeft.extract(d, i); R rightVal = extRight.extract(d, i); this.left2Right.put(leftVal, rightVal); </code></pre> <p>So far so good... But I have problems implementing the extractor-objects. They are instanciated by a factory pattern. I did it like this (simplyfied):</p> <pre><code>abstract class Extractor&lt;E&gt; { abstract E extract(DTable source, int row); static &lt;E&gt; Extractor&lt;E&gt; getInstance(int type) { if(type == 0) return new IntExtractor(); else return new StringExtractor(); } } class IntExtractor extends Extractor&lt;Integer&gt; { @Override Integer extract(DTable source, int row) { int value = 5; return new Integer(value); } } class StringExtractor extends Extractor&lt;String&gt; { @Override String extract(DTable source, int row) { String retVal = "hello"; return retVal; } } </code></pre> <p>It compiles, but I get Unsave cast warnings on casting the Integer/String to E. What am I doing wrong? I know, I can simply supress the warnings. But I thought exactly this should be the advantage of Java generics? I cannot make this cast save, because I dont know, which type 'E' really "is"...</p> <p>Or am I doing something basically wrong?</p> <p>Note: I edited my code to a "new" question after I used some information from the first answer...</p>
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    1. COI do have the feeling you are not telling us everything there is to this problem…
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      1. POHow to avoid unsafe cast warnings with Java Generics
      1. USBombe
 

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