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POStrategy Design Pattern, Generics and TypeSafety
 

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  1. POStrategy Design Pattern, Generics and TypeSafety
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    <p>I want to create the following Strategy Pattern combined with Factory, but I want it to be typesafe. I have done the following till now:</p> <pre><code>public interface Parser&lt;T&gt; { public Collection&lt;T&gt; parse(ResultSet resultSet); } public class AParser implements Parser&lt;String&gt; { @Override public Collection&lt;String&gt; parse(ResultSet resultSet) { //perform parsing, get collection Collection&lt;String&gt; cl = performParsing(resultSet); //local private method return cl; } } public class ParserFactory { public enum ParserType { APARSER } public static &lt;T&gt; Parser&lt;T&gt; createParser(ParserType parserType) { Parser&lt;?&gt; parser = null; switch (parserType) { case APARSER: parser = new AParser(); break; } //unchecked cast happens here return (Parser&lt;T&gt;) parser; } } public class Context { public &lt;T&gt; Collection&lt;T&gt; getResults(String query, ParserType parserType) { ResultSet resultSet() = getResultSet(query); //local private method Parser p = ParserFactory.createParser(parserType); Collection&lt;T&gt; results = p.parse(resultSet) } } </code></pre> <p>In general whatever I attempt, somewhere I will have an unchecked cast. Anyone have an idea how I can refactor the code to be typesafe?</p> <p>Checking Effective Java I also stumbled upon this pattern:</p> <pre><code>public final class ParserFactory { private ParserFactory() { } private static class AParser implements Parser&lt;String&gt; { @Override public Collection&lt;String&gt; parse(ResultSet resultSet) { //... return new ArrayList&lt;&gt;(); } } public static final Parser&lt;String&gt; APARSER = new AParser(); } </code></pre> <p>So now I can use as Ingo suggested</p> <pre><code>public &lt;T&gt; Collection&lt;T&gt; getResults(String query, Parser&lt;T&gt; p) </code></pre> <p>as</p> <pre><code>getResults("query", ParserFactory.APARSER); </code></pre> <p>Or would this be better with enums?</p>
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    1. COYou can't get this type safe as long as the `createParser` method (or any other method) is supposed to create T out of thin air (so to speak). (Because this says literally: this method will give you a parser of any type, which can't be true). You must pass some input that has T, only then can the compiler check it. Conventionally, one would pass a Class<T>.
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    2. COBut in your case, it is maybe possible to have the type argument on the ParserType.
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    3. COSadly, enums and generics don't mix well.
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      1. USPaul Bellora
 

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