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POHow can i make this work with java generics
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  1. POHow can i make this work with java generics
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    copied!<p>Suppose I have this interfaces and concretes:</p> <pre><code>public interface MeterValue {} public class MeterValueA implements MeterValue {} public class MeterValueB implements MeterValue {} public interface ReportBuilder&lt;T extends MeterValue&gt; {} public class ReportBuilderA implements ReportBuilder&lt;MeterValueA&gt; {} public class ReportBuilderB implements ReportBuilder&lt;MeterValueB&gt; {} </code></pre> <p>And I have a global instance: </p> <pre><code>ReportBuilder&lt;MeterValue&gt; reportBuilder; </code></pre> <p>I would like to vary the reportBuilder instance as follows:</p> <pre><code>if (isA()) reportBuilder = new ReportBuilderA(); else reportBuilder = new ReportBuilderB(); reportBuilder.desiredMethod(); </code></pre> <p>Doing this I get "Type Missmatch". How can I get my desired functionality? Or how could I improve my class design to be more flexible?</p> <p><strong>Edit:</strong></p> <p>The wildcard ? solved the problem. But also introduced other problems. Say i have this methods in the interface:</p> <pre><code>public interface ReportBuilder&lt;T extends MeterValue&gt; { public String getStuff(List&lt;T&gt; values); } public class ReportBuilderA implements ReportBuilder&lt;MeterValueA&gt; { @Override public String getStuff(List&lt;MeterValueA&gt; values) { return "A"; } } public class ReportBuilderB implements ReportBuilder&lt;MeterValueB&gt; { @Override public String getStuff(List&lt;MeterValueB&gt; values) { return "B"; } } </code></pre> <p>Say I have this instance:</p> <pre><code> ReportBuilder&lt;? extends MeterValue&gt; reportBuilder = new ReportBuilderA(); </code></pre> <p>And:</p> <pre><code> List&lt;? extends MeterValue&gt; list = new ArrayList&lt;MeterValueA&gt;(); String s = reportBuilder.getStuff(list); </code></pre> <p>The call to getStuff(...) will not work since list contain wildcards. One fix would be to modify the ReportBuilder interface method to: </p> <pre><code>public String getStuff(List&lt;? extends MeterValue&gt; values); </code></pre> <p>But then I break the concrete ReportBuilders. (list is also a global instance where the content may vary)</p> <p>Any ideas?</p>
 

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