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copied!<pre><code>public interface MeterValue {} public class MeterValueA implements MeterValue {} public class MeterValueB implements MeterValue {} public interface ReportBuilder<T extends MeterValue> { void desiredMethod(); } public class ReportBuilderA implements ReportBuilder<MeterValueA> { @Override public void desiredMethod() {} } public class ReportBuilderB implements ReportBuilder<MeterValueB> { @Override public void desiredMethod() {} } void f() { ReportBuilder<? extends MeterValue> reportBuilder = null; if (Math.random() > 0.5) reportBuilder = new ReportBuilderA(); else reportBuilder = new ReportBuilderB(); reportBuilder.desiredMethod(); } </code></pre> <p>Where: <code>ReportBuilder<? extends MeterValue></code> means generic parameter can extend <em>MeterValue</em> interface.</p> <p>See <a href="http://docs.oracle.com/javase/tutorial/extra/generics/wildcards.html" rel="nofollow">tutorial</a> to read more explanations about the wildcard declaration.</p> <hr> <p>Note: please review this code in your particular case to avoid <a href="http://sourcemaking.com/refactoring/parallel-inheritance-hierarchies" rel="nofollow">Parallel inheritance</a> smell. If to add some new MeterValue you should add ReportBuilder it should mean you have probably this problem.</p> <hr> <p><strong>EDIT:</strong></p> <p>You cannot pass array with subtypes of MeterValue to ReportBuilderA which requeires at least MeterValueA. It is not type safe technic. Instantiate concrete class after inheritance is more appropriate for C++. In Java it is better do not change restricted type after inheritance.</p> <p>Probable solution, maybe applicable for your concrete case:</p> <p>1) Use single interface for MeterValue. All differences move to ReportBuilder inheritance. This means you have the parallel inheritance in your project.</p> <p>2) Use concrete <code>List<MeterValueA></code> when call <code>getStuff()</code> method. All shared logic for processing <code>List<MeterValueA></code> and <code>List<MeterValueB></code> move to methods with signature: <code>public void sharedLogic(List<? extends MeterValue> list)</code></p>

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