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copied!<p>The concept at work here is called <a href="http://docs.oracle.com/javase/tutorial/java/generics/erasure.html" rel="noreferrer"><em>type erasure</em></a>. <code>HashSet</code> defines a method <code>add(T)</code>, and you define a method <code>add(Object)</code>. At a glance one might think this is OK; that your method just overloads <code>add</code>. However, the erasure of <code>T</code> is <code>Object</code> and so the two have the same erased signature. </p> <p>Now, that would be fine if your method properly overrode the method from <code>HashSet</code>. But to do so you should be using <code>add(Integer)</code> and not <code>add(Object)</code>. You're not properly overriding the parent method, so instead it is reported as a conflict since a class cannot provide two methods with the same signature.</p> <p>Your <code>Abc</code> example follows the same reasoning. The two methods you declared have the same erased signature so they clash.</p> <h2>Further Reading</h2> <p>Angelika Langer Generics FAQ </p> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/TechnicalDetails.html#FAQ813" rel="noreferrer">When does a method override its supertype's method?</a></li> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/TechnicalDetails.html#Can%20a%20method%20of%20a%20non-generic%20subtype%20override%20a%20method%20of%20a%20generic%20supertype?" rel="noreferrer">Can a method of a non-generic subtype override a method of a generic supertype?</a></li> </ul>

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