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copied!<p>Abstract classes can provide <em>default</em> implementations.</p> <p>Consider the <code>MouseListener</code> in the Java API, and the corresponding abstract class <code>MouseAdapter</code>.</p> <p>Using "pure" (non-abstract) <code>MouseAdapter</code> does not make sense: all methods are implemented as no-ops. However, the class is very convenient to use as abstract parent for a concrete listener, because you only have to <em>override</em> the one method that you are interested in, instead of having to reimplemet lots of methods as no-ops.</p> <p>It's not necessary to make <code>MouseAdapter</code> abstract. It is a complete class. It just doesn't make <em>sense</em> to use it without overriding at least one method. But otherwise, you would need to have <code>MouseListenerWithoutMouseClicked</code>, if you wanted it to have at least one <code>abstract</code> method.</p> <p>Another example would be the <a href="https://en.wikipedia.org/wiki/State_pattern" rel="nofollow">state pattern</a>. The interface defines the actual API. The abstract class would implement (some or all) transition functions as the <em>default operation</em>. Each state would only override those methods, where it diverts from the default behavior.</p> <p>In general, any "abstract" method declaration <em>could</em> (and often: should) be moved to an interface instead. The only reason why I often declare them as <code>@Override abstract</code> nevertheless is documentation purposes, to emphasize which methods need to be implemented for a concrete instance. Any method inherited from an interface will be effectively declare an abstract method.</p>

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