StackOverflow2013

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copied!<blockquote> <p>Q1. As interfaces are having only abstract methods (no code) so how can we say that if we are implementing any interface then it is inheritance ? We are not using its code.</p> </blockquote> <p>We can't. Interfaces aren't used to achieve multiple inheritance. They replace it with safer, although slightly less powerful construct. Note the keyword <code>implements</code> rather than <code>extends</code>.</p> <blockquote> <p>Q2. If implementing an interface is not inheritance then How interfaces are used to achieve multiple inheritance ?</p> </blockquote> <p>They are not. With interfaces a single class can have several "<em>views</em>", different APIs or capabilities. E.g. A class can be <code>Runnable</code> and <code>Callable</code> at the same time, while both methods are effectively doing the same thing.</p> <blockquote> <p>Q3. Anyhow what is the benefit of using Interfaces ? They are not having any code. We need to write code again and again in all classes we implement it.</p> </blockquote> <p>Interfaces are kind-of multiple inheritance with no problems that the latter introduces (like the <a href="http://en.wikipedia.org/wiki/Diamond_problem"><em>Diamond problem</em></a>).</p> <p>There are few use-cases for interfaces:</p> <ol> <li><p>Object effectively has two identities: a <code>Tank</code> <em>is</em> both a <code>Vehicle</code> and a <code>Weapon</code>. You can use an instance of <code>Tank</code> where either the former or the latter is expected (polymorphism). This is rarely a case in real-life and is actually a valid example where multiple inheritance would be better (or traits).</p></li> <li><p>Simple responsibilities: an instance of <code>Tank</code> object in a game is also <code>Runnable</code> to let you execute it in a thread and an <code>ActionListener</code> to respond to mouse events.</p></li> <li><p>Callback interfaces: if object implements given callback interface, it is being notified about its life-cycle or other events.</p></li> <li><p>Marker interfaces: not adding any methods, but easily accessible via <code>instanceof</code> to discover object capabilities or wishes. <code>Serializable</code> and <code>Cloneable</code> are examples of this.</p></li> </ol> <p>What you are looking for are trait (like in Scala), unfortunately unavailable in Java.</p>

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