An interface specifies a collection of methods (behaviors) without implementing their bodies (akin to giving the API).
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public interface Storable {
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public abstract void store(Stream s);
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public abstract void retrieve(Stream s);
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}
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Any other class which implements the interface is guaranteeing that it will have the set of behaviors, and will give concrete bodies to the methods of the interface.
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Interfaces solve some of the same problems as multiple inheritance, without as much overhead at runtime.
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There is a small performance penalty because interfaces involve dynamic method binding.
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Interfaces can be implemented by classes on unrelated inheritance trees, making it unnecessary to add methods to common superclass.
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