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Clarification on interfaces

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Joined: 2011-01-24

I am new to Java and have some questions regarding 'interfaces' and how they are used. I understand why they are used (the example in the Java Tutorials describing a proprietary program used as a navigation system, and manufacturers like GM, Toyota etc... using an interface to implement it in their own way was very descriptive).
What I don't understand is why they are used as opposed to just leaving a text file with your software describing the methods your class uses and how to call them, as well as any constants they use. My understanding is that interfaces act as a 'contract' or 'blueprint' for a class to implement your methods. But I know there's more to it than that that I'm missing.
Can I, for example write a class with twelve methods, then write two or three different interfaces to process the data in different ways? So, if wrote a class to calculate sales data for a store; could I write my class like the example above and write one interface for say 'weekdays', one for 'weekends', and one for 'holidays', substituting different constants and methods types so I can compensate for the fluctuation in customers?
It seems however, that I could write my class this way anyway, with specific methods to call for 'weekends', 'weekdays', and 'holidays' and leave out interfaces entirely.
So I am wondering if someone could shed some more light on interaces and their use...or at least point me in the right direction to continue my own research. I did search around this forum and couldn't find any references that answered my question.

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Joined: 2011-01-04

Interfaces, the way I've come to understand them, allow you to reference classes that function similarly-but-differently.

For instance, I have several games that I've made, and made a program that enables the user to load any one of them in the same window. This works because all of the games ipmlement the Game interface; it'd be impractical to make them instances of the same class, but as the same interface, my loader can handle all of them.