A contract or specification that defines what methods a class must implement without specifying how they should be implemented. Interfaces ensure that different classes can be used interchangeably if they follow the same interface contract.
From Latin 'inter' (between) + 'facies' (face), meaning the surface between two things. Originally used in physical sciences to describe boundaries between different materials. Computing adopted it in the 1970s to describe the boundary and interaction points between software components.
Interfaces are like job descriptions - they specify what skills you need (methods to implement) but don't care about your background or how you learned those skills. Whether you're a cat or a dog, if you can both 'makeSound(),' you can implement the Animal interface!
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