abstraction

From a FP course: type Set = Int => Boolean // Predicate /** * Indicates whether a set contains a given element. */ def contains(s: Set, elem: Int): Boolean = s(elem) Why would that make sense? assert(contains(x => true, 100)) Basically what it does is provide the value 100 to the function x => true . I.e., we provide 100, it returns true . But how is this related to sets? Whatever we put, it returns true . Where is the sense of it? I understand that we can provide our own set implementation/function as a parameter that would represent the fact that provided value is inside a set (or not) -

Why use package visibility (the default), unless the class should be public in java As Rostislav Matl said, it's useful for when you want to make something that doesn't form part of your package's interface. As an example, imagine you have a package and it provides an interface and at least one concrete implementation of a service. People who use this service are going to care about the interface you provide and use one of the concrete classes you provide but they aren't going to care about much else beyond that. Our service has to talk to a database and it needs to be able to map the result

I am little confused about abstraction in java. I have checked many pages stating that abstraction is data hiding(Hiding the implementation). What I understand about abstraction is it is 'partial implementation'. Just define what you are going to need in an abstract class/interface and afterwards extend/implement them and add your own functionality. What I don't understand is how this is a data hiding? You are going to get access to the code once you implement the class/interface and you will modify it according to your need. I have checked many questions, articles on this but still confused

Is there a Python equivalent / pseudo-equivalent to java's OutputStream or PrintWriter ? I want to be able to have a handle that represents either a stream like stdout/sterr, or a file, or something else (a pipe or a socket or a dummy sink) and abstract away what kind of thing it is, so I can just send output to it. How can I do this? "Abstracting away what type it is" happens automatically in Python - it's called 'duck typing'. Just pass any file-like object to the function, and have it use the interface of file-like objects. FWIW, the standard input/output/error streams are represented by

So if a language provides higher order procedure then I can have procedure that returns procedure. Something like: (define (Proc a b c) (lambda (x) ( #| method body here in terms of a b c and x |# ))) To create new procedure, I would just do something like: (define ProcA (Proc a1 b1 c1)) ; Would create ProcA that has 1 argument Similar task could be done in a language which does not support higher order procedure by defining Proc that takes 4 instead of 3 arguments and calling this procedure to define ProcA , like: (define (Proc a b c x) ( #| method body -- does not return any procedure |# )

I'm reading the book "Clean Code" and am struggling with a concept. When discussing Objects and Data Structures, it states the following: Objects hide their data behind abstractions and expose functions that operate on that data. Data Structures expose their data and have no meaningful functions. So, what I'm getting from this is that I shouldn't have any public properties on my object, I should only have methods that perform operations on the properties. If I do need to access properties, they should be on a Data Structure, which could be returned from a method on my object? With this