@FunctionalInterface annotation is useful for compilation time checking of your code. You cannot have more than one method besides static, default and abstract methods that override methods in Object in your @FunctionalInterface or any other interface used as a functional interface. But you can use lambdas without this annotation as well as you can override … Read more
Lambda expressions are a simpler syntax for anonymous delegates and can be used everywhere an anonymous delegate can be used. However, the opposite is not true; lambda expressions can be converted to expression trees which allows for a lot of the magic like LINQ to SQL. The following is an example of a LINQ to … Read more
Change x.append(lambda : pv(v)) to x.append(lambda v=v: pv(v)). You expect “python lambdas to bind to the reference a local variable is pointing to, behind the scene”, but that is not how Python works. Python looks up the variable name at the time the function is called, not when it is created. Using a default argument … Read more
Yes, the code is standard conforming. The + triggers a conversion to a plain old function pointer for the lambda. What happens is this: The compiler sees the first lambda ([]{}) and generates a closure object according to §5.1.2. As the lambda is a non-capturing lambda, the following applies: 5.1.2 Lambda expressions [expr.prim.lambda] 6 The … Read more
Resolution of variables in lambdas is done when lambda is executed. At this time, for all buttons i=5. To rectify this issue do as follows: make_button = Tkinter.Button(frame, text =”make!”, command= lambda i=i: makeId(i)) This creates i as a local variable in a lambda. This local variable will hold correct value of i from the … Read more
I find that if you’re familiar with SQL syntax, using the LINQ query syntax is much clearer, more natural, and makes it easier to spot errors: var id = 1; var query = from post in database.Posts join meta in database.Post_Metas on post.ID equals meta.Post_ID where post.ID == id select new { Post = post, … Read more
Although other answers prove the requirement, they don’t explain why the requirement exists. The JLS mentions why in §15.27.2: The restriction to effectively final variables prohibits access to dynamically-changing local variables, whose capture would likely introduce concurrency problems. To lower risk of bugs, they decided to ensure captured variables are never mutated.
Based on Collectors documentation it’s as simple as: Map<String, Choice> result = choices.stream().collect(Collectors.toMap(Choice::getName, Function.identity()));
Please read about minimal examples. Without reading your code, I believe you have run into a well known issue addressed in previous questions and answers that needs 2 lines to illustrate. Names in function bodies are evaluated when the function is executed. funcs = [lambda: i for i in range(3)] for f in funcs: print(f()) … Read more
You’ll need to do one of the following. If it’s your code, then define your own functional interface that declares the checked exception: @FunctionalInterface public interface CheckedFunction<T, R> { R apply(T t) throws IOException; } and use it: void foo (CheckedFunction f) { … } Otherwise, wrap Integer myMethod(String s) in a method that doesn’t … Read more