I’ll go through a scenario where we should use Supplier<LocalDate> instead of LocalDate. Code that directly makes calls to static methods like LocalDate.now() is very difficult to unit test. Consider a scenario where we want to unit test a method getAge() that calculates a person’s age: class Person { final String name; private final LocalDate … Read more
You’re looking for BooleanSupplier. https://docs.oracle.com/javase/8/docs/api/java/util/function/BooleanSupplier.html
The reason for this, is that these invocations are in fact, invocations to two different overloaded methods available in ExecutorService; each of these methods taking a single argument of different types: <T> Future<T> submit(Callable<T> task); 2. Future<?> submit(Runnable task); Then what happens is that the compiler is converting the lambda in the first case of … Read more
From the same page you linked to: The interface Comparator is functional because although it declares two abstract methods, one of these—equals— has a signature corresponding to a public method in Object. Interfaces always declare abstract methods corresponding to the public methods of Object, but they usually do so implicitly. Whether implicitly or explicitly declared, … Read more
But, a 1-arg constructor for T that takes a String is compatible with Function<String,T>: Function<String, Foo> fooSupplier = Foo::new; Which constructor is selected is treated as an overload selection problem, based on the shape of the target type.
The reason you’re having difficulty grasping the meaning of functional interfaces such as those in java.util.function is that the interfaces defined here do not have any meaning! They are present primarily to represent structure, not semantics. This is atypical for most Java APIs. The typical Java API, such as a class or interface, has meaning, … Read more
@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