Stop Writing if-checks: Refinement Types in Java 8+

The Code That Lives in Every Java Codebase Every Java developer has written this code. You have written this code. Probably this week. A method starts with five lines of null checks, blank checks, and range checks before a single line of business logic executes. The validation is scattered, duplicated, and — worst of all — invisible at the API boundary. public void createUser(String name, int age, List<String> roles) { if (name == null || name.trim().isEmpty()) { throw new IllegalArgumentException("name must not be blank"); } if (age <= 0) { throw new IllegalArgumentException("age must be positive"); } if (roles == null || roles.isEmpty()) { throw new IllegalArgumentException("roles must not be empty"); } // finally, the actual logic... } You know the problems. The checks get copy-pasted across service layers. Someone forgets one in a new method. A bug slips through because the validation was in the controller but not in the domain service. The types lie — String promise