[Java] Optional

Referring to java in Nutshell
Reffering to 더 자바, Java 8 written by whiteship in inflearn(https://www.inflearn.com/course/the-java-java8?inst=6fcc1e30)

1. Description
   • This is a value-based class; Optional is primarily intended for use as a method return type where there is a clear need to represent “no result,” and where using null is likely to cause errors.
     in JavaDocs
2. Usage

        public static void main(String[] args){
            Optional<Student> st = getStudentById(10);
       
        }
        class Student{
            public int id;
            public String name;
        }
   

3. Caution
   • for Return, not param
   • not for collection(collections already are boxed)
   • use of identity-sensitive operations (including reference equality (==), identity hash code, or synchronization) on instances of Optional may have unpredictable results and should be avoided.
4. APIs
   • For Creating
     • Optional.of()
     • Optional.ofNullable()
     • Optional.empty()
   • For Empty Checking
     • isPresent()
     • isEmpty() (after Java 11)
   • For Get inner data and with Predicate
     • get()
     • ifPresent(Consumer)
     • ofElse(T)
     • orElseGet(Supplier)
     • orElseThrow()
     • Optional.filter()
   • For Converting
     • map(Function)
     • flatMap(function) : Optional<Optional<T>>