public static void main(String[] args) {

OptionalExample o = new OptionalExample();

Optional<Integer> val1 = Optional.ofNullable(2);

Optional<Integer> val2 = Optional.ofNullable(5);

System.out.println("val1: " + val1.get());

System.out.println("val2: " + val2.get());

o.nameNotNull();

o.nameNullable();

o.nameIsPresent(null);

o.nameIsPresent("George");

o.nameIsPresent("GG");

o.useEmpty();

o.useOrElse();

o.useOrElseThrow();

o.listInOptional();

o.mapInOptional();

o.ObjectDataExample();

}

public void nameNotNull() {

// ! of() 不能塞 null

String name = "GEORGE";

Optional<String> optName = Optional.of(name);

System.out.println(optName.get());

}

public void nameNullable() {

// ! ofNullable() 可以塞 null

String name = null;

// Optional<String> optName = Optional.of(name); // 使用of會有錯誤

Optional<String> optName = Optional.ofNullable(name);

// System.out.println(optName.get()); // 因為是null，會出現NoSuchElementException狀況。

System.out.println(optName);

}

public void nameIsPresent(String n) {

// ! isPresent() 可拿來判斷是否有此物件存在

String name = n;

Optional<String> optName = Optional.ofNullable(name);

if (optName.isPresent()) {

System.out.println(optName.get());

} else {

System.out.println("name value is null.");

}

}

public void useEmpty() {

String name = null;

// ! 使用三元運算子 + optional object

Optional<String> optName = (name == null) ? Optional.empty() : Optional.of(name);

System.out.println(optName); // optName.get() 因為null, 所以無法抓取到值會有錯誤。

}

public void useOrElse() {

// 取代 if

// ! orElse() 處理 null 後續動作

String name = null;

Optional<String> optName = Optional.ofNullable(name);

System.out.println(optName.orElse("Name is null."));

}

public void useOrElseGet() {

String name = null;

Optional<String> optName = Optional.ofNullable(name);

// ! 可透過 orElseGet，讓null值時多做一些事情

System.out.println(optName.orElseGet( () -> "what!!! null !!" )); // lambda.

}

public void useOrElseThrow() {

// ! 可透過null值，跳出exception處理

String name = null;

Optional<String> optName = Optional.ofNullable(name);

try {

System.out.println(optName.orElseThrow( () -> new Exception("what the hell.") ));

} catch (Exception e) {

System.out.println(e.getMessage());

}

}

// * example for chaining the above functions.

public void situation1() {

// TODO: think the process of situation 1.

}

public void situation2() {

// TODO: think the process of situation 2.

}

public List<String> putCollectionIntoOptional() {

List<String> list = new ArrayList<>();

list.add("George");

list.add("May");

list.add("HaHa");

list.add("JJ");

list.add("PP");

list.add("John");

list.add("Amy");

return list;

}

public Map<String, String> putMapIntoOptional() {

Map<String, String> map = new HashMap<>();

map.put("P001", "GG");

map.put("P002", "Haha");

map.put("P003", "Amy");

return map;

}

public void listInOptional() {

// * put list into the optional object.

System.out.println("-------------------------");

System.out.println("execute listInOptional().");

System.out.println("-------------------------");

putCollectionIntoOptional().forEach(s -> {

System.out.println(s);

});

Optional<List<String>> result = Optional.ofNullable(putCollectionIntoOptional());

System.out.println(result);

}

public void mapInOptional() {

// * put map into the optional object.

System.out.println("-------------------------");

System.out.println("execute mapInOptional().");

System.out.println("-------------------------");

putMapIntoOptional().forEach((i, n) -> {

System.out.println(i + ", " + n);

});

}

// TODO: json into optional.

// TODO: any object or collection into optional.

public void ObjectDataExample() {

List<Student> students = new ArrayList<>();

List<Student> students2 = new ArrayList<>();

for (int i=0; i<10; i++) {

Student student = new Student("student" + i, "F00" + i, (int) (Math.random() * 100 + i));

students.add(student);

}

students.forEach(s -> {

System.out.println(s + ", name: " + s.getName() + "; id: " + s.getId() + "; age: " + s.getAge());

});

Optional<List<Student>> result = Optional.ofNullable(students);

System.out.println("result: " + result);

students.clear();

Optional<List<Student>> result2 = Optional.ofNullable(students);

System.out.println("result2: " + result2);

// ! 清空 student 依舊會存在，除非 null。

System.out.println(result2.isPresent());

// ! 把 list object 設定為 null。

students2 = null;

Optional<List<Student>> result3 = Optional.ofNullable(students2);

System.out.println("result3: " + result3.isPresent());

System.out.println("result3: " + result3.isEmpty());

System.out.println("----");

students.forEach(s -> {

System.out.println(s);

});

}

// ? THINK: stream, collection, iterable, iterator, ..., etc.

private String name;

private String id;

private int age;

Student(String name, String id, int age) {

this.name = name;

this.id = id;

this.age = age;

}

Student() {

}

public String getName() {

return name;

}

public String getId() {

return id;

}

public int getAge() {

return age;

}