使用 Optional 類可以通過檢查空值來防止程式碼汙染，也可以根據條件過濾非空值。

import java.util.NoSuchElementException;
import java.util.Optional;

public class OptionalDemo {

    public static void main(String[] args) {
        Optional<String> name = Optional.of("Sanaulla");
        Optional empty = Optional.ofNullable(null);
        if (name.isPresent()) {
            System.out.println(name.get());
        }

        try {
            System.out.println(empty.get());
        } catch (NoSuchElementException ex) {
            System.out.println(ex.getMessage());
        }

//        如果有值，則呼叫引數 Consumer
        name.ifPresent((value) -> {
            System.out.println("The length of the value is: " + value.length());
        });

//        有值則返回，否則返回預設值或生成的預設值
        System.out.println(empty.orElse("There is no value present!"));
        System.out.println(name.orElse("There is some value!"));

        System.out.println(empty.orElseGet(() -> "Default Value"));
        System.out.println(name.orElseGet(() -> "Default Value"));

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

//        Map：如果有值則執行方法
        Optional<String> upperName = name.map((value) -> value.toUpperCase());
        System.out.println(upperName.orElse("No value found"));

        upperName = name.flatMap((value) -> Optional.of(value.toUpperCase()));
        System.out.println(upperName.orElse("No value found"));

//        Filter：對 Optional 例項的值進行過濾
        Optional<String> longName = name.filter((value) -> value.length() > 6);
        System.out.println(longName.orElse("The name is less than 6 characters"));

        Optional<String> anotherName = Optional.of("Sana");
        Optional<String> shortName = anotherName.filter((value) -> value.length() > 6);
        System.out.println(shortName.orElse("The name is less than 6 characters"));
    }

}