JAVA8之妙用Optional解决判断Null为空的问题

      CustomerPoJo customerPoJo = null;
      customerPoJo = Optional.ofNullable(customerPoJo).orElse(new CustomerPoJo());
      customerPoJo = Optional.ofNullable(customerPoJo).orElseGet(() -> new CustomerPoJo());
      Optional.ofNullable(customerPoJo).orElseThrow(()->new Exception("客户不存在"));

      CustomerPoJo customerPoJo = new CustomerPoJo();
      customerPoJo.setName("毛亮");
      // 取name的写法
      String name = Optional.ofNullable(customerPoJo).map(u->u.getName()).get();

    User user = new User();
    user.setName("maoliang");
    String name = Optional.ofNullable(user).flatMap(user1 -> user1.getName()).get();
    System.out.println("name=======" + name);

@Data
public class User {

    private String name;

    public Optional<String> getName(){
        return Optional.ofNullable(name);
    }
}

5、filter(Predicate<? super T> predicate)

filter 方法接受一个 Predicate 来对 Optional中包含的值进行过滤，如果包含的值满足条件，那么还是返回这个 Optional；否则返回 Optional.empty。

    User user = new User();
    user.setName("maol");
    Optional<User> user1 =
        Optional.ofNullable(user).filter(user2 -> user2.getName().get().length() < 6);
    System.out.println("user1=========="+user1);

结果为：

user1==========Optional[User(name=Optional[maol])]

如果user的name的长度是小于6的，则返回。如果是大于6的，则返回一个EMPTY对象。

    User user = new User();
    user.setName("maoliang");
    Optional<User> user1 =
        Optional.ofNullable(user).filter(user2 -> user2.getName().get().length() < 6);
    System.out.println("user1=========="+user1);

结果为：

user1==========Optional.empty

以前写法：

    public String getCity(User user){
        if (user != null) {
            if (user.getAddress() != null) {
                Address address = user.getAddress();
                if (address.getCity() != null) {
                    return address.getCity();
                }
            }
        }
        return "";
    }

Java8写法：

    public String getCity(User user) throws Exception {
        return Optional.ofNullable(user)
                .map(user1 -> user1.getAddress())
                .map(a->a.getCity())
                .orElseThrow(()->new Exception("取值错误"));
    }

例二：

    public static void main(String[] args) throws Exception {

        User user = new User();
        if (user != null) {
            doSomething();
        }

        Optional.ofNullable(user).ifPresent(user1 -> {
            doSomething();
        });
    }

    public static void doSomething(){

    }

例三：

    public User getUser(User user) {
        if (user != null) {
            if ("maoLiang".equals(user.getName())) {
                return user;
            }
        }
        User user1 = new User();
        user1.setName("maomao");
        return user1;
    }