Java 8 之 Optional

背景介绍：在我们实际开发中，经常会遇到各种 NullPointerException 报错，为了避免此类问题，需要在业务代码上添加各种if判断，使代码看起来很臃肿，而且很不美观

使用Optional 提供的各种方法，能够很好的解决上述的问题，下面介绍一下Optional的基础API

准备工作：新建一个User类及Address类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class User {

    private String id;

    private String name;

    private Address address;
}

@NoArgsConstructor
@AllArgsConstructor
@Data
public class Address {

    private String province;

    private String city;

}

1.orElse

当value为null时，取value1，否则取value

User user = null;
// 1.user为null时
User user1 = new User("001", "user1", null);
User result = Optional.ofNullable(user).orElse(user1);
System.out.println(result); //  输出 User(id=001, name=user1, address=null)

// 2.user 不为null时
user = new User("000","user",null);
result = Optional.ofNullable(user).orElse(user1);
System.out.println(result); //输出 User(id=000, name=user, address=null)

2.orElseThrow

当value为null时，抛出参数中指定的异常

User user = null;
// 写法1
User result = Optional.ofNullable(user).orElseThrow(NullPointerException::new);
// 写法2
 User result = Optional.ofNullable(user).orElseThrow(()->new Exception("user为空"));

3.orElseGet

如果存在则返回该值，否则调用other值并返回该调用的结果。

User user = null;
User result = Optional.ofNullable(user).orElseGet(()->new User("1111","other",null));
System.out.println(result); //  输出 User(id=1111, name=other, address=null)

4.isPresent

如果存在值，则返回true ，否则返回false

User user = null;
boolean present = Optional.ofNullable(user).isPresent();
System.out.println(present); // false

User user1 = new User();
boolean present1 = Optional.ofNullable(user1).isPresent();
System.out.println(present1); // true

5.ifPresent

如果存在值，则执行定义代码，否则什么也不做。

User user = null;
Optional<User> user1 = Optional.ofNullable(user);
user1.ifPresent(u -> System.out.println(u)); // 什么都不输出

User user = new User();
Optional<User> user1 = Optional.ofNullable(user);
user1.ifPresent(u -> System.out.println(u)); // User(id=null, name=null, address=null)

6.map

如果存在值，则对其应用提供的映射函数，如果结果为非 null，则返回描述结果的Optional 。否则返回一个空的Optional

Address address = new Address("浙江","杭州");
User user = new User();
user.setId("001");
user.setName("ll");
user.setAddress(address);

String s = Optional.ofNullable(user)
  .map(u -> u.getAddress())
  .map(address1 -> address1.getCity())
  .orElseThrow(() -> new Exception("111"));
System.out.println(s); // 杭州

7.flatMap

如果存在值，则将提供的Optional函数应用于它，返回该结果，否则返回空Optional 。与map的不同是 flatMap提供的函数必需是Optional的

Address address = new Address("浙江","杭州");
User user = new User();
user.setId("001");
user.setName("ll");
user.setAddress(address)
Optional<String> s1 = Optional.ofNullable(user)
  .flatMap(user1 -> Optional.of(user1.getAddress()))
  .flatMap(address1 -> Optional.of(address1.getCity()));
System.out.println(s1.orElse("null")); // 杭州

8.filter

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。

Address address = new Address("浙江","杭州");
User user = new User();
user.setId("001");
user.setName("ll");
user.setAddress(address);

String a = Optional.ofNullable(user)
  .filter(user1 -> user1.getId().equals("001"))
  .map(user1 -> user1.getId())
  .orElse("null");
System.out.println(a); // 001

String b = Optional.ofNullable(user)
  .filter(user1 -> user1.getId().equals("002"))
  .map(user1 -> user1.getId())
  .orElse("null");
System.out.println(b); // null

流光幻影