Java 8 之 Optional

流光幻影發表於2022-03-19
Java

Java 8 之 Optional

背景介紹：在我們實際開發中，經常會遇到各種 NullPointerException 報錯，為了避免此類問題，需要在業務程式碼上新增各種if判斷，使程式碼看起來很臃腫，而且很不美觀

使用Optional 提供的各種方法，能夠很好的解決上述的問題，下面介紹一下Optional的基礎API

準備工作：新建一個User類及Address類

@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

當value為null時，取value1，否則取value

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

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

2.orElseThrow

當value為null時，丟擲引數中指定的異常

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

3.orElseGet

如果存在則返回該值，否則呼叫other值並返回該呼叫的結果。

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

4.isPresent

如果存在值，則返回true ，否則返回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，則執行定義程式碼，否則什麼也不做。

User user = null;
Optional<User> user1 = Optional.ofNullable(user);
user1.ifPresent(u -> System.out.println(u)); // 什麼都不輸出

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，則對其應用提供的對映函式，如果結果為非 null，則返回描述結果的Optional 。否則返回一個空的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，則將提供的Optional函式應用於它，返回該結果，否則返回空Optional 。與map的不同是 flatMap提供的函式必需是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

對 Optional 中包含的值進行過濾，如果包含的值滿足條件，那麼還是返回這個 Optional；否則返回 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

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