optional

2019-10-17

orElse

User user = Optional
        .ofNullable(getUserById(id))
        .orElse(new User(0, "Unknown"));

orElseGet

User user = Optional
        .ofNullable(getUserById(id))
        .orElseGet(() -> new User(0, "Unknown"));

orElseThrow

User user = Optional
        .ofNullable(getUserById(id))
        .orElseThrow(() -> new EntityNotFoundException("id 为 " + id + " 的用户没有找到"));

map

Optional<String> username = Optional
        .ofNullable(getUserById(id))
        .map(user -> user.getUsername())
        .map(name -> name.toLowerCase())
        .map(name -> name.replace('_', ' '));
        
System.out.println("Username is: " + username.orElse("Unknown"));

flatMap

use Optional as output

Optional<String> username = Optional
        .ofNullable(getUserById(id))
        .flatMap(user -> Optional.of(user.getUsername()))
        .flatMap(name -> Optional.of(name.toLowerCase()));
        
System.out.println("Username is: " + username.orElse("Unknown"));

filter

Optional<String> username = Optional
        .ofNullable(getUserById(id))
        .filter(user -> user.getId() < 10)
        .map(user -> user.getUsername());
        
System.out.println("Username is: " + username.orElse("Unknown"));