如何从 java 中的 class object 列表中获取第一个匹配元素的索引（不使用任何第三方库）

Question

I have a List of class object (List). I want to get the index of the first user from the list with a particular user status.

public enum UserStatus {
    CREATED("CREATED"),
    AVAILABLE("AVAILABLE"),
    CHECKIN("CHECKIN"),
    CHECKOUT("CHECKOUT"),
    ON_TRIP("ONTRIP"),
    UNAVAILABLE("UNAVAILABLE");
 }

  Class Users{
        private Long id;
        private Long userId;        
        private UserStatus userStatus;
        private Date createdAt;
    }

Class Main{
    public static void main(String args[]){
        List<Users> userList = getListofUser(); // this method will give me list of users

        // here I want to find the first user whose status is AVAILABLE

    }
}

I am looking for similar to List indexOf() method

Answer 1

You can do this using Java streams and optionals

Optional<User>firstMatchingUserOptional = userList.stream.filter(user -> user.getUserStatus().equals(UserStatus.AVAILABLE)).findFirst();

if(firstMatchingUserOptional.isPresent())
    int firstMatchingUserIndex = userList.indexOf(firstMatchingUserOptional.get());

Answer 2

Just iterate through list and when you find AVAILABLE return from the loop.

Users getFirstAvailableUser(List<Users> userList ) {

   Iterator<Users> it = userList.iterator();
   while ( it.hasNext() ) {
      Users curr = it.next();
      if ( curr.getUserStatus() == UserStatus.AVAILABLE ) {
         return curr;
      }
   }

   return null;
}

Answer 3

You can use

int index = IntStream.range(0, userList.size())
    .filter(ix -> userList.get(ix).getUserStatus() == UserStatus.AVAILABLE)
    .findFirst().orElse(-1);

If you don't want to use the Stream API or suspect the list not to have efficient random access, you can use

int index = -1;
for(ListIterator<Users> i = userList.listIterator(); i.hasNext(); ) {
    if(i.next().getUserStatus() == UserStatus.AVAILABLE) {
        index = i.previousIndex();
        break;
    }
}

---

As a side note, the class name should reflect what a single instance of the class represents, ie User instead of Users when one object represents a single user. Further, there is no need to pass the names of the enum constants to themselves. They do already know their names.

When you declare the type as

public enum UserStatus {
   CREATED,
   AVAILABLE,
   CHECKIN,
   CHECKOUT,
   ON_TRIP,
   UNAVAILABLE
}

calling UserStatus.AVAILABLE.name() will give you the string "AVAILABLE" and calling UserStatus.valueOf("AVAILABLE") will return the constant UserStatus.AVAILABLE . You don't need to implement such logic yourself.

Answer 4

you can use for loop;

something like

`for(User tempUser : userList){
 if(tempUser.UserStatus.equals(UserStatusTemp)){
 //do job
 break;
  }
  }`