使用Java 8将String列表转换为Object列表

Question

There is a stream read from file, the content of each line is like:

 {"uid":"5981865218","timestamp":1525309552069,"isHot":true}

The class of User:

public class User {
    private String uid;
    private long timestamp;
    private boolean isHot;

    public String getUid() {
        return uid;
    }

    public void setUid(String uid) {
        this.uid = uid;
    }

    public long getTimestamp() {
        return timestamp;
    }

    public void setTimestamp(long timestamp) {
        this.timestamp = timestamp;
    }

    public boolean getIsHot() {
        return isHot;
    }

    public void setIsHot(boolean isHot) {
        this.isHot = isHot;
    }
}

The code that I want to get a list of Object 'List' from file stream:

BufferedReader targetBr = null;
targetBr = new BufferedReader(new FileReader(targetUsersFile));
List<User> tmpUsers = targetBr.lines().?I don't know how process in there?.collect(Collectors.toList());

Answer 1

You need to deserialize the String to User objects. I have used Jackson ObjectMapper here (you could very well use others like Gson).

ObjectMapper objectMapper = new ObjectMapper();
...
targetBr.lines()
        .map(line -> {
            try {
                return objectMapper.readValue(line, User.class);
            } catch (IOException e) {
                e.printStackTrace();
                throw new RuntimeException(e);
            }
        }).collect(Collectors.toList());

The map part of the stream takes a Function and hence you cannot throw a checked exception from there. Hence, I have wrapped the IOException that Jackson readValue throws (could throw) into a RuntimeException. You might have to change that part according to your need.

This is just a start. Think about what to do when there is an invalid entry that cannot be deserialized into a User . Some specific corner cases:

1. What if there is an unrecognized field (a property that is not present in the User class) in the input string. ObjectMapper in this case throws an UnrecognizedPropertyException exception. You can find ways to ignore it.
2. What if one or more of fields in the User class is missing in the String... Are some/all of them mandatory...?