删除文本文件中的ArrayList重复项-Java

Question

I want to remove the duplicates of userNames in the ArrayList. I've tried to convert the ArrayList to an HashSet, but for some reason it doesn't work. The reason why I chose to convert the ArrayList into a HashSet is because it does not allow duplicated values, however, when I use it on my code, it only changes the order in the list:

My code output:

Choreography - Imran Sullivan
Goodfella - Khalil Person
DarknianIdeal - Sophia Jeffery
DarknianIdeal - Clyde Calderon
Frolicwas - Taylor Vargas
Reliable - Aryan Hess
DarknianIdeal - Liyah Navarro
Deservedness - Eadie Jefferson
Reliable - Angel Whitehouse
Choreography - Priya Oliver

How the output should be:

Choreography - Imran Sullivan
Goodfella - Khalil Person
DarknianIdeal - Sophia Jeffery
Frolicwas - Taylor Vargas
Reliable - Aryan Hess
Deservedness - Eadie Jefferson

This is the code. I've splitted the data into an Array so I can print out the data individually.

import java.util.*;
import java.io.*;

public class Task1 {
    public static void main(String[] args) {   
        List<Person> personFile = new ArrayList<>();
        Set<Person> splitUserNameList = new HashSet<>(personFile);

        try {            
            BufferedReader br = new BufferedReader(new FileReader("person-data.txt"));
            String fileRead = br.readLine();
            while (fileRead != null) {
                String[] personData = fileRead.split(":");                
                String personName = personData[0];
                String userNameGenerator = personData[1];
                String[] userNameSplit = userNameGenerator.split(",");                
                String newUserNameSplit = userNameSplit[0];                
                Person personObj = new Person(personName, newUserNameSplit);
                splitUserNameList.add(personObj);                
                fileRead = br.readLine();
            }
            br.close();            
        }  
        catch (FileNotFoundException ex) {            
            System.out.println("File not found!");
        } 
        catch (IOException ex) {             
            System.out.println("An error has occured: " + ex.getMessage());
        }

        for (Person userNames: splitUserNameList) {                
            System.out.println(userNames);           
        }           
    } 
}

/* Person Class */

public class Person {
    private String personName;
    private String userNameGenerator;

    public Person(String personName, String userNameGenerator) {
        this.personName = personName;
        this.userNameGenerator = userNameGenerator;
    }

    public String getPersonName() {
        return personName;
    }

    public String getUserNameGenerator() {
        return userNameGenerator;
    }

    @Override
    public String toString() {        
        return userNameGenerator + " - " + personName;
    }
}

Answer 1

You need to override the equals and hashCode methods on your Person object in order for Set to know which objects are considered the same.

It seems you want any two people with the same userNameGenerator field to be considered equal. In that case, the following will suit your needs:

public class Person {
    private String commonName;
    private String userNameGenerator;

    ...

    @Override
    public int hashCode() 
    {
        return userNameGenerator.hashCode();
    }

    @Override
    public boolean equals(Object o) 
    {
        if (this == o) return true;

        if (this.getClass() != o.getClass()) return false;

        final Person that = (Person) o;

        return this.userNameGenerator.equals(that.userNameGenerator);
    } 
}

Some important things to note:

• In the case of duplicates, the Set will only allow in the first one so insertion order becomes important .

• Sets should not contain mutable elements because mutability destroys their internal consistency. Your Person class is immutable (has no setters) which is good but you may want to enforce this immutability even further by declaring all of it's fields final .