Java对对象数组进行排序

Question

I have a member application, the data is going into an object and then the object is being stored in the array. I am using string and integers in the object.

I am writing a method to display all male members and sort them by their score but I am struggling with the logic for this.

I am thinking I might need to create another array to store the male members in once I have got them out of the main array but am not really sure.

Could anyone help please.

public static void displayAllMaleMembers(){     
    System.out.println("List of male members:\n");

    for (int i=0; i < memberCount; i++) {                    
        Member member = memberList[i];                  
        if (member.getGender().equals("male") && member.getScore() > 50){
            System.out.println("Name:  " +member.getName());
            System.out.println("Score: " +member.getScore());
            System.out.println();   
        }           
        Arrays.sort(memberList, Collections.reverseOrder()); 
    }
    System.out.println("The sorted names by score are:");

    for (Member member : memberList) {
        System.out.println("Name:     " +member.getName());
        System.out.println("Handicap: " +member.getScore());
    }
}

Answer 1

The Problem

1. You're sorting the entire memberList whereas you should first find all the students satisfying the said criteria and then sort the list once.
2. You should not sort the memberList in each iteration as it's wrong and very inefficient.
3. When it comes to printing, again you're printing the entire list not just the male members with scores greater than 50 .

The Solution

First, you will need to copy only the male members with scores greater than 50 to a list:

List<Member> result = new ArrayList<>();
for(Member member : memberList)
    if ("male".equals(member.getGender()) 
                && member.getScore() > 50)
          result.add(member);

then sort it descending by member score:

result.sort(Comparator.comparingInt(Member::getScore).reversed());

then print:

result.forEach(member -> {
     System.out.println("Name:  " +member.getName());
     System.out.println("Score: " +member.getScore()); 
     System.out.println();   
});

Full code:

List<Member> result = new ArrayList<>();
for(Member member : memberList)
    if ("male".equals(member.getGender()) 
                && member.getScore() > 50)
      result.add(member);

result.sort(Comparator.comparingInt(Member::getScore).reversed());
result.forEach(member -> {
     System.out.println("Name:  " +member.getName());
     System.out.println("Score: " +member.getScore()); 
     System.out.println();   
});

Since you only want a result of male members sorted descending based on some criteria, you could also go with the stream approach simply because it's more readable and easier for this type of use case ie filtering and then applying further logic:

Arrays.stream(memberList) // create a Stream<Member>
      .filter(m -> "male".equals(m.getGender()) &&
            m.getScore() > 50) // retain only male members with score greater than 50
      .sorted(Comparator.comparingInt(Member::getScore).reversed()) // sort based on score descending i.e. highest to lowest
      .forEachOrdered(m -> {
             System.out.println("Name:     " +m.getName());
             System.out.println("Handicap: " +m.getScore());
             System.out.println();   
      }); // print the result

1. Arrays.stream creates a Stream<Member>
2. filter retains only the male members with score greater than 50
3. sorted sorts the members based on their scores from highest to lowest.
4. forEachOrdered then prints the results.

Answer 2

You need to have your Member class implement Comparable and then implement the compareTo function however you want

https://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html

Answer 3

First, the sort operation should be outside the for loop as tyou want to do that only one time. Second, you can sort the list in this way:

memberList.sort(Comparator.comparing(Member::getScore));

If your data are not stored in a List you can sort them in this way:

Arrays.sort(memberList, Comparator.comparingInt(Member::getScore));

Otherwise let your Member class implement Combarable interface.

Answer 4

If your Java version is 1.8 or greater , you can use Stream with an easiest way.

final List<Member> memberListWhichIsMaleAndScoreGreaterThan50 = Arrays.stream(memberList)
        .filter(member -> member.getGender().equals("male") && member.getScore() > 50)
        .collect(Collectors.toList());
//do  whatever you want on this list.

Here filter is filtering the list with gender and score . After that collecting to list to make an operation if you want.

Answer 5

I have a nice solution for you. Using constructors and getters and setters, without complex functions. Due to you only need to print name and score you can concatenate them as a single string, sort them and finally divide the string.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;

public class foo {

public Member me = new Member();

public void displayAllMaleMembers() {

    System.out.println("List of male members:");
    for (Member member : me.getMembers()) {
        if (member.getGender().equals("male") && member.getScore() > 50) {
            System.out.println("Name:  " + member.getName());
            System.out.println("Score: " + member.getScore());
            System.out.println();
        }
    }

    System.out.println("The sorted names by score are:");
    String[] sort = new String[me.getMembers().size()];
    // concatenate name + comma + score
    for (int i = 0; i < sort.length; i++) {
        sort[i] = me.getMembers().get(i).getName() + "," + me.getMembers().get(i).getScore();
    }
    Arrays.sort(sort, Collections.reverseOrder()); // sort the concatenated string
    // prints
    for (String s : sort) {
// Split() divides the concatenated string, 
// [0] is the first position of the string[] returned by spit(",");
        System.out.println("Name:     " + s.split(",")[0]); // string before comma: name
        System.out.println("Handicap: " + s.split(",")[1]); // string after comma: score
    }
}

//Main
public static void main(String[] args) {

    foo f = new foo();
    ArrayList<Member> m = new ArrayList<Member>();
    // Creates an arraylist using the constructor field from Member class
    m.add(new Member("Criss", 80, "male"));
    m.add(new Member("Frank", 20, "male"));
    m.add(new Member("Ann", 80, "femmale"));

    // adds the arraylist through setMembers method
    f.me.setMembers(m);
    // print data
    f.displayAllMaleMembers();
}

}

Getters and setters class: The arraylist<Members> stores all attributtes you need (score, name, gender)

import java.util.ArrayList;
public class Member {

String Name;
int Score;
String Gender;
ArrayList<Member> members = new ArrayList<>();

//Constructor using fields
public Member(String name, int score, String gender) {
    super();
    Name = name;
    Score = score;
    Gender = gender;
} 

// default
Member(){
}

public ArrayList<Member> getMembers() {
    return members;
}

// Creates an arrayList of Members
public void setMembers(ArrayList<Member> members) {
    this.members.addAll(members); //adds all arraylist items
}

public String getName() {
    return Name;
}

public void setName(String name) {
    Name = name;
}

public int getScore() {
    return Score;
}

public void setScore(int score) {
    Score = score;
}

public String getGender() {
    return Gender;
}

public void setGender(String gender) {
    Gender = gender;
}
}

Output:

List of male members:
Name:  Criss
Score: 80

The sorted names by score are:
Name:     Frank
Handicap: 20
Name:     Criss
Handicap: 80
Name:     Ann
Handicap: 80