使用Java中的Comparator对List进行排序

Question

I have two class

public class PopularSports {    
    public int Rank;    
    public String Sport;
}

public class Medals {
    public String Sport    
    public String Medal    
    public int Year    
    public String Game    
    public String Athlete
}

When i try to lead their csv and get the result in List of each class, I want to sort them in following order.

The PopularSports class should sort in the following order:

1. Rank
2. Sport

The Medals class should sort in the following order:

1. Sport
2. Medal
3. Year
4. Game
5. Athlete

I have created compactor like this

Collections.sort(medalReportList, new Comparator<Medals >() {
  @Override
  public int compare(Medals o1, Medals o2) {
   return getRankOfSportBySportName(o1.getSport()) > getRankOfSportBySportName(o2.getSport())
          ? +1
          : getRankOfSportBySportName(o1.getSport()) < getRankOfSportBySportName(o2.getSport())
          ? -1
          : o1.getAthlete().compareToIgnoreCase(o2.getAthlete());
    }
  });

But it is not making it happen, it sorts on athlete names.

Answer 1

You need to nest those checks. I'll write the PopularSports method and leave the other for you.

I'd recommend that you learn that Java coding standards and make those class names singular: PopularSport and Medal . I'll demonstrate below:

public int compareTo(PopularSport other) {
    if (this.rank < other.rank) return -1;
    else if (this.rank > other.rank) return +1;
    else {
        // This is where the nesting happens to break the ties.
        if (this.sport.compareTo(other.sport) < 0) return -1;
        else if (this.sport.compareTo(other.sport) > 0) return +1;
        else return 0;  // You'll have deeper nesting for the other values in Medal.
    }
}

The order you choose determines which ones are checked first. You nest in the else clause to break the ties.

Answer 2

I would simplify you logic a bit.

public int compare(Medals o1, Medals o2) {
  int rankDifference = getRankOfSportBySportName(o1.getSport()) 
                             - getRankOfSportBySportName(o2.getSport());
  return rankDifference != 0 
      ? rankDifference
      : o1.getAthlete().compareToIgnoreCase(o2.getAthlete());
  }

Answer 3

Others already gave you a solution with ternary operator. I, on the other hand, discourage you from using it in complicated situations - it's simply not readable and hard to maintain.

Either go with if/else blocks (which will look pretty bad with so many fields to compare) or, even better, use a third party library. In Guava you can use ComparisionChain like this:

new Comparator<Medals>() {
    @Override
    public int compare(Medals m1, Medals m1) {
        return ComparisonChain.start()
         .compare(m1.getSport(), m2.getSport())
         .compare(m1.getMedal(), m2.getMedal())
         .compare(m1.getYear(), m2.getYear())
         .compare(m1.getGame(), m2.getGame())
         .compare(m1.getAthlete(), m2.getAthlete(), String.CASE_INSENSITIVE_ORDER)
         .result();
    }
}

Isn't that more readable?!

Answer 4

Sorting first list will no problem to you. If you want to sort as if sportName is same then sort according to medal and if medal name is same then sort according to year then solution is :

Best way:

public void sortAccordingToAboveRequirement(List<Medal> medals){

 Collections.sort(medals,myComparator);

 }


  static Comparator<Medal> myComparator=new Comparator<Medal>() {
    @Override
        public int compare(Medal medal1, Medal medal2) {
        int value=0;
        value=medal1.getSport().compareTo(medal2.getSport());
        if(value!=0) return value;
        value=medal1.getMedal().compareTo(medal2.getMedal());
        if(value!=0) return value;
        value=medal1.getYear().compareTo(medal2.getYear());
        if(value!=0) return value;
        value=medal1.getGame().compareTo(medal2.getGame());
        if(value!=0) return value;
        value=medal1.getAthlete().compareTo(medal2.Athlete());
        return value;
    }
};

Use compareToIgnoreCase() if you wish :)