内部类中的比较器（int）

Question

I have some problem with comparator. I make very simple class with 2 fields and i need make inner classes with comparators for this two fields.

public class KlassA {

    private int i;
    private String tekst;  

    public int getI(){
        return i;

This comparator works ok.

   public static class KomparatorText implements Comparator<KlassA>{
        public int compare(KlassA a1, KlassA a2) {
            return a1.getTekst().compareTo(a2.getTekst());
        }

   }

In this comparator i have some issue: "Cannot invoke compareTo(int) on the primitive type int"

   public static class KomparatorI implements Comparator<KlassA>{
        public int compare(KlassA a1, KlassA a2) {
            return a1.getI().compareTo(a2.getI());
        }
   }

I also try CompareTo but then i need to implements field or method and it's not work

   public static class KomparatorI implements Comparable<KlassA>{

        private int i = this.i;

        public int compareTo(KlassA o) {
           if (this.i < o.i)
              return -1;
           else if (this.getI() == o.getI())
              return 0;
           else
              return 1;
       }
   }
}

I really don't know how to fix it. I'm already serach the other inquiries but all sollutions don't work for my example. Pls help.

Answer 1

You could force everything to Integers as in the other answers, but, IMO, better to keep your current class structure with ints and to use a built in method, Integer.compare()

public static class KomparatorI implements Comparator<KlassA>{
   public int compare(KlassA a1, KlassA a2) {
      return Integer.compare(a1.getI(), a2.getI());
   }
}

Likewise, if you wish to use Comparable instead, use it to save typing (and avoid mistakes in the logic):

public static class KomparatorI implements Comparable<KlassA>{
   public int compareTo(KlassA o) {
      return Integer.compare(this.i, o.i);
   }
}

Answer 2

Lets see what you gout here

a1.getI().compareTo(a2.getI());

it looks like getI() returns int insteed of Integer .

Change mehod definition to return Integer insteed and all wil be fine. So inside your KlaasA

public Integer getI(){ return i};

Answer 3

In Java, primitive types like int , float , or boolean are not objects like the correspondent Integer , Float , or Boolean .This means you cannot call methods on them.

Try using,

Integer.valueOf(this.i).compareTo(o.i);

Your confusion probably comes from the fact that the compiler tries to covert primitive types to their corresponding object classes automatically. In the sample above this happens for the argument oi , which is converted automatically to an Integer . Try searching for "java autoboxing".

Note : You can actually use Integer.compare(this.i, oi) which is better, for that case.