sorting string lengths using comparator

Question

While trying to sort an array based on its element string lengths, I am struck with a compile error. I have an set to start with, ,

Set<String> arraycat = new HashSet<String>();
//add contents to arraycat
String[] array = arraycat.toArray(new String[0]);
//array looks like this now:
//array=[cat,cataaaa,cataa,cata,cataaa]

I would ideally want to sorted to

array=[cat,cata,cataa,cataaa,cataaaa]

so I have a comparator of type

class comp implements Comparator {

    public int compare(String o1, String o2) {
        if (o1.length() > o2.length()) {
            return 1;
        } else if (o1.length() < o2.length()) {
            return -1;
        } else {
            return 0;
        }
    }
}

and then I call the class by

Collections.sort(array, new comp());

but then, it throws me two compile errors:

comp is not abstract and does not override abstract method   compare(java.lang.Object,java.lang.Object) in java.util.Comparator
class comp implements Comparator {
^
testa.java:59: cannot find symbol
symbol  : method sort(java.lang.String[],comp)
location: class java.util.Collections
Collections.sort(array, new comp());
^2 errors

I would appreciate any clues to solve the problem.

Laurence Gonsalves · Accepted Answer · 2015-04-13 16:04:30Z

17

You need to specify a type parameter for Comparator for your implementation to work.

class comp implements Comparator<String> {
  public int compare(String o1, String o2) {
    if (o1.length() > o2.length()) {
      return 1;
    } else if (o1.length() < o2.length()) {
      return -1;
    } else {
      return 0;
    }
  }
}

In Java 1.7 and later, you can also simplify the body of this method to:

class comp implements Comparator<String> {
  public int compare(String o1, String o2) {
    return Integer.compare(o1.length(), o2.length());
  }
}

Also, Collections.sort sorts List objects. Since you're sorting an array, you should use Arrays.sort:

Arrays.sort(array, new comp());

edited Apr 13, 2015 at 16:04

answered Dec 26, 2011 at 4:15

Laurence Gonsalves

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7 Comments

JohnJ Over a year ago

Hi Lawrence.. thanks for this.. That has solved the first compile error: but, Ive still got another compile error saying: "cannot find symbol symbol : method sort(java.lang.String[],comp) location: class java.util.Collections Collections.sort(array, new comp());" I have imported the util* package, not sure why I am getting this error

Rob Hruska Over a year ago

sort() takes a List, not a String[] array. Use Arrays.asList() to convert it.

Rob Hruska Over a year ago

Also, for a simpler comparator, consider using Integer.signum(int) instead of the if/else-if/else; e.g. return Integer.signum(o1.length() - o2.length());

Master_ex Over a year ago

this question is too old but anyway... there is no need to convert the array to list, you can sort it like that: Arrays.sort(array, new comp());

Chthonic Project Over a year ago

@RobHruska If JDK >= 1.7, one can even use Integer.compare(o1.length(), o2.length()).

|

KV Prajapati · Accepted Answer · 2011-12-26 04:56:20Z

7

You need to use Arrays.sort() method if data source is an array.

For instance,

String []array={"first","second","third","six"};

Arrays.sort(array,new Comparator<String>()
{
  public int compare(String s1,String s2)
   {
    return s1.length() - s2.length();
    }
});

Or convert array to List to use Collections.sort() method,

Collections.sort(Arrays.asList(array),new Comparator<String>()
{
  public int compare(String s1,String s2)
   {
    return s1.length() - s2.length();
    }
});

answered Dec 26, 2011 at 4:56

KV Prajapati

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Comments

Dhruvan Ganesh · Accepted Answer · 2018-04-24 13:43:45Z

6

One Liners

Ascending

Arrays.sort(words, Comparator.comparingInt(String::length));

Descending

Arrays.sort(words, Comparator.comparingInt(String::length).reversed());

answered Apr 24, 2018 at 13:43

Dhruvan Ganesh

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Comments

Michael Lorton · Accepted Answer · 2011-12-26 04:15:09Z

2

Should be

class comp implements Comparator<String> { ...

or even better

Collections.sort(array, new Comparator<String> { ...

(and not even name the class that's only used once)

answered Dec 26, 2011 at 4:15

Michael Lorton

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1 Comment

Michael Lorton Over a year ago

You need to specify the type-variable! Comparator <String>

Harshank · Accepted Answer · 2021-03-31 18:12:38Z

1

Achieved same by using stream and comparator as below-

Arrays.stream(array)
      .sorted(Comparator.comparingInt(String::length))
      .forEach(a -> System.out.print(a + " "));

answered Mar 31, 2021 at 18:12

Harshank

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Comments

Kara · Accepted Answer · 2014-06-11 20:51:40Z

I had a similar assignment recently and I'll provide you with an additional example.

import java.util.Arrays;
import java.util.Comparator;
//for more information: http://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html

/**
 *
 * @author Xilef
 */
public class StringKorter implements Comparator<String> {
    @Override
    public int compare(String s1, String s2){
        if (s1.length() > s2.length())
         return 1;
        else if (s1.length() < s2.length())
         return -1;
        else return 0;
    }
    public static void main(String[] args) {
        String[] woorden = { "boot", "kinderen", "stoel", "volwassenen", "ei", "stoel", "kop", "zeshoek", "stoel", "ei" };
        System.out.println("woorden: " + Arrays.toString(woorden));//before sorting by length
        Arrays.sort(woorden, new StringKorter());
        System.out.println("Array woorden after sorting by length: " + Arrays.toString(woorden));
    }
}

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