MyComp

package ch.fhnw.claudemartin;

import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;

public class SomeClass {

  public static void main(final String[] arrg) {
    final int[] arr = { 2, 5, 2, 8, 5, 6, 8, 8 };
    final int[] sorted = Arrays.stream(arr).distinct().boxed()
        .sorted(new MyComp(arr)).mapToInt(i -> i).toArray();
    System.out.println(Arrays.toString(sorted));
  }

  static class MyComp implements Comparator<Integer> {

    final HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
    final int[]                     data;

    private boolean                 initialized = false;
    public MyComp(final int[] data) {
      this.data = data;
    }

    private void init() {
      if(this.initialized) return;
      this.initialized = true;

      for (final int i : this.data) {
        // could use map.computeIfAbsent(key, mappingFunction)
        if (this.map.get(i) == null) {
          this.map.put(i, 1);
        } else {
          int count = this.map.get(i);
          this.map.put(i, ++count);
        }
      }

    }

    @Override
    public int compare(final Integer o1, final Integer o2) {
      this.init();
      final int count1 = this.map.get(o1);
      final int count2 = this.map.get(o2);
      return Integer.compare(count2, count1);
    }
  }
}