W3Schools Tryit Editor

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

class SortEvenFirst implements Comparator {
  public int compare(Object obj1, Object obj2) {
  	// Make sure the objects are integers
  	Integer a = (Integer)obj1;
    Integer b = (Integer)obj2;
    
    // Check each number to see if it is even
    // A number is even if the remainder when dividing by 2 is 0
    boolean aIsEven = (a % 2) == 0;
    boolean bIsEven = (b % 2) == 0;
    
    if (aIsEven == bIsEven) {
    
      // If both numbers are even or both are odd then use normal sorting rules
      if (a < b) return -1;
      if (a > b) return 1;
      return 0;
      
    } else {
    
      // If a is even then it goes first, otherwise b goes first
      if (aIsEven) {
      	return -1;
      } else {
        return 1;
      }
    }
  }
}

public class Main {
  public static void main(String[] args) {
    ArrayList<Integer> myNumbers = new ArrayList<Integer>();
    myNumbers.add(33);
    myNumbers.add(15);
    myNumbers.add(20);
    myNumbers.add(34);
    myNumbers.add(8);
    myNumbers.add(12);

Comparator myComparator = new SortEvenFirst();
    Collections.sort(myNumbers, myComparator);

for (int i : myNumbers) {
      System.out.println(i);
    }
  }
}