java comparator for simple custom logic

Question

How I can find line 4,1 and 6 in example below?
And is the use of Collection.sort() with Comparator reasonable in this case?

       a -  b - c - d

1.)    6    8   16  18   
2.)    38  40   55  57  
3.)    6    8   25  27  
4.)    1    5   11  15  
5.)    6    8    3   5  
6.)    9   12   19  22   
7.)    18  20    1   3  
8.)    23  25   15  17 

Example on the top is a List with object meets following criteria:
- every object contains 4 integer(a,b,c,d),
- every object in the list is unique,
- a < b and c < d.

Answer 1

What you need to do is implement a Comparator interface. Look up the JavaDocs for that interface. You will need to write a class that implements that interface. This involves writing one method (you don't need to reimplement equals()).

The method gets passed two objects. Look at what value you need to return from the method to show the two objects are 'equal' according to your requirements. Then write the code to return that value when they are 'equal', according to your requirements.

If any of that is unclear you will need to look up a basic Java textbook on writing methods, writing classes or using interfaces.

Answer 2

It seems you are confused where you would use a comparator. DJClayworth describes exactly HOW to create one. You would use one in, for instance, a sort mechanism:

Collections.sort(myList, myComparator);

You use this because you can define the comparison algorithm to sort through the collection. Hope this helps clarify somewhat.

Answer 3

@DJClayworth: Thank you and everybody for trying to help. Below is not working example, but my way of thinking, how I can expect comparator to work for finding expected object.

Pls check if this the right way to continue.

public class Row_Filter implements Comparable<Row_Filter>{
    int a,b,c,d;
    public Row_Filter(int a, int b, int c, int d) {
        this.a = a; this.b = b; this.c = c; this.d = d;
    }
   static class FilterAccordingAB implements Comparator<Row_Filter> {
        public int compare(Row_Filter o1, Row_Filter o2) {
            return o2.a - o1.b+1;
        }
    }
   static class FilterAccordingCD implements Comparator<Row_Filter> {
        public int compare(Row_Filter o1, Row_Filter o2) {
            return o2.c - o1.d+1;
        }
    }
   static class FilterAccordingABCD implements Comparator<Row_Filter> {
        public int compare(Row_Filter o1, Row_Filter o2) {
            FilterAccordingAB abF=null;    FilterAccordingCD cdF=null;
            if((abF.compare(o1, o2)==0) && (cdF.compare(o1, o2)==0)){
                return 1;
            }
            return -1;
        }
    }
}