key1:key2
key4:key2
key1:key3

key1:key3
key1:key2
key4:key2

public class myTest {
    public static void main(String a[]){
        Comparator<Empl> relationshipComparator = new Comparator<Empl>(){

            public int compare(Empl e1, Empl e2) {
                String e1Left = e1.getLeft();
                String e1Right = e1.getRight();

                String e2Left = e2.getLeft();
                String e2Right = e2.getRight();
                if (
                        e1Left.equals(e2Left) ||
                        e1Left.equals(e2Right) ||
                        e1Right.equals(e2Left) ||
                        e1Right.equals(e2Right))
                {
                    return 0;

                }else {
                    return -1;
                }
            }
        };

        LinkedList<Empl> list = new LinkedList<Empl>();
        list.add(new Empl("key1", "key2"));
        list.add(new Empl("key4", "key2"));
        list.add(new Empl("key1", "key3"));

        System.out.println("after: ");
        for(Empl e:list){
            System.out.println(e.getLeft()+":"+e.getRight());
        }
        Collections.sort(list,relationshipComparator);
        System.out.println();
        System.out.println("before: ");
        for(Empl e:list){
            System.out.println(e.getLeft()+":"+e.getRight());
        }
    }

}

public class Empl {
    private String left;
    private String right;

    public Empl (String left, String right){
        this.left=left;
        this.right=right;
    }

    public String getLeft() {
        return left;
    }

    public void setLeft(String left) {
        this.left = left;
    }

    public String getRight() {
        return right;
    }

    public void setRight(String right) {
        this.right = right;
    }
}