The java compare

public int compare (Object o1, Object o2)
the original order is the order parameter o1, O2;
if the sequence returns -1 holding exchange sequence returns 1, do nothing returns 0;
therefore, in ascending order, then, If o1 <o2, returns -1.

Example: two-dimensional array num achieved by the first column in ascending order, the first column of the second row by the same descending order.

Arrays.sort (Yan, (O1, O2) -> { 
    ! IF (O1 [0] = O2 [0]) { 
        return O1 [0] - O2 [0]; // first column in ascending order 
    } the else { 
        return o2 [1] - o1 [1 ]; // second column in descending order 
    } 
});