Question

I almost finished the code, but still I need some help. This code calculates newTime based on adding minutes to startTime. The minutes might have values greater than 60. The problem is that in the current example it must output “00:10”, but it outputs “23:70″…

    String startTime = "23:40";
    int minutes = 30;
    String[] hm = startTime.split(":");
    int h = minutes / 60 + Integer.parseInt(hm[0]);
    int m = minutes % 60 + Integer.parseInt(hm[1]);
    if (m<0) {
        if (h==0) 
            h=23;
        else 
            h = h-1;
        m = 60+m;
    }
    String newTime = String.format("%02d", h)+":"+String.format("%02d", m); 
    System.out.println(newTime);

Answer 1

I suggest calculating the total number of minutes since midnight as a single value, then adding the desired amount, and then recalculating the resulting hours and minutes:

String startTime = "23:40";
int minutes = 30;

String[] hm = startTime.split(":");
int h = Integer.parseInt(hm[0]);
int m = Integer.parseInt(hm[1]);

int t = h * 60 + m;      // total minutes
t += minutes;            // add the desired offset

while (t < 0) {          // fix `t` so that it's never negative
  t += 1440;             // 1440 minutes in a day
}

int nh = (t / 60) % 24;  // calculate new hours
int nm = t % 60;         // calculate new minutes

String newTime = String.format("%02d:%02d", nh, nm); 

You absolutely should not be adding your minutes variable to the calculation of h – they’re incompatible units.