Why my String is returning as a blank string?(JAVA)

Question

I am trying to return a string with time but somehow the returned string blank. I don't know what is the right way to do it.

static String timeConversion(String s) { 
     int hr=Integer.parseInt(s.substring(0,2));
     String min=(s.substring(3,5));
     String sec=(s.substring(6,8));
     String ap=s.substring(8,10);
     String res="";
     if(ap=="AM")
     {
         if(hr==12)
         {
             res.concat("00"+":"+min+":"+sec);
         }
         else
         {
             res.concat(hr+":"+min+":"+sec);
         }
     }
     else if(ap=="PM")
     {
         hr=hr+12;
         res.concat(hr+":"+min+":"+sec);
     }
     return res;
}

I can't remember in Java, but doesn't `concat` return a String (since Strings are immutable)? — Chris Forrence, Jan 08 '20 at 23:08

score 2 · Answer 1 · answered Jan 08 '20 at 23:14

There are two issues:

Since strings are immutable, you must assign the result of .concat(String) to the variable if you want it to be updated.
Comparing strings must be done using .equals()

With that in mind...

static String timeConversion(String s)
{
    // Expecting a string in the format "HH:MM:SSAM" or "HH:MM:SSPM"
    int hr = Integer.parseInt(s.substring(0, 2));
    String min = s.substring(3, 5);
    String sec = s.substring(6, 8);
    String ap = s.substring(8, 10);
    String res = "";
    if (ap.equals("AM")) {
        if (hr == 12) {
            res = res.concat("00" + ":" + min + ":" + sec);
        } else {
            res = res.concat(hr + ":" + min + ":" + sec);
        }
    } else if (ap.equals("PM")) {
        hr = hr + 12;
        res = res.concat(hr + ":" + min + ":" + sec);
    }
    return res;
}

As another note, while concat is OK in small doses, if you're concatenating multiple strings in a function/method (for example, in a loop), use a StringBuilder instead.

score 0 · Accepted Answer · answered Jan 08 '20 at 23:12

 int hr=Integer.parseInt(s.substring(0,2));
 String min=(s.substring(3,5));
 String sec=(s.substring(6,8));
 String ap=s.substring(8,10);
 String res="";

 System.out.println(ap) //see the value of ap in the console
 if(ap=="AM")// do ap.equalsIgnoreCase("AM")
 {
     if(hr==12)
     {
         res.concat("00"+":"+min+":"+sec);
         System.out.println(res) //see console and try res = res.concat ...

     }
     else
     {
         res.concat(hr+":"+min+":"+sec);
         System.out.println(res) // see console

     }
 }
 else if(ap=="PM")
 {
     hr=hr+12;
     res.concat(hr+":"+min+":"+sec);
     System.out.println(res)

 }
  System.out.println(res)
  return res;
}

you can also try to use + e.g res = res + ("00"+":"+min+":"+sec); you can also try using stringBuilder

i think your question is more like.. "how can i debug my code"; with the above solution. you will figure out where the problem is

Why my String is returning as a blank string?(JAVA)

2 Answers2