ADDITIONAL SYSTEM INFORMATION :
String temp1 = "2";
String temp2 = "19";
int result1 = temp1.compareTo(temp2);
The test code is as above:
According to the default value of result1, it should be a negative number, but the result is a positive number. I looked at the source code of compareTo, which converts two String types into Char[] arrays to compare the sizes. When compared to char[0] = 2, and When temp2's char[0]=1, the result has been returned. There is no comparison of the digits, but a simple comparison from beginning to end.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
  public static void main(String[] args) {
         //actual quantity
         String temp1 = "2";
         //Actual quantity received
         String temp2 = "19";
         //The number of masters to be issued
         String temp3 = "2.0000";
         //The number of real issuers
         String temp4 = "21.0000";

         int result1 = temp1.compareTo(temp2);
         int result2 = temp3.compareTo(temp4);

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Default result:
result1 is a negative number
result2 is a negative number
ACTUAL -
actual results:
result1 is a positive number
result2 is a negative number
---------- BEGIN SOURCE ----------

public class CompaerNumTest {
    public static void main(String[] args) {
        //actual quantity
         String temp1 = "2";
         //Actual quantity received
         String temp2 = "19";
         //The number of masters to be issued
         String temp3 = "2.0000";
         //The number of real issuers
        String temp4 = "21.0000";

        int result1 = temp1.compareTo(temp2);
        int result2 = temp3.compareTo(temp4);

        if (result1>=0 && result2>=0){
            System.out.println(result1);
            System.out.println(result2);
        }

    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
For the time being, I convert the String type to Double type for size comparison.