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I want to read a number from the user and then sum the last seven digits of the entered number. What is the best way to do this? This is my code, but unfortunately it does not work:

class ersteAufgabe 
 public static void main (String args)
 
 Scanner s = new Scanner(System.in);
 double a = new double[10];
 for (int i = 0;i<10;i++)
 
 a[i]=s.nextInt();

 
 s.close();
 System.out.println(a[0]);
 

I wanted only one number to be read and used as an array. Only now he expects 10 inputs from me.

public static int lastDigitsSum(int total) 
 try (Scanner scan = new Scanner(System.in)) 
 String str = scan.next();
 int count = 0;

 for (int i = str.length() - 1, j = 0; i >= 0 && j < total; i--, j++) 
 if (Character.isDigit(str.charAt(i)))
 count += str.charAt(i) - '0';
 else
 throw new RuntimeException("Input is not a number: " + str);
 

 return count;
 

First you have to recognize if the entered value is a number and has at least 7 digits. Unless you have to output an error message. Convert the entered value to String and use the class Character.isDigit(); to check if the characters are numbers. Then you can use some methods from the String class like substring(..). At the end do a Unit-Test with erroneous/valid values to see if your code is robust. Close the BufferedReader and Resources when you are done by using finally br.close() . Push your code in methods and use an instance class erste-Aufgabe (first exercise).. When you are really really done use JFrame for a GUI-Application.

private static final int SUM_LAST_DIGITS = 7;

public void minimalSolution() 
 String enteredValue = "";
 showInfoMessage("Please enter your number with at least " + SUM_LAST_DIGITS + " digits!");
 try (Scanner scan = new Scanner(System.in)) 
 enteredValue = scan.next();
 if (enteredValue.matches("^[0-9]" + SUM_LAST_DIGITS + ",$")) 
 showInfoMessage(enteredValue, lastDigitsSum(enteredValue));
 else 
 showErrorMessage(enteredValue);
 
 catch(Exception e) 
 showErrorMessage(e.toString());
 


public int lastDigitsSum(String value) 
 int count = 0;
 for (int i = value.length() - 1, j = 0; i >= 0 && j < SUM_LAST_DIGITS; i--, j++)
 count += value.charAt(i) - '0';
 return count;


public void showInfoMessage(String parMessage) 
 System.out.println(parMessage);


public void showInfoMessage(String parValue, int parSum) 
 System.out.println("Your entered value: [" + parValue + "]");
 System.out.println("The summed value of the last 7 digits are: [" + parSum + "]");


public void showErrorMessage(String parValue) 
 System.err.println("Your entered value: [" + parValue + "] is not a valid number!");