Java：不兼容的类型

Question

import java.text.*;
import java.util.*;
public class Proj3 {
    public static void main(String[]args){
     // DecimalFormat df = new DecimalFormat("#0.00”);
     Scanner s = new Scanner(System.in);
     int TotalHours = 0;
     int TotalGrade = 0;

     System.out.print("How many courses did you take? ");
     int Courses = Integer.parseInt(s.nextLine());
     System.out.println("");

     int CourseNumber = Courses - (Courses - 1);
     while (Courses > 0){
         System.out.print("Course (" + CourseNumber +"): How many hours? ");
         int Hours = Integer.parseInt(s.nextLine());
         TotalHours = TotalHours + Hours;

         System.out.print("Course (" + CourseNumber +"): Letter grade? ");
         char Grade = s.nextLine().charAt(0);

         if (Grade == 'A'){
             TotalGrade = TotalGrade + (4 * Hours);
            }
         if (Grade == 'B'){
             TotalGrade = TotalGrade + (3 * Hours);
            }
         if (Grade == 'C'){
             TotalGrade = TotalGrade + (2 * Hours);
            }
         if (Grade == 'D'){
             TotalGrade = TotalGrade + (1 * Hours);
            }

         Courses = Courses - 1;
         CourseNumber = CourseNumber + 1;
        }
     Double GPA = TotalGrade / TotalHours;
     System.out.println(df.format(GPA));

    }
}

This is for an assignment and I don't know how to fix my problem. The Double GPA = TotalGrade / ToutalHours; line is coming up with the Incompatible Types error.

Also I'm supposed to include the DecimalFormat df = new DecimalFormat("#0.00”); line at the beginning of the main but its not working.

Anything is very helpful. Thanks

Answer 1

 Double GPA = TotalGrade / TotalHours;

Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes. Hence Double is wrapper class for primitive double and Integer is wrapper class for primitive int .

Your TotalGrade / TotalHours is resulting in an int but you are trying to assign it to object with type Double . Do either:

double GPA = (double)TotalGrade / TotalHours;

or,

Double GPA = (double)TotalGrade / TotalHours;

Answer 2

You are using a wrapper class Double . While your variables TotalGrade / ToutalHours are primitive types int. Hence they are incompatible. Using primitive double will solve this issue.

double GPA = TotalGrade / ToutalHours;

Same for DecimalFormat issue, the format method doesn't accept wrapper Double . So when you use primitive double it will solve the issue.

Answer 3

There are two problems in your code

1. Java allows type promotion but you are trying to promote an int value to Double which is not possible this can be resolved in following two way

change your Double GPA = TotalGrade / TotalHours;

to Double GPA = (double)TotalGrade / TotalHours; //this is explicit type casting

or double GPA = TotalGrade/ TotalHours // this type promotion

Second problem is

// DecimalFormat df = new DecimalFormat("#0.00”);

in this line see the closing quotes its wrong change it to "

Answer 4

You are trying to convert int To Wrapper class Double.

Replace

Double GPA = TotalGrade / TotalHours;

With

double GPA = TotalGrade /TotalHours;

AND you can write

NumberFormat formatter = new DecimalFormat("#0.000");

At begining of main if you want and print it like

System.out.println("The Decimal Value is:"+formatter.format(GPA));