How can I efficiently convert a user-input char into a numerical value (double) in Java?

Question

I'm new to programming, especially in Java, so please bear with what may be a very noob-ish question. My main goal here is to create a simple program that calculates the user's GPA by taking three character inputs and displaying the result as a double. For this program I was only required to represent basic letter grades (no A+ or A- or anything like that, just A-F) with their corresponding numerical value (4.0-0.0).

I was able to get the program to work however my question lies in the way that I had to convert, or more so associate, the user's input into/with a decimal.

My approach to this, as you can see in the code below, was to have a user input their letter grade, create a separate variable that would hold the numerical value of each character for each class and use if statements in order to associate that value with it's corresponding character in it's corresponding class. Although it seems to work okay, is there a more efficient way of doing this? I found the if statements sort of inefficient as it seems like a lot of code for what I'm trying to accomplish.

Also, on a side note, I'm using Eclipse as my main IDE for Java and I keep on getting the yellow warning squiggly line message under the first "in" when I type "Scanner in = new Scanner(System.in);" It says "Resource leak: 'in' is never closed" and I have no idea what that means. My programs always run but I hate seeing the warning sign on my files in the package explorer :/

* Lab9.java
package labs;
import java.util.Scanner;

public class Lab9 {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.print("Please input 3 letter grades: ");
        @SuppressWarnings("resource")
        Scanner in = new Scanner(System.in);
        char class1 = in.next() .charAt(0);
        char class2 = in.next() .charAt(0);
        char class3 = in.next() .charAt(0);
    
        double g1 = 0, g2 = 0, g3 = 0;
    
        if (class1 == 'A') {
            g1 = 4.0;
        }
        if (class1 == 'B') {
            g1 = 3.0;
        }
        if (class1 == 'C') {
            g1 = 2.0;
        }
        if (class1 == 'D') {
            g1 = 1.0;
        }
        if (class1 == 'F') {
            g1 = 0.0;
        }
    
        if (class2 == 'A') {
            g2 = 4.0;
        }
        if (class2 == 'B') {
            g2 = 3.0;
        }
        if (class2 == 'C') {
            g2 = 2.0;
        }
        if (class2 == 'D') {
            g2 = 1.0;
        }
        if (class2 == 'F') {
            g2 = 0.0;
        }
    
        if (class3 == 'A') {
            g3 = 4.0;
        }
        if (class3 == 'B') {
            g3 = 3.0;
        }
        if (class3 == 'C') {
            g3 = 2.0;
        }
        if (class3 == 'D') {
            g3 = 1.0;
        }
        if (class3 == 'F') {
            g3 = 0.0;
        }
    
        System.out.print("Your GPA is a "+((g1 + g2 + g3)/3));
    }
}

Here is the output of a test run:

Please input 3 letter grades: A C B
Your GPA is a 3.0

Thank you for the help!

Use switch instead of 'if' and wrap your code of converting grade into a double/float inside a function. — Vaibhav Pratihar
– Vaibhav Pratihar, Commented Jan 21, 2021 at 4:11
One rule-of-thumb in programming is that if you repeat code, it should be turned into a loop or a new method. — NomadMaker
– NomadMaker, Commented Jan 21, 2021 at 4:22
As for the resource leak warning, It is a good practice to always close an object, that you no longer need after it's usage. In this case, after you are done with reading the input, the Scanner object is no longer needed. Although, after your program exits, garbage collector would collect the non-referenced memory, Its still a good practice to manually close your objects. So after you are done with user input, just call sc.close() — Arun123
– Arun123, Commented Jan 21, 2021 at 5:37
@Arun123 thank you for explaining this, it was driving me crazy seeing so many warnings. — ETHN
– ETHN, Commented Jan 21, 2021 at 16:39

jacobeng3l · Accepted Answer · 2021-01-21 05:06:14Z

While if statements work just fine, there are some slightly more java like implementations, mainly using the switch ... case statements in Java (more can be read about them on Oracle's Page).

Essentially though, the if statements could be replaced with

...
switch(class1) {
                 case 'A':       g1 = 4.0;
                                 break;
                 case 'B':       g1 = 3.0;
                                 break;
                 case 'C':       g1 = 2.0;
                                 break;
                 case 'D':       g1 = 1.0;
                                 break;
                 default:        g1 = 0.0;
                                 break;
         }

         switch(class2) {
                 case 'A':       g2 = 4.0;
                                 break;
                 case 'B':       g2 = 3.0;
                                 break;
                 case 'C':       g2 = 2.0;
                                 break;
                 case 'D':       g2 = 1.0;
                                 break;
                 default:        g2 = 0.0;
                                 break;
        }
          switch(class3) {
                 case 'A':       g3 = 4.0;
                                 break;
                 case 'B':       g3 = 3.0;
                                 break;
                 case 'C':       g3 = 2.0;
                                 break;
                 case 'D':       g3 = 1.0;
                                 break;
                 default:        g3 = 0.0;
                                 break;
         }
...

The default statement would also serve as some error handling, taking in any rogue values and assigning them 0.0. break is a more complex term, but it is needed in the switch case statement.

Beyond this, you could create a method to reduce duplicate code.

public class Lab9 {
     /**
      * @param letter
      */
      private static double letterValue(char letter) {
         double value = 0.0;
         switch(letter) {
                 case 'A':       value = 4.0;
                                 break;
                 case 'B':       value = 3.0;
                                 break;
                 case 'C':       value = 2.0;
                                 break;
                 case 'D':       value = 1.0;
                                 break;
                 default:        value = 0.0;
                                 break;
         }
         return(value);
     }
     /**
      * @param args
      */
     public static void main(String[] args) {
         // TODO Auto-generated method stub
         System.out.print("Please input 3 letter grades: ");
         @SuppressWarnings("resource")
         Scanner in = new Scanner(System.in);
         char class1 = in.next() .charAt(0);
         char class2 = in.next() .charAt(0);
         char class3 = in.next() .charAt(0);
         double g1 = 0, g2 = 0, g3 = 0;
         g1 = Lab9.letterValue(class1);
         g2 = Lab9.letterValue(class2);
         g3 = Lab9.letterValue(class3);
         System.out.print("Your GPA is a "+((g1 + g2 + g3)/3));
     }
 }

To do it with minimal code, you could actually leverage the fact that characters have numeric representations (normally using the ASCII Enconding framework). This would allow you to simplify the method. This uses some more advanced topics such as type casting, so be wary.

     private static double letterValue(char letter) {
         double value = 0.0;
         if((int)letter < 70) {
                 value = (((int)letter) - 69) * -1;
         }
         return(value);
     }

Essentially this takes the numeric values of the letter (A: 65, B: 66, ...) and subtracts 69 to make them the correlated negative values, and then flips them to be positive. It assigns anything out of the range a value of 0.0 (including Es, which is a strange edge case). This is not an intuitive representation of what the code is doing, so I would not recommend it.

All that being said, none of these are significantly more efficient than any other. They all function reasonably similarly (ignoring the last numeric operations), and therefore runtime wouldn't be significantly impacted. Some look cleaner than others, and that is worth something.

Thank you for the detailed explanation! I greatly appreciate your help with this :)

Trần Trí Nhân · Accepted Answer · 2021-01-21 04:33:47Z

you can use "switch case" for this:

public static void main(String[] args) {

    System.out.print("Please input 3 letter grades: ");
    @SuppressWarnings("resource")
    Scanner in = new Scanner(System.in);
    char class1 = in.next().charAt(0);
    char class2 = in.next().charAt(0);
    char class3 = in.next().charAt(0);

    double g1 = 0, g2 = 0, g3 = 0;

    g1 = charToDouble(class1);
    g2 = charToDouble(class2);
    g3 = charToDouble(class3);

    System.out.print("Your GPA is a " + ((g1 + g2 + g3) / 3));
}

static double charToDouble(char className) {
    switch (className) {
        case 'A':
            return 4.0;
        case 'B':
            return 3.0;
        case 'C':
            return 2.0;
        case 'D':
            return 1.0;
        case 'F':
            return 0.0;
        default:
            throw new IllegalStateException("Unexpected value: " + (className));
    }
}

AcidiC · Accepted Answer · 2021-01-21 05:44:42Z

Since your grades are characters, you could directly translate them into corresponding integers (Look up ascii tables on google, you'd find that 'A' = 65, 'B' = 66 and so on). This means that we can directly do comparison and arithmetic on chars:

double getGrade(char class) {
    if (class == 'F') return 0;       // We first check the outlier 'F'
    if (class >= 'A' && class <= 'D') // This ensures the value class is 'A', 'B', 'C' or 'D'
        return 4 - (class - 'A');     // if class is 'A', then class - 'A' is 0,
                                      // if class is 'B', then class - 'A' is 1, etc.
    return -1; // invalid case, maybe throw an error here instead of returning -1
}

As for the resource leak, it is common practice to close your scanner after you are done using it. In your case, this would be calling in.close() at the end of your main method.

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