问Java等级类，字母等级到数字

EN

if (grade == null || !grade.matches("[A-F][+-]?")) // if grade wasn't specified, or
{// if grade is not between A to F *optionally* followed by a + or - sign
    System.out.println("Invalid letter grade");
    return gradeNum;
}

String letterGrade = grade.substring(0, 1); // pass 1 to exclude + or -

if (letterGrade.equalsIgnoreCase("A"))
{
    gradeNum = 4.0;
}
else if(letterGrade.equalsIgnoreCase("B"))
{
    gradeNum = 3.0;
}
else if(letterGrade.equalsIgnoreCase("C"))
{
  gradeNum = 2.0;
}
else if(letterGrade.equalsIgnoreCase("D"))
{
    gradeNum = 1.0;
}
else if(letterGrade.equalsIgnoreCase("F"))
{
    gradeNum = 0.0; // return from here if you want F = F- = F+
    return gradeNum; // if you want to invalidate F- and F+ look below
}
/* else // not required; already validated by using regex above
{
    System.out.println("Invalid letter grade");
    return gradeNum; // don't forget to RETURN from here
}*/

String suffix;
suffix = grade.substring(1);
if (suffix.isEmpty()) { // return early if there's no suffix
    return gradeNum;
} else if (gradeNum == 0.0) { // to invalidate F- and F+
    System.out.println("Invalid letter grade");
    return gradeNum;
}
if(suffix.equals("+") && gradeNum != 4.0) // don't increment if grade is A+ (= A = 4.0)
{
    gradeNum = gradeNum + .3;
}
else if (suffix.equals("-"))
{
    gradeNum = gradeNum - .3;
}
return gradeNum;

/**
 * Grade encapsulates symbols with values
 * @author Michael
 * @link http://stackoverflow.com/questions/16744116/java-grade-class-letter-grade-to-number/16744210#16744210
 * @since 5/24/13 6:01 PM
 */
public enum Grade {
    APLUS("A+", 4.3),
    A("A", 4.0),
    AMINUS("A-", 3.7),
    BPLUS("B+", 3.3),
    B("B", 3.0),
    BMINUS("B-", 2.7),
    CPLUS("C+", 2.3),
    C("C", 2.0),
    CMINUS("C-", 1.7),
    DPLUS("D+", 1.3),
    D("D", 1.0),
    DMINUS("D-", 0.7),
    F("F", 0.0);

    private final String symbol;
    private final double value;

    Grade(String symbol, double value) {
        this.symbol = symbol;
        this.value = value;
    }

    public String getSymbol() {
        return symbol;
    }

    public double getValue() {
        return value;
    }
}

String letterGrade = grade.substring(0);

String letterGrade = grade.substring(0,1);  // the end position is exclusive