为什么这个switch语句没有识别subString？

我创建一个类Course代表有三个属性的课程对象courseID,letterGrade和numberGrade.

numberGrade是letterGrade使用该computeGrade()方法计算的.我正在测试我的Course课程并发现使用的switch语句computeGrade无法识别相关内容substring.

为什么会这样？

public void computeGrade() {
    switch (letterGrade.substring(0,1)) {
        case "A": numberGrade = 4.0;
        case "B": numberGrade = 3.0;
        case "C": numberGrade = 2.0;
        case "D": numberGrade = 1.0;
        case "F": numberGrade = 0.0;
        default: numberGrade = 999;
    }
    if(letterGrade.endsWith("+") && !letterGrade.startsWith("A") && !letterGrade.startsWith("F")) {
        numberGrade += 0.3;
    }
    else if(letterGrade.endsWith("-") && !letterGrade.startsWith("D") && !letterGrade.startsWith("F")) {
        numberGrade -= 0.3;
    }
}

我的测试人员循环了一系列可能的等级

public static void main(String[] args) {
    String[] grades = {"A", "A-", "B+", "B", "B-", "C+", "C", "C-", "D+", "D", "F"};
    Course course = new Course("COP2253", " ");
    String toString;
    for (int i = 0; i < grades.length; i++) {
        course.updateGrade(grades[i]);
        toString = course.toString();
        System.out.println(toString + "\n");

    }

具有以下输出

COP2253     A   999.0
COP2253     A-  998.7
COP2253     B+  999.3
COP2253     B   999.0
COP2253     B-  998.7
COP2253     C+  999.3
COP2253     C   999.0
COP2253     C-  998.7
COP2253     D+  999.3
COP2253     D   999.0
COP2253     F   999.0

PS我相当肯定构造函数,

public Course(String courseID, String letterGrade) {
    this.courseID = courseID;
    this.letterGrade = letterGrade;
    computeGrade();
}

updateGrade(),

public void updateGrade(String grade) {
    letterGrade = grade;
    computeGrade();
}

和 toString()

public String toString() {
    String grade = courseID + "\t\t" + letterGrade + "\t" + numberGrade;
    return grade;
}

不是问题.