Question

for (String userName : studentEnrollments.keySet()) {
    System.out.println("Student: " + userName);
    ArrayList<String> courses = studentEnrollments.get(userName);
    String courseMessage = "Courses: ";
    for (String singleCourse : courses) {
        courseMessage += singleCourse + ", ";
        System.out.println(courseMessage);
    }
}

在for循环之后，它会打印出学生正在学习的每门课程。在没有最后一个逗号的情况下用逗号打印字符串的正确方法是什么？

例如：AC130，AC140，AC150，AC130，AC140，AC150

Answer 1

使用Iterator而不是for循环使用while，然后仅当hasNext（）返回true时才添加昏迷。

String courseMessage = "Courses: ";
Iterator<String> c = studentEnrollments.get(userName).iterator();

while (c.hasNext()) {
    courseMessage += c.next();
    if (c.hasNext()) {
        courseMessage += ",";
    }
}

Answer 2

请注意，在Java 8中，您可以使用StringJoiner：

 String commaSeparatedCourses = courses.stream().collect(Collectors.joining(", "));

Answer 3

您可以使用apache中的StringUtils.join方法

示例：

StringUtils.join(studentEnrollments, ", ");

Answer 4

这是另一种方法：

将空字符串定义为分隔符
打印分隔符和数组元素（按此顺序，首先是分隔符）
将分隔符更改为逗号（或任何您想要的内容）

此方法可以解决此问题：除了在最后一个元素之外的每个元素之后添加逗号，不如在第一个元素之外的每个元素之前添加逗号。

这是一个实现

String separator = "";
for (String singleCourse : courses) {
    courseMessage += separator + singleCourse;
    System.out.println(courseMessage);
    separator = ",";
}

Answer 5

如果您使用的是java 8

，则可以使用StringJoiner

StringJoiner joiner = new StringJoiner(", ");
for (String singleCourse : courses) {
    joiner.add(singleCourse);
}
System.out.println(joiner.toString());

否则你只需在字符串前添加,而不是附加它。

String courseMessage = "Courses: " + courses.get(0);
for (int i = 1; i<courses.size();++i) {
    courseMessage += ", " + courses.get(i);
}

Answer 6

使用StringBuilder：

for (String userName : studentEnrollments.keySet()) {
    System.out.println("Student: " + userName);
    ArrayList<String> courses = studentEnrollments.get(userName);
    StringBuilder sb = new StringBuilder();
    for (String singleCourse : courses) {
        if(sb.length()>0){
            sb.append(", ");
        }
        sb.append(singleCourse);
    }
    System.out.println(sb.insert(0, "Courses: ").toString());
}

Answer 7

使用StringBuilder

for (String userName : studentEnrollments.keySet()) {
    System.out.println("Student: " + userName);
    ArrayList<String> courses = studentEnrollments.get(userName);
    StringBuilder courseMessage = "Courses: ";
    for (String singleCourse : courses) {
        sb.append(singleCourse).append(", ");
    }
    if (sb.length() > 2) 
        sb.setLength(sd.length() -2);
    String courseMessage = sb.toString();
    System.out.println(courseMessage);
}

甚至更好地使用StringUtils（来自apache-common）

for (String userName : studentEnrollments.keySet()) {
    System.out.println("Student: " + userName);
    ArrayList<String> courses = studentEnrollments.get(userName);
    String courseMessage = sb.toString();
    System.out.println(StringUtils.join(courses, ", ");
}

Answer 8

对于Java 8之前的解决方案，您还可以查看此代码段作为起点

String[] courses ={ "AC130", "AC140", "AC150" };
for (int i = 0; i < courses.length - 1; i++) {
    System.out.append(courses[i]).append(", ");
}
System.out.println(courses[courses.length-1]);

小解释：对于所有元素，但最后一个元素打印“元素”+“，”，对于最后一个元素，它只打印“元素”。您需要确保自己不会处理空数组。

Answer 9

使用string.replaceAll(", $", "")，如下所示：

for (String userName : studentEnrollments.keySet()) {
            System.out.println("Student: " + userName);
            ArrayList<String> courses = studentEnrollments.get(userName);
            String courseMessage = "Courses: ";
            for (String singleCourse : courses) {
                courseMessage += singleCourse + ", ";
            }
  System.out.println(courseMessage.replaceAll(", $", ""));
}

如何在没有最后一个逗号的情况下打印出一串数组？

9 个答案: