我如何在没有最后一个逗号的情况下打印出一串数组？

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修读的每门课程。 用逗号将字符串打印出来而末尾没有逗号的正确方法是什么？

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

Answer 1

与Iterator一起使用一会儿，而不是for循环，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

您可以从apache使用StringUtils.join方法

范例：

StringUtils.join(studentEnrollments, ", ");

Answer 4

这是另一种方法：

• 定义一个空字符串作为分隔符
• 打印分隔符和数组元素（按此顺序，首先分隔符）
• 将分隔符更改为逗号（或任何您想要的）

此方法可以解决问题：您可以在除第一个元素之外的每个元素之前添加一个逗号，而不是在最后一个元素之后添加一个逗号。

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，您可以只在字符串前面添加,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

改用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);
}

甚至与StringUtils更好（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，您也可以看看此片段作为起点

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]);

简短说明：对于除最后一个元素以外的所有元素，它只打印“ element” +“，”，对于最后一个元素它仅打印“ element”。 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(", $", ""));
}