用逗号分隔列表的最简单方法? [英] The simplest way to comma-delimit a list?

问题描述

在 Java 中用逗号分隔列表的最清晰方法是什么?

What is the clearest way to comma-delimit a list in Java?

我知道几种方法，但我想知道最好的方法是什么(最好"的意思是最清晰和/或最短，而不是最有效.

I know several ways of doing it, but I'm wondering what the best way is (where "best" means clearest and/or shortest, not the most efficient.

我有一个列表，我想遍历它，打印每个值.我想在每个项目之间打印一个逗号，但不是在最后一个之后(也不在第一个之前).

I have a list and I want to loop over it, printing each value. I want to print a comma between each item, but not after the last one (nor before the first one).

List --> Item ( , Item ) *
List --> ( Item , ) * Item

示例解决方案 1:

boolean isFirst = true;
for (Item i : list) {
  if (isFirst) {
    System.out.print(i);        // no comma
    isFirst = false;
  } else {
    System.out.print(", "+i);   // comma
  }
}

示例解决方案 2 - 创建子列表:

Sample solution 2 - create a sublist:

if (list.size()>0) {
  System.out.print(list.get(0));   // no comma
  List theRest = list.subList(1, list.size());
  for (Item i : theRest) {
    System.out.print(", "+i);   // comma
  }
}

示例解决方案 3:

  Iterator<Item> i = list.iterator();
  if (i.hasNext()) {
    System.out.print(i.next());
    while (i.hasNext())
      System.out.print(", "+i.next());
  }

这些特别对待第一项；可以改为特别对待最后一个.

These treat the first item specially; one could instead treat the last one specially.

顺便说一下，这是在 Java 1.6 中 List toString is 的实现方式(它继承自 AbstractCollection):

Incidentally, here is how List toString is implemented (it's inherited from AbstractCollection), in Java 1.6:

public String toString() {
    Iterator<E> i = iterator();
    if (! i.hasNext())
        return "[]";

    StringBuilder sb = new StringBuilder();
    sb.append('[');
    for (;;) {
        E e = i.next();
        sb.append(e == this ? "(this Collection)" : e);
        if (! i.hasNext())
            return sb.append(']').toString();
        sb.append(", ");
    }
}

它提前退出循环以避免在最后一项之后出现逗号.BTW:这是我第一次记得看到(本系列)"；这是激发它的代码:

It exits the loop early to avoid the comma after the last item. BTW: this is the first time I recall seeing "(this Collection)"; here's code to provoke it:

List l = new LinkedList();
l.add(l);
System.out.println(l);

我欢迎任何解决方案，即使他们使用了意外的库(正则表达式?)；以及 Java 以外语言的解决方案(例如，我认为 Python/Ruby 有一个 intersperse 函数 - 那个 是如何实现的?).

I welcome any solution, even if they use unexpected libraries (regexp?); and also solutions in languages other than Java (e.g. I think Python/Ruby have an intersperse function - how is that implemented?).

澄清:通过库，我指的是标准 Java 库.对于其他库，我考虑将它们与其他语言一起使用，并且很想知道它们是如何实现的.

Clarification: by libraries, I mean the standard Java libraries. For other libraries, I consider them with other languages, and interested to know how they're implemented.

EDIT 工具包提到了一个类似的问题:Lastjava中增强for循环的迭代

EDIT toolkit mentioned a similar question: Last iteration of enhanced for loop in java

还有一个:循环中的最后一个元素是否应该单独治疗?

推荐答案

Java 8 及更高版本

使用StringJoiner 类和 forEach 方法:

StringJoiner joiner = new StringJoiner(",");
list.forEach(item -> joiner.add(item.toString());
return joiner.toString();

使用Stream 和 收集器:

return list.stream().
       map(Object::toString).
       collect(Collectors.joining(",")).toString();

Java 7 及更早版本

另见#285523

String delim = "";
for (Item i : list) {
    sb.append(delim).append(i);
    delim = ",";
}

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