以逗号分隔列表最清晰的方法？

问题描述：

用Java逗号分隔列表最清晰的方法是什么？

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（其中最好意味着最清晰和/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，我想循环它，打印每个值。我想要在每个项目之间打印逗号，但不是在最后一个项目之后（也不是在第一个项目之前）。

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());
  }

这些特别处理第一项;一个人可以专门对待最后一个。

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

顺便提一下，这里是 List toString在Java 1.6中实现 （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逗号后面的逗号最后一项。顺便说一句：这是我第一次回忆起（这个收藏品）;这里是激发它的代码：

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，即使他们使用意外的库（regexp？）;还有Java以外的语言解决方案（例如我认为Python / Ruby有散布功能 - 实现了怎么样？）。

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，我的意思是标准的Java库。对于其他库，我认为它们与其他语言一样，并且有兴趣了解它们是如何实现的。

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 工具包提到了类似的问题：最后一次使用java增强for循环

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

另一个：
循环中的最后一个元素是否需要单独处理？

答

对于前Java -8：

For pre Java-8:

另见＃285523

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

自Java 8以来的更新：

Update since Java 8:

使用 StringJoiner ：

StringJoiner joiner = new StringJoiner(",");
for (Item item : list) {
    joiner.add(item.toString());
}
return joiner.toString();

使用流，收藏家：

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

以逗号分隔列表最清晰的方法？

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