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

用Java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，但是我想知道最好的方法是什么(最好"是指最清晰和/或最短而不是最有效的方法.

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，但不要在最后一个项目之后(也不在第一个项目之前).

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-创建一个子列表:

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());
  }

这些特别对待第一个项目；可以改而特别地对待最后一个.

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中实现List toString 的方法(它是从AbstractCollection继承的):

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(", ");
    }
}

它尽早退出循环以避免最后一项后面的逗号.顺便说一句:这是我第一次记得看到(本收藏集)"；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，即使它们使用意外的库(regexp?)；以及Java以外的其他语言的解决方案(例如，我认为Python/Ruby具有 intersperse 函数- 的实现方式是什么?).

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Java库.对于其他库，我认为它们可以与其他语言一起使用，并且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 工具箱提到了类似的问题:最后Java中增强的for循环的迭代

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

还有另一个: 循环中的最后一个元素是否值得单独使用治疗?

推荐答案

Java 8及更高版本

使用 StringJoiner 类:

Java 8 and later

Using StringJoiner class :

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

使用 Stream ，和 Collectors :

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

Java 7和更早版本

另请参见#285523

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

这篇关于用逗号分隔列表的最简单方法?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！