Question

Using only the standard Java library, what is a simple mechanism to join strings up to a limit, and append an ellipsis when the limit results in a shorter string?

Efficiency is desirable. Joining all the strings and then using String.substring() may consume excessive memory and time. A mechanism that can be used within a Java 8 stream pipeline is preferable, so that the strings past the limit might never even be created.

For my purposes, I would be happy with a limit expressed in either:

Maximum number of strings to join
Maximum number of characters in result, including any separator characters.

For example, this is one way to enforce a maximum number of joined strings in Java 8 with the standard library. Is there a simpler approach?

final int LIMIT = 8;

Set<String> mySet = ...;
String s = mySet.stream().limit( LIMIT ).collect( Collectors.joining(", "));
if ( LIMIT < mySet.size()) {
    s += ", ...";
}

Answer 1

您可以为此编写自定义收集器。这个基于another I wrote for a similar case：

private static Collector<String, List<String>, String> limitingJoin(String delimiter, int limit, String ellipsis) {
    return Collector.of(
                ArrayList::new, 
                (l, e) -> {
                    if (l.size() < limit) l.add(e);
                    else if (l.size() == limit) l.add(ellipsis);
                },
                (l1, l2) -> {
                    l1.addAll(l2.subList(0, Math.min(l2.size(), Math.max(0, limit - l1.size()))));
                    if (l1.size() == limit) l1.add(ellipsis);
                    return l1;
                },
                l -> String.join(delimiter, l)
           );
}

在此代码中，我们保留了所有受欢迎的字符串ArrayList<String>。当一个元素被接受时，当前列表的大小将根据限制进行测试：严格小于它，添加元素;等于它，省略了省略号。对于组合器部分也是如此，这有点棘手，因为我们需要正确处理子列表的大小而不超过限制。最后，终结者只是使用给定的分隔符加入该列表。

此实现适用于并行Streams。它将保留Stream in encounter order的head元素。请注意，即使在达到限制后没有添加任何元素，它也会消耗Stream中的所有元素。

工作示例：

List<String> list = Arrays.asList("foo", "bar", "baz");
System.out.println(list.stream().collect(limitingJoin(", ", 2, "..."))); // prints "foo, bar, ..."

Answer 2

虽然使用第三方代码不是提问者的选择，但对其他读者来说可能是可以接受的。即使编写自定义收集器，您仍然有一个问题：整个输入将被处理，因为标准收集器不能短路（特别是它不可能处理无限流）。我的StreamEx库增强了收藏家的概念，使得创建短路收集器成为可能。 Joining收集器也很容易提供：

StreamEx.of(mySet).collect( 
    Joining.with(", ").ellipsis("...").maxChars(100).cutAfterDelimiter() );

保证结果不超过100个字符。可以使用不同的计数策略：您可以通过字符，代码点或字形来限制（组合Unicode字符将不计算在内）。您也可以在任何位置（“First entry，second en ...”）或单词（“First entry，second ...”）或分隔符（“First entry，...”）之后剪切结果，或者在分隔符之前（“第一次进入，第二次进入...”）。它也适用于并行流，但在有序情况下可能效率不高。

Answer 3

Using only the standard Java library

I don't believe there is anything in there that can do what you ask.

You need to write your own Collector. It won't be that complicated, so I don't see why writing your own would be an issue.

Joining strings with limit

3 个答案: