问题描述
我有一个 HashMap< Integer，Integer> .我将其内容写入文件，因此它的每一行都包含 hashmapKey ::: hashmapValue .我现在是这样的:
I have a HashMap<Integer, Integer>. I write its content to the file, so each line of it contains hashmapKey:::hashmapValue. This is how I do it now:
List<String> mLines = new ArrayList<String>();
mHashMap.forEach((key, value) -> mLines.add(key + DATA_SEPARATOR + value));
Files.write(mOutputPath, mLines, StandardCharsets.UTF_8);
我非常怀疑是否需要将整个 HashMap 复制到字符串列表中，我确信在处理大量数据时会给我带来性能问题.我的问题是:如何使用Java 8将 HashMap 内容写入文件，而避免在另一个列表中复制值?
I very doubt that I need to copy entire HashMap to the list of strings, I am sure it will give me performance issues when working with big amounts of data. My question is: how can I write HashMap contents to the file using Java 8 avoiding copying values in another list?
推荐答案
最简单，非复制，最精简"的解决方案是
The simplest, non-copying, most "streamish" solution is
Files.write(mOutputPath, () -> mHashMap.entrySet().stream()
.<CharSequence>map(e -> e.getKey() + DATA_SEPARATOR + e.getValue())
.iterator());
虽然Stream不实现 Iterable ，但是可以执行lambda表达式来执行Stream操作，该操作以在流上调用 iterator()结尾.它会履行合同，因为lambda表达式将与Stream不同，在每次调用时都会产生一个新的 Iterator .
While a Stream does not implement Iterable, a lambda expression performing a Stream operation that ends with calling iterator() on the stream, can be. It will fulfill the contract as the lambda expression will, unlike a Stream, produce a new Iterator on each invocation.
请注意，我删除了显式的 UTF-8 字符集说明符，因为 java.nio.Files 将在没有字符集的情况下使用 UTF-8 是指定的(与旧的io类不同).
Note that I removed the explicit UTF-8 character set specifier as java.nio.Files will use UTF-8 when no charset is specified (unlike the old io classes).
上述解决方案的优点是I/O操作包装了Stream处理，因此在Stream内部，我们不必处理已检查的异常.相反， Writer + forEach 解决方案需要处理 IOException s，因为不允许 BiConsumer 进行检查例外情况.结果，使用 forEach 的可行解决方案如下所示:
The neat thing about the above solution is that the I/O operation wraps the Stream processing, so inside the Stream, we don’t have to deal with checked exceptions. In contrast, the Writer+forEach solution needs to handle IOExceptions as a BiConsumer is not allowed to throw checked exceptions. As a result, a working solution using forEach would look like:
try(Writer writer = Files.newBufferedWriter(mOutputPath)) {
mHashMap.forEach((key, value) -> {
try { writer.write(key + DATA_SEPARATOR + value + System.lineSeparator()); }
catch (IOException ex) { throw new UncheckedIOException(ex); }
});
} catch(UncheckedIOException ex) { throw ex.getCause(); }
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