Java 如何将计数器插入 Stream<String> .forEach()？
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How to insert a counter into a Stream<String> .forEach()?
提问by Vega
FileWriter writer = new FileWriter(output_file);
int i = 0;
try (Stream<String> lines = Files.lines(Paths.get(input_file))) {
lines.forEach(line -> {
try {
writer.write(i + " # " + line + System.lineSeparator());
} catch (Exception e) {
e.printStackTrace();
}
}
);
writer.close();
}
I need to write the line with the line number, so I tried to add a counter into the .forEach(), but I can't get it to work. I just don't know where to put the i++; into the code, randomly screwing around didn't help so far.
我需要用行号写一行，所以我试图在 .forEach() 中添加一个计数器，但我无法让它工作。我只是不知道把 i++ 放在哪里；进入代码，随机乱搞到目前为止没有帮助。
采纳答案by Erik Vesteraas
This is a good example of where you should rather use a good old fashioned for loop. While Files.lines()specifically gives a sequential stream, streams can be produced and processed out of order, so inserting counters and relying on their order is a rather bad habit to get into. If you still really want to do it, remember that anywhere you can use a lambda you can still use a full anonymous class. Anonymous classes are normal classes, and can as such have state.
这是一个很好的例子，你应该使用一个好的老式 for 循环。虽然Files.lines()专门给出了一个顺序流，但流可以无序地生成和处理，因此插入计数器并依赖它们的顺序是一个相当糟糕的习惯。如果您仍然真的想这样做，请记住，在任何可以使用 lambda 的地方，您仍然可以使用完整的匿名类。匿名类是普通类，因此可以具有状态。
So in your example you could do like this:
所以在你的例子中，你可以这样做：
FileWriter writer = new FileWriter(output_file);
try (Stream<String> lines = Files.lines(Paths.get(input_file))) {
lines.forEach(new Consumer<String>() {
int i = 0;
void accept(String line) {
try {
writer.write((i++) + " # " + line + System.lineSeparator());
} catch (Exception e) {
e.printStackTrace();
}
}
});
writer.close();
}
回答by OldCurmudgeon
You can use an AtomicIntegeras a mutable finalcounter.
您可以将AtomicInteger用作可变final计数器。
public void test() throws IOException {
// Make sure the writer closes.
try (FileWriter writer = new FileWriter("OutFile.txt") ) {
// Use AtomicInteger as a mutable line count.
final AtomicInteger count = new AtomicInteger();
// Make sure the stream closes.
try (Stream<String> lines = Files.lines(Paths.get("InFile.txt"))) {
lines.forEach(line -> {
try {
// Annotate with line number.
writer.write(count.incrementAndGet() + " # " + line + System.lineSeparator());
} catch (Exception e) {
e.printStackTrace();
}
}
);
}
}
}
回答by akhil_mittal
As stated in Java doc:
正如Java 文档中所述：
Any local variable, formal parameter, or exception parameter used but not declared in a lambda expression must either be declared final or be effectively final (§4.12.4), or a compile-time error occurs where the use is attempted.
任何使用但未在 lambda 表达式中声明的局部变量、形式参数或异常参数必须声明为 final 或有效 final (§4.12.4)，否则在尝试使用时会发生编译时错误。
It means your variable has to be final or effectively final. You want to add a counter into forEachand for that you can use an AtomicIntegeras suggested by OldCurumudgeon which is IMO a preferred approach.
这意味着您的变量必须是最终的或有效的最终变量。您想添加一个计数器forEach，为此您可以使用AtomicIntegerOldCurumudgeon 的建议，这是 IMO 的首选方法。
I believe you can also use an array having only one value 0which you can use as counter. Check and let me know if the following example works for you:
我相信您也可以使用只有一个值的数组0作为计数器。检查并让我知道以下示例是否适合您：
public void test() throws IOException {
FileWriter writer = new FileWriter("OutFile.txt");
final int[] count = {0};
try (Stream<String> lines = Files.lines(Paths.get("InFile.txt"))) {
lines.forEach(line -> {
try {
count[0]++;
writer.write(count[0] + " # " + line + System.lineSeparator());
} catch (Exception e) {
e.printStackTrace();
}
}
);
writer.close();
}
}
