不好用非常大的字符串？ （JAVA） [英] Bad to use very large strings? (Java)

问题描述

创建大字符串是否有任何负面影响？例如，如果我们从一个可能很大的文本文件中读取文本：

Are there any negatives to creating huge strings? For instance, if we're reading in text from a potentially huge text file:

while (scanner.hasNext()) {
  someString += scanner.next();
}
// do something cool with someString

处理文件行by line（通常）是一个更好的解决方案，为什么？

Would processing the file line by line be (generally) a better solution, and why?

谢谢

推荐答案

Streaming vs not

当您可以流式传输时，您可以处理任何大小的文件（假设你真的可以忘记你已经看过的所有数据。你最终会得到一个自然的O（n）复杂性，这是一件非常好的事情。你不会因内存不足而中断。

When you can stream, you can handle files of any size (assuming you really can forget all the data you've already seen). You end up with a naturally O(n) complexity, which is a very good thing. You don't break by running out of memory.

流媒体很可爱......但在每种情况下都不起作用。

Streaming is lovely... but doesn't work in every scenario.

StringBuilder

因为似乎对 StringBuilder有一定的争议建议，这是显示效果的基准。为了让慢速版本在合理的时间内完成，我不得不减小基准的大小。

As it seems there's been a certain amount of controversy over the StringBuilder advice, here's a benchmark to show the effects. I had to reduce the size of the benchmark in order to get the slow version to even finish in a reasonable time.

首先是结果，然后是代码。这是一个非常粗略和准备好的基准，但结果足够引人注目......

Results first, then code. This is a very rough and ready benchmark, but the results are dramatic enough to make the point...

c:\Users\Jon\Test>java Test slow
Building a string of length 120000 without StringBuilder took 21763ms

c:\Users\Jon\Test>java Test fast
Building a string of length 120000 with StringBuilder took 7ms

代码......

class FakeScanner
{
    private int linesLeft;
    private final String line;

    public FakeScanner(String line, int count)
    {
        linesLeft = count;
        this.line = line;
    }

    public boolean hasNext()
    {
        return linesLeft > 0;
    }

    public String next()
    {
        linesLeft--;
        return line;
    }
}

public class Test
{    
    public static void main(String[] args)
    {
        FakeScanner scanner = new FakeScanner("test", 30000);

        boolean useStringBuilder = "fast".equals(args[0]);

        // Accurate enough for this test
        long start = System.currentTimeMillis();

        String someString;
        if (useStringBuilder)
        {
            StringBuilder builder = new StringBuilder();
            while (scanner.hasNext())
            {
                builder.append(scanner.next());
            }
            someString = builder.toString();
        }
        else
        {
            someString = "";     
            while (scanner.hasNext())
            {
                someString += scanner.next();
            }        
        }
        long end = System.currentTimeMillis();

        System.out.println("Building a string of length " 
                           + someString.length()
                           + (useStringBuilder ? " with" : " without")
                           + " StringBuilder took " + (end - start) + "ms");
    }
}

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