不好用非常大的字符串？(JAVA)

Question

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:

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

逐行处理文件(通常)是一个更好的解决方案,为什么？

谢谢

Answer 1

流媒体与否

当你可以流,你可以处理任何大小的文件(假设你真的可以忘记你已经看到的所有数据).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.

流媒体很可爱......但在每种情况下都不起作用.

StringBuilder的

由于似乎对这些StringBuilder建议存在一定程度的争议,这里有一个显示效果的基准.我不得不减小基准的大小,以便让慢速版本在合理的时间内完成.

结果首先是代码.这是一个非常粗略和准备好的基准,但结果是戏剧性的,足以说明问题......

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

而代码......

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");
    }
}