为什么 for 循环在给定的 Java 代码中花费太多时间

Question

how to reduce time taken by for loop, remove the for loop by using replaceAll(,) method if possible :

String extractText(String s) throws IOException
{
    String html = fj.toHtmlString(s); //extracted html source code from wikipedia
    String filtered_text="";
    System.out.println("extracted \n\n");
    String []html_text = html.split("\n");
    long start = System.currentTimeMillis();

    for(String h:html_text)
    {   //System.out.println("ky4"+h);
        if(Pattern.compile("</strong>", Pattern.CASE_INSENSITIVE + Pattern.LITERAL).matcher(h).find())
        {

        }
        else if(Pattern.compile("<strong", Pattern.CASE_INSENSITIVE + Pattern.LITERAL).matcher(h).find())
        {

        }
        else
        {
            filtered_text += h;
            filtered_text += "\n";
        }
    }
    long end = System.currentTimeMillis();
    System.out.println("loop end in "+(end-start)/1000+" seconds"+" or "+(end-start)+" miliseconds");//System.out.println(++i2+" th loop end in "+(end-start)/1000+" seconds");
    return filtered_text;
}

Answer 1

Let's take a look a the loop:

for(String h:html_text)
{   //System.out.println("ky4"+h);
    if(Pattern.compile("</strong>", Pattern.CASE_INSENSITIVE + Pattern.LITERAL).matcher(h).find())
    {

    }
    else if(Pattern.compile("<strong", Pattern.CASE_INSENSITIVE + Pattern.LITERAL).matcher(h).find())
    {

    }
    else
    {
        filtered_text += h;
        filtered_text += "\n";
    }
}

You compile at least one regex each time you loop, which is very time consuming. Use variables instead :

Pattern endingTag = Pattern.compile("</strong>", Pattern.CASE_INSENSITIVE + Pattern.LITERAL);
Pattern startTag = Pattern.compile("<strong", Pattern.CASE_INSENSITIVE + Pattern.LITERAL)
for(String h:html_text)
{   //System.out.println("ky4"+h);
    if(endingTag.matcher(h).find())
    {}
    else if(startTag.matcher(h).find())
    {}
    else
    {
        filtered_text += h;
        filtered_text += "\n";
    }
}

This will save you a lot of time. Also, note that when you measure performence, always run in Release mode.

Answer 2

You can try to compile your patterns before the loop and not in it as:

String extractText(String s) throws IOException
{
    String html = fj.toHtmlString(s); //extracted html source code from wikipedia
    String filtered_text="";
    System.out.println("extracted \n\n");
    String []html_text = html.split("\n");
    long start = System.currentTimeMillis();
    Pattern end = Pattern.compile("</strong>", Pattern.CASE_INSENSITIVE + Pattern.LITERAL)
    Pattern start = Pattern.compile("<strong", Pattern.CASE_INSENSITIVE + Pattern.LITERAL)
    for(String h:html_text)

Answer 3

my problem is resoled by following code :

String extractText(String s) throws IOException
{
    String html = fj.toHtmlString(s); 
    String filtered_text="";
    System.out.println("extracted \n\n");
    html=html.replaceAll("(?i)</strong>", "");
    html=html.replaceAll("(?i)<strong[^>]*>", "");
    filtered_text = html;        
    long end = System.currentTimeMillis();
    System.out.println("loop end in "+(end-start)/1000+" seconds"+" or "+(end-start)+" miliseconds");//System.out.println(++i2+" th loop end in "+(end-start)/1000+" seconds");
    return filtered_text;
}