Java如何在特定字符串之后找到特定行？

Question

I got a text from a BufferedReader and I need to get a specific line after a specific string

This is the text for example:

<img src="https://example.com/image1.jpg">
<img src="https://example.com/image2.jpg">
<img src="https://example.com/image3.jpg">
<img src="https://example.com/image4.jpg">
<all>
<img src="https://example.com/image9.jpg">
<img src="https://example.com/image11.jpg">
<img src="https://example.com/image15.jpg">

example with linebreak

<img src="https://example.com/image1.jpg">
<img src="https://example.com/image2.jpg">
<img src="https://example.com/image3.jpg">
<img src="https://example.com/image4.jpg">
<all>

<img src="https://example.com/image9.jpg">
<img src="https://example.com/image11.jpg">
<img src="https://example.com/image15.jpg">

example with other character

<img src="https://example.com/image1.jpg">
<img src="https://example.com/image2.jpg">
<img src="https://example.com/image3.jpg">
<img src="https://example.com/image4.jpg">
<all>
UTUYGBLK
<img src="https://example.com/image9.jpg">
<img src="https://example.com/image11.jpg">
<img src="https://example.com/image15.jpg">

I only want the img src line after 'all' which cont and end loop after that. the image number are random so I cant possible hit any of those.

currently my code is as follows.

while ((line = rd.readLine()) != null) {

  if (line.contains("all"))  {    
    line = rd.readLine();   
    System.out.println(line);
  }
 }

but if there is a linebreak after the 'all' i will get a null return... or funny character UTUYGBLK

Answer 1

You will need to loop after the element then until the result is not null. The code below should protect you against reading lines after end of file and also stop after the first line is shown after is read.

boolean loop = true;
// global loop
while ((line = rd.readLine()) != null && loop) {
  // line with "all" in it is found?
  if(line.contains("all"))  {    
    // Continue going through the input until end of input
    while((line = rd.readLine()) != null && loop) {  
      // When the line is not empty you have found the line after all!
      if(!line.isEmpty()) {
        System.out.println(line);
        // to stop the loops
        loop = false; 
      }
    }
  }
}

Answer 2

Then just skip the blank lines.

while ((line = rd.readLine()) != null) {

    if (line.contains("all"))  {
        while ((line = rd.readLine()) != null && line.trim().isEmpty());
        System.out.println(line);
        break;
    }
}

Answer 3

String line = "";
while (!line.contains("all"))
    line = rd.readLine();
line = rd.readLine();
while ("".equals(line.trim()))
    line = rd.readLine();
return line;
//or System.out.println(line);

This will:

1. Set line to the empty string.
2. Read lines until it gets one containing all .
3. Read the next line.
4. Read lines until it gets a non-empty one.
5. Return the last thing it read.

This operates on the assumption that there really will be a line containing all , and that there really will be a non-blank somewhere after it.

Answer 4

You can go into a second loop to check for nulls.

while ((line = rd.readLine()) != null) {

        if (line.contains("all"))  {    
            while ((line = rd.readLine()) == null || line.isEmpty()) {
                  //keep looping while line is null

                  //check if it is no longer ready to avoid infinite loop
                  if (!rd.ready()) {break;}
            }
            //line should not be null here
            System.out.println(line);
       }

Or something to that affect.

Answer 5

If you use contains() if you have an image with a name containing 'all' you will have incorrect results. Better use equals() after trimming the line. So the code will look like this:

while ((line = rd.readLine()) != null ) {
    if (line.trim().equals("<all>"))  {
        while ((line = rd.readLine()).isEmpty());
        System.out.println(line);
        break;
    }
}

Answer 6

You can use the method

line.isEmpty()

to handle that case