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Get the list of all URLs on the website using Java [closed]

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There are many libraries(eg. Jsoup) which can do this task in a go but how can I get all the URLs present in the HTML content of any website using Java without using any external libraries?
Edit 1: Can anyone explain what scanner.useDelimiter("\Z") actually does and what is the difference between scanner.useDelimiter("\Z") and scanner.useDelimiter("\z").

I am answering my own question as I was trying to find the accurate answer on StackOverflow but couldn't find one.
Here is the code:
URL url;
ArrayList<String> finalResult = new ArrayList<String>();
try {
String content = null;
URLConnection connection = null;
try {
connection = new URL("https://yahoo.com").openConnection();
Scanner scanner = new Scanner(connection.getInputStream());
scanner.useDelimiter("\\Z");
content = scanner.next();
scanner.close();
} catch (Exception ex) {
ex.printStackTrace();
}
String regex = "(https?|ftp|file)://[-a-zA-Z0-9+&##/%?=~_|!:,.;]*[-a-zA-Z0-9+&##/%=~_|]";
Pattern p = Pattern.compile(regex);
Matcher m = p.matcher(content);
while (m.find()) {
if(!finalResult.contains((m.group())))
finalResult.add(m.group());
}
} finally {
for(String res: finalResult){
System.out.println(res);
}
}

You can try using a regEx.
Here is an example of a regEx that checks if any test is a URL or not.
https://www.regextester.com/96504.
But I can't stop my self to say that Jsoup is what fits for this. but it's an external library.

how to read the number of columns in text file in java [closed]

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how to read the number of columns in text file in java.
Example text file as below. Comma separated. In this i need to get the total column as count 4
ABC,BBC,12-10-2018,1234
ABC,BBC,12-10-2018,1234
ABC,BBC,12-10-2018,1234
ABC,BBC,12-10-2018,1234

The simplest way is to use a Scanner and read the 1st line.
By using split() with , as a delimeter you get an array and its length is what you want.
public static int getFileColumnsNumber(String filename) {
File file = new File(filename);
Scanner scanner;
try {
scanner = new Scanner(file);
} catch (FileNotFoundException e) {
e.printStackTrace();
return -1;
}
int number = 0;
if (scanner.hasNextLine()) {
number = scanner.nextLine().split(",").length;
}
scanner.close();
return number;
}
public static void main(String[] args) {
String filename = "test.txt";
System.out.println(getFileColumnsNumber(filename));
}

java how to catch an exception in main? [closed]

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Closed 8 years ago.
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I need to read lines from an input path (file).
to do so the main calls a class that uses BufferedReader , it iterates over each line and adds it to an Array.
the problem is:
I want to catch all exceptions thrown from the method in the class in the main.
public static void main (String[] args){
if (args.length != 2){
System.err.print("ERROR");
return;
}
MyFileScript.sourceDir = args[SOURCE_DIR_INDEX];
MyFileScript.commandFile = args[COMMAND_INDEX];
try (FileReader file = new FileReader(MyFileScript.commandFile);
BufferedReader reader = new BufferedReader(file)){
fileParsing = new CommandFileParser(reader);
sectionList = fileParsing.parseFile();
}catch (FileNotFoundException error){
System.err.print(ERROR_MESSAGE);
return;
}catch(IOException error){
System.err.print(ERROR_MESSAGE);
return;
}catch(ErrorException error){
System.err.print(error.getMessage());
return;
}
}
public class CommandFileParser {
public CommandFileParser (BufferedReader reader){
this.reader = reader;
}
/**
* read all lines from a file.
*
* #return a string array containing all file lines
*/
public String[] readFileLines(){
ArrayList<String> fileLines = new ArrayList<String>();
String textLine;
while ((textLine = this.reader.readLine()) != null){
fileLines.add(textLine);
}
String[] allFileLines = new String[fileLines.size()];
fileLines.toArray(allFileLines);
return allFileLines;
}
in the while loop I get a compilation error for unhandling the IOException.
How can I catch all exceptions in main,
and so the class takes only one string argument?

your readFileLines method is lacking a throws clause.
public String[] readFileLines() throws IOException {

Alternate code for Throws IOException [closed]

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What changes do I need to make in the code If I remove the line "throws IOException"??
import java.io.*;
class Buffered_class{
public static void main(String[] args)
throws IOException // remove this line
{
char c;
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.print("Enter characters, 'q' to quit");
do{
c= (char)br.read();
System.out.println(" you entered : " + c );
}while(c !='q');
}
}

You need to catch the exception
import java.io.*;
class Buffered_class{
public static void main(String[] args)
{
char c;
try{
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.print("Enter characters, 'q' to quit");
do{
c= (char)br.read();
System.out.println(" you entered : " + c );
}while(c !='q');
}catch(IOException e){
// do something
}finally{
br.close();
}
}

java.io.File getting extra char while writing into file [closed]

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Why I am getting extra char while writing into file for the following code? If I am using writeBytes(String) than the below code is working file. Then what is the problem with dos.writeChars() method?
File fileObj = new File("student.txt");
try {
// writing into file
FileOutputStream fos = new FileOutputStream(fileObj);
String msg = "This is student file";
DataOutputStream dos = new DataOutputStream(fos);
dos.writeChars(msg);
//reading from file
FileInputStream fis = new FileInputStream(fileObj);
DataInputStream dis = new DataInputStream(fis);
System.out.println(dis.readLine());
for (int i = 0; ((i = dis.read()) != -1); i++) {
System.out.println(i);
}
fos.close();
dos.close();
} catch (FileNotFoundException e) {
System.err.println("File not found!");
} catch (IOException e) {
e.printStackTrace();
}

writeChars() uses 2-byte chars (UTF-16). So each char you write will result in two bytes written.
If you want another encoding use getBytes() on the String and write it as bytes.