How to store the last element in an ArrayList into another ArrayList (Java)

Question

I have a text file that reads

NFR1: (some text) 
NFR2: (some text)
NFR3: (some text)

I wanted to group all strings in a text file that had the pattern"N".

This is code I have so far:

import java.util.List;
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main {

public static void main(String[] args) throws FileNotFoundException {
    List<String> result = new ArrayList<String>();
    Pattern patt = Pattern.compile("^N.*");
    BufferedReader r = new BufferedReader(new FileReader("fileName"));

    String line;
    try {
        while ((line = r.readLine()) != null) {
          Matcher m = patt.matcher(line);
          while (m.find()) {

            int start = m.start(0);
            int end = m.end(0);
            result.add(m.group());
            System.out.println(result);
          }
        }
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

}
}

The output is as follows:

[NFR1:]
[NFR1:,NFR2:]
[NFR1:,NFR2:,NFR3]

How do I get the array to read just

[NFR1:,NFR2:,NFR3]

without the other two elements above?

When I add the line of code to get the last element:

if (result != null && !result.isEmpty()) {
              System.out.println("Last element is:");
              System.out.println(result.get(result.size()-1));
             }

The output is:

The last element is:
NFR1:
The last element is:
NFR2:
The last element is:
NFR3:

What am I doing wrong?

Use System.out.println(result); at the end of try-catch block? You are printing the arraylist in every iteration of the loop. — S.K.
– S.K., Commented Nov 1, 2018 at 3:22

Karthik R · Accepted Answer · 2018-11-01 03:22:09Z

1

You are not doing any logic wrong. Just that you are printing inside the while loop rather than outside.

while ((line = r.readLine()) != null) {
  Matcher m = patt.matcher(line);
  while (m.find()) {

    int start = m.start(0);
    int end = m.end(0);
    result.add(m.group());
    //System.out.println(result); printing inside and so first u add 1 then 2 then 3.
  }
}

System.out.println(result);//Move here
if(!result.isEmpty()){//result != null is redundant. You have already initialized.
  System.out.println("Last element is:" + result.get(result.size() - 1));
}

answered Nov 1, 2018 at 3:22

Karthik R

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Comments

Huy Nguyen · Accepted Answer · 2018-11-01 03:23:38Z

1

Because you print the output in loop, each loop, it will have a output.

To correct it just move the output out of loop.

For example

List<Integer> rest = new ArrayList<Integer>();
for(int i=0;i<2;i++){
rest.add(i);
System.out.println(result);
}

output

i=0: [0]
i=1: [0,1]

to correct it:

List<Integer> rest = new ArrayList<Integer>();
for(int i=0;i<2;i++){
rest.add(i);
}
System.out.println(result);

The result will be output [0,1]

answered Nov 1, 2018 at 3:23

Huy Nguyen

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1 Comment

Tom Wilson Over a year ago

Thank you. I can't believe I didn't see that!

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