How to append a sentence with prefix and suffix in java?

I am trying to read an input file that contains the following:

input.txt

Hello world. Welcome,
to the java.

And, I have to append the sentence with prefix(BEGIN) and suffix(END) and the output should like the following:

output expected:

BEGIN_Hello world_END.BEGIN_ Welcome,
to the java_END.

Following is my input file reading function. I am reading an entire file and storing it in array list:
InputDetails.java

private List<String> readInput = new ArrayList<>();
public void readFile() throws IOException {
   while((inputLine = input.readLine()) != null ) {
      readInput.add(inputLine);
   }
}

//Getter to return input file content
public List<String> getReadInput() {
   return readInput;
}

And following is my code for appending the string with BEGIN and END:

public void process() {
   InputDetails inputD = new InputDetails();
   for(int i=0;i<inputD.getReadInput().size();i++) {
            String sentence = inputD.getReadInput().get(i);
            String splitSentence[] = sentence.split("\\.");
            for(int j=0;j<splitSentence.length;j++) {
                System.out.println(splitSentence[j]);
                splitSentence[j] = "BEGIN_"+splitSentence[j]+"__END";
            }
            sentence = String.join(".",splitSentence);
            inputD.writeToFile(sentence);
        }
}

output getting:

BEGIN_SENTENCE__Hello world__END_SENTENCE.BEGIN_SENTENCE__Welcome
to the java.

Note: Each sentence is separated by a "." (period) character. The output Sentence should be prefixed with BEGIN_ and suffixed with __END. The period character is not considered a part of the sentence. And, input file are delimited by one or more spaces. The sentence is complete when it has period(.) Even if it means the sentence completes on the new line(just as the input that i specified above). All, the special chars position should be retained in the output. There can also be a space between period(.) or a comma(,) and a word. for eg: java . or Welcome ,

Can Anyone help me fix this? Thanks

First, you'll need to join your string list input into a single string. Then, you can use the String.split() method to break up your input into parts delimited by the . character. You can then choose to either run a loop on that array or use the stream method (as shown below) to iterate over your sentences. On each part, simply append the required BEGIN_ and _END blocks to the sentence. You can use manual string concatenation using the + operator or use a string template with String.format() (as shown below). Finally, reintroduce the . delimiter used to break the input by joining the parts back into a single string.

String fullString = String.join("", getReadInput());
Arrays.asList(fullString).split("\\.")).stream()
  .map(s -> String.format("BEGIN_%s_END", s))
  .collect(Collectors.joining("."));