public static void main(String[] args)
  {
  while (true)
  {
        System.out.println("Enter a sentence:");
        Scanner keyboard = new Scanner(System.in);
        String sentence = keyboard.nextLine();

        if (sentence.isEmpty())      // quit the program when user enter an empty string
        {
           break;
        }
        else
        {
        StringTokenizer st = new StringTokenizer(sentence);

        List<String> sentenceElement = new ArrayList<String>();

        while (st.hasMoreTokens())
        {
          sentenceElement.add(st.nextToken());
        }

        System.out.println(sentenceElement);
        }
  }

public static void main(String[] args){
    Scanner keyboard = new Scanner(System.in);
    String[] myPhrase = keyboard.nextLine().split(" ");
    HashMap<String, Integer> myWordsCount = new HashMap<String, Integer>();
    for (String s : myPhrase){
        if (myWordsCount.containsKey(s)) myWordsCount.replace(s, myWordsCount.get(s) + 1);
        else myWordsCount.put(s, 1);
    }
    System.out.println(myWordsCount);
}

One two three four and again three and four
{four=2, and=2, One=1, again=1, two=1, three=2}

import java.util.Arrays;
import java.util.stream.Collectors;

public class StringFrequencyMap {
    public static void main(String... args) {
        String[] wordArray = {"Apple", "is", "an", "apple", "is", "a", "phone"};
        var freqCaseSensitive = Arrays.stream(wordArray)
                         .collect(Collectors.groupingBy(x -> x, Collectors.counting()));
        //If you want case insensitive then use
        var freqCaseInSensitive = Arrays.stream(wordArray)
                .collect(Collectors.groupingBy(String::toLowerCase, Collectors.counting()));

        System.out.println(freqCaseSensitive);
        System.out.println(freqCaseInSensitive);
    }
}

{a=1, apple=1, Apple=1, phone=1, is=2, an=1}
{a=1, apple=2, phone=1, is=2, an=1}