How do you declare a string array of unknown size?

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Declaring an array of unknown size

I'm working in Java and I am trying to input a sentence into a string array. I am tokenizing it and determining the word count. However, I need to add each word into a string array in order to determine if there are duplicates or not. I am not sure how to initialize my array if I don't know the word count until later in the program.

  //Declares variables
  Scanner scan = new Scanner (System.in);
  int withoutdup = 0, wordCount = 0;
  String line, word; 
  StringTokenizer tokenizer;
  List<String> sentence = ArrayList<String>;

  //Asks user for input
  System.out.println ("Please enter text. Enter DONE to finish.");
  line = scan.next();


  //Tokenizes the string and counts the number of character and words
while (!line.equals("DONE"))
 {
     tokenizer = new StringTokenizer (line);
     while (tokenizer.hasMoreTokens())
     {
        word = tokenizer.nextToken();
        wordCount++;
        sentence += word; 
     }
     line = scan.next();
 }