Given an arbitrary text document written in English, this solution will generate an alphabetical list of all word occurrences with their frequencies and the sentence numbers in which they appear.

WordFrequencies.java

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.text.BreakIterator;
import java.util.ArrayList;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.TreeMap;

public class WordFrequencies {

	private Locale locale;
	private Map<String, List<Integer>> concordanceMap;  // Key = Word, Value = list of sentence numbers in which word appears

	public WordFrequencies(Locale locale) {
		this.locale = locale;
		this.concordanceMap = new TreeMap<String, List<Integer>>();
	}

	public void build(String text) {
		BreakIterator sentenceIterator = BreakIterator.getSentenceInstance(this.locale);
		BreakIterator wordIterator = BreakIterator.getWordInstance(this.locale);

		int sentenceCounter = 0;

		int sentenceIdx = 0;
		sentenceIterator.setText(text);

		// Go through sentences
		while (BreakIterator.DONE != sentenceIterator.next()) {
			String sentence = text.substring(sentenceIdx, sentenceIterator.current());
			sentenceCounter++;

			int wordIdx = 0;
			wordIterator.setText(sentence);

			// Go through words
			while (BreakIterator.DONE != wordIterator.next()) {
				String word = sentence.substring(wordIdx, wordIterator.current()).toLowerCase();

				if (Character.isLetterOrDigit(word.charAt(0))) {
					if (this.concordanceMap.containsKey(word)) {
						this.concordanceMap.get(word).add(sentenceCounter);
					} else {
						List<Integer> sentenceNumbers = new ArrayList<Integer>();
						sentenceNumbers.add(sentenceCounter);
						this.concordanceMap.put(word, sentenceNumbers);
					}
				}
				wordIdx = wordIterator.current();
			}
			sentenceIdx = sentenceIterator.current();
		}
	}

	public void print() {
		for (String word : this.concordanceMap.keySet()) {
			List<Integer> sentenceNumbers = this.concordanceMap.get(word);
			System.out.println(word + " {" + sentenceNumbers.size() + ":" + sentenceNumbers.toString() + "}");
		}
	}

	public static void main(String[] args) {
		String fileName = (args.length == 0 ? "input.txt" : args[0]);
		Charset inputEncoding = StandardCharsets.UTF_8;

		try {
			String text = new String(Files.readAllBytes(Paths.get(fileName)), inputEncoding);

			if (null != text && !text.trim().isEmpty()) {
				WordFrequencies solution = new WordFrequencies(Locale.US);
				// Build concordance Map
				solution.build(text);
				// Print Result
				solution.print();
			} else {
				throw new IllegalArgumentException();
			}
		} catch (IOException e) {
			System.out.println("Error: Could not load file " + fileName);
		} catch (IllegalArgumentException e) {
			System.out.println("Text file '" + fileName + "' is empty!");
		}
	}
}