Find all permutations of source within input

GetPermutations.java

public class Main {

	public static void main(String[] args) {
		System.out.println(getPermutations("bbca", "abbca"));
	}

	private static Set<String> getPermutations(String source, String input) {
		if (source == null || input == null || source.isEmpty() || input.isEmpty()) {
			return null;
		}
		if (source.length() > input.length()) {
			throw new IllegalArgumentException();
		}

		Set<String> result = new HashSet<>();
		Map<Character, Integer> sourceMap = new HashMap<>();
		for (int i = 0; i < source.length(); i++) {

			char currentChar = source.charAt(i);
			if (sourceMap.containsKey(currentChar)) {
				sourceMap.put(currentChar, sourceMap.get(currentChar) + 1);
			} else {
				sourceMap.put(currentChar, 1);
			}

		}

		Map<Character, Integer> buffer = new HashMap<>();
		for (int i = 0; i <= input.length() - source.length(); i++) {

			int jEnd = i + source.length();

			for (int j = i; j < jEnd; j++) {

				char currentChar = input.charAt(j);
				if (buffer.containsKey(currentChar)) {
					buffer.put(currentChar, buffer.get(currentChar) + 1);
				} else {
					buffer.put(currentChar, 1);
				}

			}

			if (buffer.equals(sourceMap)) {
				result.add(input.substring(i, jEnd));
			}
			buffer.clear();

		}
		return result;

	}

}