JAVA Sort String Array Add Missing Numbers

In java, I have a string array. It contains strings with different numbers. For example: x1,x3,x5,x9,y1,y3,y6,y9,z14,z17,z22,z50. How can I sort this array by both letters and numbers, and add the missing string elements (x2...) in order?

A simple way to do this would be the following:

1. Sort the array.
2. Get the minimum and the maximum number of each letter and save them in separate maps.
3. For each letter, add to a list from the specified saved range of it.

Here is the solution:

public static void main(String[] args) {
	String[] arr = new String[] {"x1","x3","x5","x9","y1","y3","y6","y9","z14","z17","z22","z50"};
	System.out.println(Arrays.toString(addMissing(arr)));
}
private static String[] addMissing(String[] arr) {
	Arrays.sort(arr);
	Map<String,Integer> min = new HashMap<>();
	Map<String,Integer> max = new HashMap<>();
	for(int i = 0; i < arr.length; i++) {
		String str = arr[i];
		int number = getNum(str);
		String letter = getChar(str);
		if(min.containsKey(letter)) {
			if(min.get(letter) > number)
				min.put(letter, number);
		}else {
			min.put(letter, number);
		}
		if(max.containsKey(letter)) {
			if(max.get(letter) < number)
				max.put(letter, number);
		}else {
			max.put(letter, number);
		}
	}
	List<String> list = new ArrayList<>();
	for(String key : min.keySet()) {
		int minNumber = min.get(key), maxNumber = max.get(key);
		for(int i = minNumber; i <= maxNumber; i++)
			list.add(key+i);
	}
	String[] res = new String[list.size()];
	for(int i = 0; i < list.size(); i++) res[i] = list.get(i);
	return res;
}
private static String getChar(String str) {
	return str.replaceAll("\\d", "");
}
private static int getNum(String str) {
	return Integer.parseInt(str.replaceAll("\\D+",""));
}

Output:

[x1, x2, x3, x4, x5, x6, x7, x8, x9, y1, y2, y3, y4, y5, y6, y7, y8, y9, z14, z15, z16, z17, z18, z19, z20, z21, z22, z23, z24, z25, z26, z27, z28, z29, z30, z31, z32, z33, z34, z35, z36, z37, z38, z39, z40, z41, z42, z43, z44, z45, z46, z47, z48, z49, z50]

Try this.

String[] arr = new String[] {"x1","x3","x5","x9","y1","y3","y6","y9","z14","z17","z22","z50"};
List<String> result = Arrays.stream(arr)
    .collect(Collectors.groupingBy(s -> s.substring(0, 1),
        Collectors.mapping(s -> Integer.parseInt(s.substring(1)), Collectors.toList())))
    .entrySet().stream()
    .flatMap(e -> IntStream.rangeClosed(
        e.getValue().stream().mapToInt(Integer::intValue).min().getAsInt(),
        e.getValue().stream().mapToInt(Integer::intValue).max().getAsInt())
        .mapToObj(i -> e.getKey() + i))
    .sorted()
    .collect(Collectors.toList());
System.out.println(result);

output

[x1, x2, x3, x4, x5, x6, x7, x8, x9, y1, y2, y3, y4, y5, y6, y7, y8, y9, z14, z15, z16, z17, z18, z19, z20, z21, z22, z23, z24, z25, z26, z27, z28, z29, z30, z31, z32, z33, z34, z35, z36, z37, z38, z39, z40, z41, z42, z43, z44, z45, z46, z47, z48, z49, z50]

Intermediate result of groupingBy

{x=[1, 3, 5, 9], y=[1, 3, 6, 9], z=[14, 17, 22, 50]}