How to generate string mask with various numbers of symbols?

Question

For example: I have a string mask with length = 5 symbols and I have a value with length = 3 symbols. All combinations are:

val__, _val_, __val

another example for mask length = 3, value length = 2:

xx_, _xx

How to generate these masks programmatically? For example in method with following signature: String[] generateMasks(int maskLength, String val);

My attempts:

private ArrayList<String> maskGenerator2(int from, char[] value) {
    ArrayList<String> result = new ArrayList<String>();
    //initial position
    char[] firstArray = new char[from];
    for (int i=0; i<from; i++) {
        if (i < value.length) firstArray[i] = value[i];
        else firstArray[i] = '_';
    }
    result.add(String.valueOf(firstArray));
    System.out.println(firstArray);

    //move value
    int k = 0;
    while (k < from - value.length) {
        char last = firstArray[from - 1];
        for (int i = from - 1; i > 0; i--) {
            firstArray[i] = firstArray[i - 1];
        }
        firstArray[0] = last;
        result.add(String.valueOf(firstArray));
        System.out.println(firstArray);
        k++;
    }
    return result;
}

Maybe are there more elegant solutions for this?

Answer 1

• Create a String consisting of repeated mask symbols to required length .. see Simple way to repeat a String in java
• Use StringBuilder to insert the input string at each possible point.

Example

public static void main(String[] args) throws Exception {
    System.out.println(createMask(5, "val"));
    System.out.println(createMask(3, "xx"));
}

private static List<String> createMask(int length, String value) {
    List<String> list = new ArrayList<String>();
    String base = new String(new char[length]).replace("\0", "_");
    for (int offset = 0; offset <= length - value.length(); offset++) {
        StringBuilder buffer = new StringBuilder(base);
        buffer.replace(offset, offset + value.length(), value);
        list.add(buffer.toString());
    }
    return list;
}

Output

[val__, _val_, __val]
[xx_, _xx]