Java (move point in string) - move

Question

I have a string = "abc";
And also I have a point ".".
How I can move this point "." in that string("abc").
Example :
Input date = "abc".
Output date = "abc", "a.bc", "ab.c", "a.b.c".
Thanks'.
public class MovePoint {
public static void main(String[] args) {
String str = "abcd";
String str1 = ".";
String[] ara = new String[str.length()];
for (int i = 0; i < str.length(); i++) {
ara[i] = str.substring(i, 1) + str1 + str.substring(1, 2);
System.out.print(Arrays.toString(ara));
}
}
}

Answer 1

Here is one way to do it. This uses StringBuilder as well as a plain char array to avoid having another loop over the array to build the last String, but it therefore consumes more memory.
First I print the first desired output, which is just the unmodified input String. Then I create a StringBuilder which can hold all chars from the input + one more for the chosen separator to avoid unnecessary array resizing. Then I initialize the StringBuilder so that it is in the form of the second desired ouput [char, sep, char, ...]. I am using StringBuilder here because it is just more convenient as it has the append() function that I need here.
Last but not least I also initialize a char array which will hold the values for the last String to avoid having to iterate over the array twice to generate that.
Now I loop over over the StringBuilder starting from one (as its already initialize to the first result with separator) to the last character. In this loop I do three things.
Print out the current value of StringBuilder
Swap the separator with the next character in the StringBuilder
Put the character and separator to the correct position in the char array as required for the last string
After the loop the last desired output is computed and I just have to print it to the console.
Runtime for this in BigO-notation would be O(n).
public static void main(String[] args) {
String str = "abcd";
char sep = '.';
movePoint(str, sep);
}
public static void movePoint(String str, char sep){
// print first desired output
System.out.println(str);
// String builder that can hold str.length + 1 characters, so no unnecessary resizing happens
var sb = new StringBuilder(str.length() + 1);
// fill with first char
sb.append(str.charAt(0));
// add separator
sb.append(sep);
// add rest of the string
sb.append(str.substring(1));
// Array that holds the last string
var lastStr = new char[str.length() + str.length() - 1];
for (int i = 1; i < sb.capacity() - 1; i++) {
System.out.println(sb);
// build current string
// swap separator with next character
var temp = sb.charAt(i);
sb.setCharAt(i, sb.charAt(i+1));
sb.setCharAt(i+1, temp);
// manipulate char array so last string is built correctly
int doubled = i << 1;
// set character at correct position
lastStr[doubled - 2] = sb.charAt(i-1);
// set separator at correct position
lastStr[doubled - 1] = sep;
}
// add last character of string to this char array
lastStr[lastStr.length - 1] = sb.charAt(sb.length() - 2);
// print last desired output
System.out.println(lastStr);
}
Expected output:
abcd
a.bcd
ab.cd
abc.d
a.b.c.d