Question

Considering the following string, I would like to get a substring in that way:

String str="abcdefghij"

Substring returned:

 acegi 

that is to say every two chars of the first string str.
Then with the same reasoning, getting a substring of every three chars of the first string str.

And the same question with a frequency of f

This is my current method but actually there is an error:

 private static String subString(String str,int freq){
        String subStr=null;

        for(int i = 0; i < str.length(); freq++){
          subStr= subStr + str[i];
        }
     return subStr;
 }

Also maybe there is some already existing method for these tasks?

Answer 1

I don’t know of a built-in method to do it. You could use a StringBuilder to avoid creating many temporary strings, and initialise it with the right capacity:

private static String subString(String str, int freq) {
    //make it the right capacity to avoid reallocations
    StringBuilder subStr = new StringBuilder(str.length() / freq + 1);

    for (int i = 0; i < str.length(); i += freq) {
        subStr.append(str.charAt(i));
    }
    return subStr.toString();
}