[Solved] Replace every nth character with “-” in java without regex

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What you could do is use a StringBuilder that will be initialized with your input String, then use setCharAt(int index, char ch) to modify a given char, this way you avoid building a new String at each iteration as you currently do with the operator += on String.

Something like:

public String everyNth(String str, int n) {
    if (n < 1) {
        throw new IllegalArgumentException("n must be greater than 0");
    }
    StringBuilder result = new StringBuilder(str);
    // This will replace every nth character with '-'
    for (int i = n - 1; i < str.length(); i += n) {
        result.setCharAt(i, '-');
    }
    return result.toString();
}

NB: Starts from n - 1 unless you want to replace the first character too if so starts from 0.

If you want to simply remove the characters, you should use the method append(CharSequence s, int start, int end) to build the content of your target String, as next:

public String everyNth(String str, int n) {
    if (n < 1) {
        throw new IllegalArgumentException("n must be greater than 0");
    }
    StringBuilder result = new StringBuilder();
    // The index of the previous match
    int previous = 0;
    for (int i = n-1; i < str.length(); i += n) {
        // Add substring from previous match to the current
        result.append(str, previous, i);
        // Set the new value of previous
        previous = i + 1;
    }
    // Add the remaining sub string
    result.append(str, previous, str.length());
    return result.toString();
}

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solved Replace every nth character with “-” in java without regex