Remove characters that are before another

Question

My teacher asked to make a method that cancels a character in a sentence using the # cancels the previous character, example:

Entree: PUO#C MIP#NO#AS

Exit: PUC MINAS

However, my code gives error when using the method replace or replaceAll to remove and swap for an empty character. Follow my code below:

public void Elimina(String frase) {

        for (int j = 0; j<frase.length(); j++) {
            if (frase.charAt(j) == '#') {
                frase = frase.replace(frase.charAt(j), '');
                frase = frase.replace(frase.charAt(j-1), '-');
            }
        }

        System.out.println(frase);      
    }

OBS:

frase = frase.replace(frase.charAt(j), ''); -> The mistake is this line on ''.

frase = frase.replace(frase.charAt(j-1), '-'); -> Here is to show that replacing the method works.

Answer 1

How is an exercise, probably the teacher wants you to use a loop, and then you can use the code of another answer.

But only to record an alternative:

public String elimina(String frase) {
    return frase.replaceAll(".#", "");    
}

I used the regex .#. The point corresponds to any character (except line breaks), so she takes any character followed by # and replaces with the empty string (which in practice is the same as removing these characters).

I also changed the method to return the string instead of printing it. So whoever calls the method gets the result and can do whatever you want with it (including printing, of course, but this responsibility should not remain in the function).

And I put the name starting with lowercase letter to be adhered to the Java code conventions.

Example of use:

System.out.println(elimina("PUO#C MIP#NO#AS"));

The above code prints "PUC MINAS".

---

The regex also already handles a corner case, which is when the string starts with #. In that case, I understand that the # should not be removed, since there is no previous character (and the above regex already does, because the substitution only occurs if there is a character before the #).

But if you want to remove the character # also if it is at the beginning of the string, just change the regex to ".?#". The ? indicates that the character before the # is optional.

Answer 2

I suggest you use a StringBuilder. You copy in it the characters one by one, but when you find one #, you delete the last.

public class Main {
    public static void main(String[] args) {
        System.out.println(elimina("PUO#C MIP#NO#AS"));
    }

    public static String elimina(String frase) {
        StringBuilder sb = new StringBuilder(frase.length());
        for (int j = 0; j < frase.length(); j++) {
            if (frase.charAt(j) == '#') {
                sb.deleteCharAt(sb.length() - 1);
            } else {
                sb.append(frase.charAt(j));
            }
        }
        return sb.toString();
    }
}

See it working on Ideone.