Strings.java

package modern.challenge;

import java.util.regex.Pattern;
import java.util.stream.Collectors;

public final class Strings {

    private Strings() {
        throw new AssertionError("Cannot be instantiated");
    }

    public static String removeCharacterV1(String str, char ch) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException
            return "";
        }

        StringBuilder sb = new StringBuilder();
        char[] chArray = str.toCharArray();
        for (char c : chArray) {
            if (c != ch) {
                sb.append(c);
            }
        }

        return sb.toString();
    }

    public static String removeCharacterV2(String str, char ch) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException
            return "";
        }

        return str.replaceAll(Pattern.quote(String.valueOf(ch)), "");
    }

    public static String removeCharacterV3(String str, char ch) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException
            return "";
        }

        return str.chars()
                .filter(c -> c != ch)
                .mapToObj(c -> String.valueOf((char) c))
                .collect(Collectors.joining());
    }

    public static String removeCharacterV4(String str, String ch) {

        if (str == null || ch == null || str.isEmpty() || ch.isEmpty()) {
            // or throw IllegalArgumentException            
            return "";
        }

        if (ch.codePointCount(0, ch.length()) != 1) {
            return ""; // there is more than 1 Unicode character in the given String
        }

        int codePoint = ch.codePointAt(0);

        return str.codePoints()
                .filter(c -> c != codePoint)
                .mapToObj(c -> String.valueOf(Character.toChars(c)))
                .collect(Collectors.joining());
    }
}