import java.text.Normalizer;
import java.util.regex.Pattern;

public static String deAccent(String str) {
    String nfdNormalizedString = Normalizer.normalize(str, Normalizer.Form.NFD);
    Pattern pattern = Pattern.compile("\p{InCombiningDiacriticalMarks}+");
    return pattern.matcher(nfdNormalizedString).replaceAll("");
}

System.out.print(deAccent("Olá, mundo!"));