在字符串中交换两个字母

Question

I want to swap two letters in a string. For example, if input is W and H then all the occurrences of W in string should be replaced by H and all the occurrences of H should be replaced by W . String WelloHorld will become HelloWorld .

I know how to replace single char:

str = str.replace('W', 'H');

But I am not able to figure out how to swap characters.

Answer 1

public String getSwappedString(String s)
{
char ac[] = s.toCharArray();
for(int i = 0; i < s.length(); i++)
{
   if(ac[i] == 'H')
     ac[i]='W';
   else if(ac[i] == 'W')
     ac[i] = 'H'; 
}

s = new String(ac);
return s;
}

Answer 2

With Java8 it's truly simple

static String swap(String str, String one, String two){
    return Arrays.stream(str.split(one, -1))
        .map(s -> s.replaceAll(two, one))
        .collect(Collectors.joining(two));
}

Usage example:

public static void main (String[] args){
    System.out.println(swap("𐌰𐌱𐌲", "𐌰", "𐌲"));
}

I urge you not to use a Character for the swap function, since it will break strings containing letters outside the BMP

In case you want to extend this to work with arbitrary Strings (not only letters), you can just quote the supplied strings:

static String swap(String str, String one, String two){
    String patternOne = Pattern.quote(one);
    String patternTwo = Pattern.quote(two);
    return Arrays.stream(str.split(patternOne, -1))
        .map(s -> s.replaceAll(patternTwo, one))
        .collect(Collectors.joining(two));
}

Answer 3

You would probably need three replace calls to get this done.

The first one to change one of the characters to an intermediate value, the second to do the first replace, and the third one to replace the intermediate value with the second replacement.

String str = "Hello World";

str = star.replace("H", "*").replace("W", "H").replace("*", "W");

Edit

In response to some of the concerns below regarding the correctness of this method of swapping characters in a String . This will work, even when there is a * in the String already. However, this requires the additional steps of first escaping any occurrence of * and un-escaping these before returning the new String .

public static String replaceCharsStar(String org, char swapA, char swapB) {
    return org
            .replace("*", "\\*")
            .replace(swapA, '*')
            .replace(swapB, swapA)
            .replaceAll("(?<!\\\\)\\*", "" + swapB)
            .replace("\\*", "*");

}

Edit 2

After reading through some the other answers, a new version, that doesn't just work in Java 8, works with replacing characters which need to be escaped in regex, eg [ and ] and takes into account concerns about using char primitives for manipulating String objects.

public static String swap(String org, String swapA, String swapB) {
    String swapAEscaped = swapA.replaceAll("([\\[\\]\\\\+*?(){}^$])", "\\\\$1");
    StringBuilder builder = new StringBuilder(org.length());

    String[] split = org.split(swapAEscaped);

    for (int i = 0; i < split.length; i++) {
        builder.append(split[i].replace(swapB, swapA));
        if (i != (split.length - 1)) {
            builder.append(swapB);
        }
    }

    return builder.toString();

}

Answer 4

A slightly nicer version of the string-scanning approach, without explicit arrays and index access:

StringBuilder sb = new StringBuilder();
for (char c : source_string.toCharArray()) {
  if (c == 'H') sb.append("W");
  else if (c == 'W') sb.append("H");
  else sb.append(c);
}
return sb.toString();

Answer 5

You could iterate over the String's character array, and swap whenever you see either of the characters:

private static String swap(String str, char one, char two) {
    char[] chars = str.toCharArray();
    for (int i = 0; i < chars.length; i++) {
        if (chars[i] == one) {
            chars[i] = two;
        } else if (chars[i] == two) {
            chars[i] = one;
        }
    }
    return String.valueOf(chars);
}

Answer 6

You could try this code also.

System.out.println("WelloHorld".replaceAll("W", "H~").replaceAll("H(?!~)", "W").replaceAll("(?<=H)~", ""));

Output:

HelloWorld

Use any character which isn't present in the input string instead of ~ .