Replacing a character in a string with another character from another string

Question

I am trying to eventually replace a sentence with another set of String. But I hit a roadblock while trying to replace a char in a String with another character of another String.

Here's what I have so far.

String letters = "abcdefghijklmnopqrstuvwxyz";
String encode = "kngcadsxbvfhjtiumylzqropwe";
// the sentence that I want to encode
String sentence = "hello, nice to meet you!";

//swapping each char of 'sentence' with the chars in 'encode'
for (int i = 0; i < sentence.length(); i++) {
    int indexForEncode = letters.indexOf(sentence.charAt(i));
    sentence.replace(sentence.charAt(i), encode.charAt(indexForEncode));
}

System.out.println(sentence);

This way of replacing characters doesn't work. Can someone help me?

score 4 · Accepted Answer · edited Mar 19 '21 at 11:24

The reason

sentence.replace(sentence.charAt(i), encode.charAt(indexForEncode));

doesn't work is that Strings are immutable (i.e., they never change). So, sentence.replace(...) doesn't actually change sentence; rather, it returns a new String. You would need to write sentence = sentence.replace(...) to capture that result back in sentence.

OK, Strings 101: class dismissed (;->).

Now with all that said, you really don't want want to keep reassigning your partially encoded sentence back to itself, because you will, almost certainly, find yourself re-encoding characters of sentence that you already encoded. Best to leave sentence in its original form while building up the encoded string one character at a time like this:

StringBuilder sb = new StringBuilder();
for (int i = 0; i < sentence.length(); i++){
    int indexForEncode = letters.indexOf(sentence.charAt(i));
    sb.append(indexForEncode != -1
            ? encode.charAt(indexForEncode)
            : sentence.charAt(i)
    );
}
sentence = sb.toString();

Hi, thank you so much! This worked for me. I wish I could upvote. However, is there an article for me to read on if the chars in 'sentence' aren't alphabets? This gives me an error if I have non alphabet characters. — hendrahjh, Mar 18 '21 at 01:54
I've updated the code to address that; maybe you're still looking at the old version. You might want to check the [Javadoc for the `indexOf` method](https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#indexOf-int-): to see the logic behind my fix. — Kevin Anderson, Mar 18 '21 at 02:00

score 0 · Answer 2 · edited Mar 19 '21 at 18:36

I would use a character array as follows. Make the changes to a character array and then use String.valueOf to get the new version of the string.

String letters = "abcdefghijklmnopqrstuvwxyz";
String encode =  "kngcadsxbvfhjtiumylzqropwe";
// the sentence that I want to encode
String sentence = "hello, nice to meet you!";

char[] chars = sentence.toCharArray();
for (int i = 0; i < chars.length; i++){
    int indexForEncode = letters.indexOf(sentence.charAt(i));
    // if index is < 0, use original character, otherwise, encode.
    chars[i] = indexForEncode < 0 ? chars[i] : encode.charAt(indexForEncode);
}
System.out.println(String.valueOf(chars));

Prints

xahhi, tbga zi jaaz wiq!

score 0 · Answer 3 · 2021-03-22T05:35:09.533

You can use codePoints method to iterate over the characters of this string and replace them with characters from another string, if any.

Try it online!

public static void main(String[] args) {
    String letters = "abcdefghijklmnopqrstuvwxyz";
    String encode = "kngcadsxbvfhjtiumylzqropwe";

    String sentence = "hello, nice to meet you!";
    String encoded = replaceCharacters(sentence, letters, encode);
    String decoded = replaceCharacters(encoded, encode, letters);

    System.out.println(encoded); // xahhi, tbga zi jaaz wiq!
    System.out.println(decoded); // hello, nice to meet you!
}

public static String replaceCharacters(String text, String from, String to) {
    // wrong cipher, return unencrypted string
    if (from.length() != to.length()) return text;
    // IntStream over the codepoints of this text string
    return text.codePoints()
            // Stream<Character>
            .mapToObj(ch -> (char) ch)
            // encrypt characters
            .map(ch -> {
                // index of this character
                int i = from.indexOf(ch);
                // if not present, then leave it as it is,
                // otherwise replace this character
                return i < 0 ? ch : to.charAt(i);
            }) // Stream<String>
            .map(String::valueOf)
            // concatenate into a single string
            .collect(Collectors.joining());
}

^{See also: Implementation of the Caesar cipher}

Replacing a character in a string with another character from another string

3 Answers3

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