Write a method to replace all spaces in a string with %20

I have implemented a solution to this Cracking the Coding Interview question:

Write a method to replace all spaces in a string with %20.
You may assume that the string has sufficient space at the end of the string to hold the additional characters,
and that you are given the true length of the string.
USE character array to perform this operation in place.

public static String conversion(String str, int length){

    char[] strChars = str.toCharArray();
    int numSpaces = 0;

    for(int i = 0; i = 0; i--){
        char c = strChars[i];
        if(c != ' '){
            newChars[i + 2 * numSpaces] = c;
        }
        else{
            newChars[i + 2 * numSpaces] = '0';
            newChars[i + 2 * numSpaces - 1] = '2';
            newChars[i + 2 * numSpaces - 2] = '%';
            numSpaces--;
        }
    }
    String newString = String.valueOf(newChars);
    return newString;
}

Can anyone give me any feedback on my implementation in regards of efficiency or improvements?

Your strategy of counting the spaces and then back-looping to shift the characters right (and replace spaces with %20) is good. The basic algorithm is probably as good as it gets as a character array system.

Your variable names are decent, and the code flows well.

On the other hand, there are some small things I would change.

Possible bug

Your code, given the input conversion("abc ", 3) you would output "abc" but you should not remove any "extra" padding in the string, you should return "abc "

In fact, you should only really have the one char[] array. The second one is making you do bad things ;-)

Enhanced fors

Use enhanced-for loops when possible, and always use {} blocks for if-statements, even 1-liners:

for(int i = 0; i < length; i++){
    if(strChars[i] == ' ')
        numSpaces++;
}

should be:

for (char c : strChars) {
    if (c == ' ') {
        numSpaces++;
    }
}

Comments

Comment unusual loops - your second for-loop is an odd one, and it often helps the reader if you comment why a loop is non-sequential (or even if you just make sure they are aware of it).

multiple indexes

Have you considered having a separate index for each position in the array - the position of the source character, and the position of where to insert it?

public static String conversion(String str, int length){

    char[] strChars = str.toCharArray();

    int numSpaces = 0;
    for (int i = 0; i = 0; i--) {
        char c = strChars[i];
        if (c == ' '){
            strChars[insert--] = '0';
            strChars[insert--] = '2';
            strChars[insert--] = '%';
        } else {
            strChars[insert--] = c;
        }
    }
    return String.valueOf(strChars);
}

The benefit of two indexes is that you can keep the logic more readable ... ("more" is relative) as each index moves by one character at a time... and a space counts as 3 characters.

StackExchange Code Review Q#156821, answer score: 5