Raw replacement of space by %20 in Java

Question

I tried coding this for practice. Is there a more memory-efficient way to work in place without using more arrays etc (using raw array operations)?

public class SpaceUrlEncoder
{
 // arguments are passed using the text field below this editor
 public static void main(String[] args)
 {
 String unencoded = "Mr John Smith ";
 int lastCharIdx = 12;
 char[] charr = unencoded.toCharArray();
 for (int i = 12; i >= 0; i--) {
 if (charr[i] == ' ') {
 encodeSpaceAt(i, lastCharIdx, charr);
 lastCharIdx += 2;
 }
 }
 System.out.println("space encoded string: " + new String(charr));
 }
 public static void encodeSpaceAt(int spaceIdx, int lastCharIdx, char[] charr) {
 for (int i = lastCharIdx; i > spaceIdx; i--) {
 charr[i+2] = charr[i];
 charr[i] = ' ';
 }
 charr[spaceIdx] = '%';
 charr[spaceIdx+1] = '2';
 charr[spaceIdx+2] = '0';
 }
}

• 200_success: Related questions: 1, 2, 3

Answer 1

We should understand what's happening when we do string-related operations in Java. When we concat strings in java, like

 String result = "";
 for(int i=0; i<100; i++){ 
 result += i+","; // bad, because each string + string creates new string 
 }

Strings are immutable, so its bad to += them, instead, StringBuilder is used. Here's another way to solve this task:

public class ReplaceSpace {
 private static final String REPLACE_SPACE = "%20";
 private static final String SPACE = " ";
 public static void main(String[] args) {
 String input = "Just some input string to perform stuff !";
 System.out.println(replaceString(input));
 }
 private static String replaceString(String input) {
 StringBuilder sb = new StringBuilder();
 int lastPoint = 0;
 int foundIndex;
 int maxIndex = input.length() - 1;
 while ((foundIndex = input.indexOf(SPACE, lastPoint)) != -1 && foundIndex <= maxIndex) {
 sb.append(input.substring(lastPoint, foundIndex - 1));
 sb.append(REPLACE_SPACE);
 lastPoint = foundIndex;
 }
 sb.append(input.substring(lastPoint));
 return sb.toString();
 }
}