字符串替换空格

public class Solution {    public String replaceSpace(StringBuffer str) {        if(str == null) return null;        int count = 0;        for(int i = 0; i < str.length(); ++i) {            if(str.charAt(i) == ' ') ++count;        }        System.out.println(str.length());        StringBuffer stringBuffer = new StringBuffer();        stringBuffer.setLength(str.length() + 2 *count);        int k = str.length() + 2 *count-1;        System.out.println(k);        for(int i = str.length()-1; i >= 0; --i) {            if(str.charAt(i) != ' ') {                stringBuffer.setCharAt(k--, str.charAt(i));            } else {                stringBuffer.setCharAt(k--, '0');                stringBuffer.setCharAt(k--, '2');                stringBuffer.setCharAt(k--, '%');            }        }        return stringBuffer.toString();    }}