去掉字符串中间的空格

/**
* Created by king on 2017/5/8.
* 功能：去掉中间的空格，保留两边的空格。
*/
public class Trimid {
public static void main(String[] args) {
String a1 = "   asd  fa  sdf   ";
String a2 = "   asd  fa  sdf";
String a3 = "asd  fa  sdf   ";
String a4 = "   asdfasdf   ";
String a5 = "asdfasdf";
String a6 = "       ";
System.out.println("--->" + trimid(a1) + "<---");
System.out.println("--->" + trimid(a2) + "<---");
System.out.println("--->" + trimid(a3) + "<---");
System.out.println("--->" + trimid(a4) + "<---");
System.out.println("--->" + trimid(a5) + "<---");
System.out.println("--->" + trimid(a6) + "<---");
}

private static String trimid(String a) {
int start = 0;
int end = a.length() - 1;
StringBuilder sb = new StringBuilder();
while (start < end && a.charAt(start) == ' ') start++;
while (start < end && a.charAt(end) == ' ') end--;
int i = start;
int j = start;
for (; i < end; i++) {
if (a.charAt(i) != ' ' && a.charAt(i + 1) == ' ') {
sb.append(a.substring(start, i + 1));
}
if (a.charAt(i) == ' ' && a.charAt(i + 1) != ' ') {
start = i + 1;
}
}
sb.append(a.substring(start, a.length())).insert(0, a.substring(0, j));
return sb.toString();
}
}