java 空格替换%20

public String replaceSpace(StringBuffer str2) {

StringBuffer str4 = new StringBuffer();

int length=str2.length();
char []str3=str2.toString().toCharArray();

for(int i=0;i<length;i++)
{
if(str3[i]!=' ')
{
str4.append(str3[i]);
}
else{

str4.append("%");
str4.append("2");
str4.append("0");

}
}
//str4.append("\0");
return str4.toString();

}