JAVA中，字符串STRING与STRINGBUILDER的效率差异

public class HelloJava {

public static void main(String[] args) {
// TODO Auto-generated method stub
String str = "a";
long starTime = System.currentTimeMillis();
for(int i = 0; i<10000;i++){
str = str + i;
}
long endTime = System.currentTimeMillis();
long time = endTime - starTime;
System.out.println("String time is " + time);
System.out.println(str);
StringBuilder builder = new StringBuilder("b");
starTime = System.currentTimeMillis();
for(int j = 0; j<10000;j++){
builder = builder.append(j);
}
endTime = System.currentTimeMillis();
time = endTime - starTime;
System.out.println("StringBuilder time is " + time);
System.out.println(builder);

}

}