Comparison of StringBuilder and StringBuffer

StringBuilder and StringBuffer are very common classes in Java.
Here, I am comparing with these two classes.
Generally speaking,
StringBuilder is
1) non-synchronized
2) be careful when using multithreads
3) faster  

StringBuffer 
1) synchronized.
2) safer but slower

>>Java test codes
public class StringDemo{
public static void main(String[] args){
int N = 100000;
StringBuffer buffer = new StringBuffer();
StringBuilder builder = new StringBuilder();
long begin = System.currentTimeMillis();
for(int i = 0; i < N; i++){
buffer.append(" ");
}
System.out.println("StringBuffer takes " + (System.currentTimeMillis() - begin) + " ms");
begin = System.currentTimeMillis();
for(int i= 0; i < N; i++){
builder.append(" ");
}
System.out.println("StringBuilder takes " + (System.currentTimeMillis() - begin) + " ms");
}
}

output >>
StringBuffer takes 8 ms
StringBuilder takes 1 ms