This is an example from a real class which is used in production in many trading systems. It’s not a commonly used but you might assume that StringBuffer gives you thread safety, when it doesn’t.

public void addProperty(String name, String value) {
 if (value != null && value.length() > 0) {
 if (sb.length() > 0) {
 sb.append(',');
 }
 sb.append(name).append('=').append(value);
 }
 }

A puzzle

 T1: addProperty("a", "b");
T2: addProperty("c", "d");
T3: sb.toString();

write a program which will generate every possible output of T3’s sb.toString() I found 89.  With thread safety, you might reduce this to 4.

Note

If you used StringBuilder it would be worse, but at least you might not assume your method is thread safe when it is not. e.g. SimpleDateFormat uses StringBuffer