String, StringBuilder, StringBuffer
String is a unchangeable object(although it does not belong to the 8 primitive types). If we change the string, we rebuild a new object that a reference “points” to it. So for an an object that always changes, the speed will slow down (JVM GC). At this time, we use StringBuffer(not thread safe) or StringBuilder(thread safe, available after JDK 5.0) instead.
StringBuilder methods， constructors(click to check official doc)
StringBuilder(int capacity/ String str)
1)append(x) x could be several types which can be transferred to char
2) insert, reverse, toString, getChar…