Why StringBuilder when there is String?
I just encountered StringBuilder for the first time and was surprised since Java already has a very powerful String class that allows appending.
Why a second String class?
Where can I learn more about StringBuilder?
Answer
String does not allow appending. Each method you invoke on a String creates a new object and returns it. This is because String is immutable - it cannot change its internal state.
On the other hand StringBuilder is mutable. When you call append(..) it alters the internal char array, rather than creating a new string object.
Thus it is more efficient to have:
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 500; i ++) {
sb.append(i);
}
rather than str += i, which would create 500 new string objects.
Note that in the example I use a loop. As helios notes in the comments, the compiler automatically translates expressions like String d = a + b + c to something like
String d = new StringBuilder(a).append(b).append(c).toString();
Note also that there is StringBuffer in addition to StringBuilder. The difference is that the former has synchronized methods. If you use it as a local variable, use StringBuilder. If it happens that it's possible for it to be accessed by multiple threads, use StringBuffer (that's rarer)