String, StringBuilder and StringBuffer on Three Axes

Three types that all hold text. Two of them can be changed in place, and the difference between those two is a decision somebody made in 1996 and a decision somebody made in 2004.

The three text types on mutability, thread safety and when to use each
Type	Can be changed in place	Safe across threads	Reach for it when
`String`	No	Yes, because nothing can change it	The text is a value you pass around
`StringBuilder`	Yes	No	You are assembling text in a loop or a method
`StringBuffer`	Yes	Yes, every method is synchronised	Two threads genuinely share one builder

java

StringBuilder row = new StringBuilder();
for (String value : values) {
    row.append(value).append(",");
}
String result = row.toString();

append writes into a character array the builder already owns, growing it in jumps when it runs out. Nothing is copied per pass, so a thousand appends cost about a thousand times one append rather than a thousand squared.

The decision rule. Use String for text you hold and hand around. Use StringBuilder the moment you are assembling text in a loop. Use StringBuffer only when two threads append to the same builder, which is rare enough that meeting it should make you check the design first.

Java
JVM
Performance