String vs StringBuffer or StringBuilder

String is immutable.Once it is created can not be changed.(previously discussed)
String x = "hi";
x.concat("bhabani");
System.out.println(x); // output is "hi"
StringBuffer is not immutable
StringBuffer sb = new StringBuffer("hi");
sb.append(" bhabani");
System.out.println(sb); // output is "hi bhabani"