You can use the + operator to concatenate a strings, and a primitive type value or an object to another string.
For example,
String str = "X" + "Y" + 12.56;
To optimize the string concatenation operation, the compiler replaces the string concatenation by a statement, which uses a StringBuilder.
The compiler replaces the above statement with the following one:
String str = new StringBuilder().append("X").append("Y").append(12.56).toString();
toString() method at the end of this statement is used to convert the final content of a StringBuilder to a String.
append() method of StringBuilder returns a reference to itself.
public class Main { public static void main(String[] args) { String str = "X" + "Y" + 12.56; System.out.println(str);//from w w w.j ava2 s .c o m str = new StringBuilder().append("X").append("Y").append(12.56).toString(); System.out.println(str); } }