String greeting = "Hello world!";

Whenever it encounters a string literal in your code, the 
compiler creates a String object with its valuein this case,
 "Hello world!'.As with any other object, you can create 
String objects by using the new keyword and a constructor. 
The String class has eleven constructors that allow you to
 provide the initial value of the string using different sources, 
such as an array of characters:
public class StringDemo{   public static void main
(String args[]){    
  char[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.'}; 
 String helloString = new String(helloArray);   
     System.out.println( helloString );  
 }}
This would produce following result:
hello
Note: The String class is immutable, so that once it
 is created a String object cannot be changed. If there
 is a necessity to make alot of modifications to Strings 
of characters then you should use
 String Buffer & String Builder Classes.
 Creating Format Strings:
You have printf() and format() methods to print output with formatted numbers. The String class has an equivalent class method, format(), that returns a String object rather than a PrintStream object.

System.out.printf("The value of the float variable is "
 + "%f, while the value of the integer " + 
  "variable is %d, and the string " +  
    "is %s", floatVar, intVar, stringVar);

you can write:

String fs;
fs = String.format("The value of the float variable is "
 +  "%f, while the value of the integer " + 
   "variable is %d, and the string " + 
 "is %s", floatVar, intVar, stringVar);
System.out.println(fs);