String is the most commonly used class in java library. String class is encapsulated under java.lang package. 

The Java platform provides the String class to create and manipulate strings.

Strings, which are widely used in Java programming, are a sequence of characters. In the Java programming language, strings are objects. i.e every string that you create is actually an object of type String

One important thing to notice about string object is that string objects are immutable that means once a string object is created it cannot be altered.

An array of characters works same as java string. For example:

 char[] ch={'H','e','l','l','o','W','o','r','l','d'};  
String s=new String(ch);  

String s="HelloWorld";  

What is an Immutable object?

An object whose state cannot be changed after it is created is known as an Immutable object. String, Integer, Byte, Short, Float, Double and all other wrapper classes objects are immutable.

Creating an Immutable class

public final class MyString
{
 final String str;
 MyString(String s)
 {
  this.str = s;
 }
 public String get()
 {
  return str;
 }
}

In this example MyString is an immutable class. MyString's state cannot be changed once it is created.

How to create String object?

There are two ways to create String object:

1. By string literal
2. Using another String object
3. By new keyword

1) String Literal

The most direct way to create a string is to write:

String strng = "Hello world!";

In this case, "Hello world!" is a string literal—a series of characters in your code that is enclosed in double quotes. Whenever it encounters a string literal in your code, the compiler creates a String object with its value—in 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 thirteen constructors that allow you to provide the initial value of the string using different sources, such as an array of characters:

char[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.' };
String helloString = new String(helloArray);
System.out.println(helloString);

The last line of this code snippet displays hello.

Note: String objects are stored in a special memory area known as string constant pool.

2) Using another String object

 String str2 = new String(str1);
 

2) Using new Keyword

 String s1=new String("HelloWorld");
 //creates two objects and one reference variable  

In such case, JVM will create a new string object in normal(non pool) heap memory and the literal "Welcome" will be placed in the string constant pool. The variable s will refer to the object in heap(non pool).

Java String Example

Program

  public class SampleString{
public static void main(String args[]){

	String s1="Hello";//creating string by java string literal

	char character[]={'a','t','n','y','l','a'};

	String s2=new String(character);//converting char array to string
	String s3=new String("welcome");//creating java string by new keyword

	System.out.println(s1);
	System.out.println(s2);
	System.out.println(s3);
	}
}
  

Program

  Hello
atnyla
welcome
Press any key to continue . . .
  

Methods used to obtain information about an object are known as accessor methods. One accessor method that you can use with strings is the length() method, which returns the number of characters contained in the string object. After the following two lines of code have been executed, len equals 17:

String palindrome = "Dot saw I was Tod";
int len = palindrome.length();

The following program is an example of length(), method String class.

Program

public class SampleString {

   public static void main(String args[]) {
      String strng = "Man can't do everything";
      int len = strng.length();
      System.out.println( "String Length is : " + len );
   }
}
  

Program

String Length is : 23
Press any key to continue . . .
  

A palindrome is a word or sentence that is symmetric—it is spelled the same forward and backward, ignoring case and punctuation. Here is a short and inefficient program to reverse a palindrome string. It invokes the String method charAt(i), which returns the i^th character in the string, counting from 0.

public class StringDemo {
    public static void main(String[] args) {
        String palindrome = "Dot saw I was Tod";
        int len = palindrome.length();
        char[] tempCharArray = new char[len];
        char[] charArray = new char[len];
        
        // put original string in an 
        // array of chars
        for (int i = 0; i < len; i++) {
            tempCharArray[i] = 
                palindrome.charAt(i);
        } 
        
        // reverse array of chars
        for (int j = 0; j < len; j++) {
            charArray[j] =
                tempCharArray[len - 1 - j];
        }
        
        String reversePalindrome =
            new String(charArray);
        System.out.println(reversePalindrome);
    }
}

Running the program produces this output:

doT saw I was toD

To accomplish the string reversal, the program had to convert the string to an array of characters (first for loop), reverse the array into a second array (second for loop), and then convert back to a string. The String class includes a method, getChars(), to convert a string, or a portion of a string, into an array of characters so we could replace the first for loop in the program above with

palindrome.getChars(0, len, tempCharArray, 0);

Concatenating Strings

The String class includes a method for concatenating two strings:

string1.concat(string2); 

This returns a new string that is string1 with string2 added to it at the end.

You can also use the concat() method with string literals, as in:

"My name is ".concat("ILoveYou");

Strings are more commonly concatenated with the + operator, as in

"World," + " Brotherhood" + "!"

which results in

"World, Brotherhoood!"

The + operator is widely used in print statements. For example:

String string1 = "you are man";
System.out.println("I," + string1 + " wow!");

which prints

I,you are man wow!

Such a concatenation can be a mixture of any objects. For each object that is not a String, its toString() method is called to convert it to a String.

String quote = 
    "Now is the time for all good " +
    "men to come to the aid of their country.";

Example of Concatenating Strings

public class SampleString {

   public static void main(String args[]) {
      String string1 = "I Hate You";
      System.out.println("Hey, " + string1 + "Girls");
   }
}
  

Program

Hey, I Hate YouGirls
Press any key to continue . . .
  

Creating Format Strings

You have seen the use of the printf() and format() methods to print output with formatted numbers. The String class has an equivalent class method, format(), that returns a Stringobject rather than a PrintStream object.

Using String's static format() method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead of

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);

String Methods

Here is the list of methods supported by String class ?