Dinesh on Java

A string in literal terms is a series of characters. Hey, did you say characters, isn’t it a primitive data type in Java. Yes, so in technical terms, the basic Java String is basically an array of characters.

Java String:
String is one of the widely used java classes. It is special in java as it has some special characteristics than a usual java class. Lets explore java String in this article and this page aims to serve you as a single point reference for all your queries on Java String.

Though you might be knowing most of the things, this will help you to recall them and I am sure you will find one or two new things. I recommend you to take your time and read through this completely as java String is a basic building block of your java programs.

Immutable Java String:
Java String is a immutable object. For an immutable object you cannot modify any of its attribute’s values. Once you have created a java String object it cannot be modified to some other object or a different String. References to a java String instance is mutable. There are multiple ways to make an object immutable. Simple and straight forward way is to make all the attributes of that class as final. Java String has all attributes marked as final except hash field.
Java String is final. I am not able to nail down the exact reason behind it. But my guess is, implementers of String didn’t want anybody else to mess with String type and they wanted de-facto definition for all the behaviors of String.

We all know java String is immutable but do we know why java String is immutable? Main reason behind it is for better performance. Creating a copy of existing java String is easier as there is no need to create a new instance but can be easily created by pointing to already existing String. This saves valuable primary memory. Using String as a key for Hashtable guarantees that there will be no need for re hashing because of object change. Using java String in multithreaded environment is safe by itself and we need not take any precautionary measures.

Java String Instantiation
In continuation with above discussion of immutability of java String we shall see how that property is used for instantiating a Sting instance. JVM maintains a memory pool for String. When you create a String, first this memory pool is scanned. If the instance already exists then this new instance is mapped to the already existing instance. If not, a new java String instance is created in the memory pool.

This approach of creating a java String instance is in sync with the immutable property. When you use ‘new’ to instantiate a String, you will force JVM to store this new instance is fresh memory location thus bypassing the memory map scan. Inside a String class what you have got is a char array which holds the characters in the String you create.

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

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

String greeting = "Hello world!";

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 );
   }
}