Difference between String literal and New String object in Java

String is a special class in Java API and has so many special behaviours which is not obvious

to many programmers. In order to master Java, first step is to master String class, and one 
way to explore is checking what kind of String related questions are asked on Java interviews.
 Apart from usual questions like why String is final, or  equals vs == operator, one of the most
frequently asked question is what is difference between String literal and String object in
 Java.For example, what is the difference between String object created in following two
 expression :

String strObject = new String("Java");

and

String strLiteral = "Java";

Both expression gives you String object, but there is subtle difference between them. When
you create String object using new() operator, it always create a new object in heap
 memory. On theother hand, if you create object using String literal syntax e.g. "Java",
 it may return an existing object from String pool (a cache of String object in Perm gen
 space, which is now moved to heap space in recent Java release), if it's already exists. 
Otherwise it will create a new string object and put in string pool for future re-use. In 
rest of this article, why it is one of the most important thing
 you should remember about String in Java.

What is String literal and String Pool

Since String is one of the most used type in any application, Java designer took a step further
to optimize uses of this class. They know that Strings will not going to be cheap, and that's
 why they come up with an idea to cache all String instances created inside double quotes
 e.g. "Java".These double quoted literal is known as String literal and the cache which 
stored these String instances are known as as String pool. In earlier version of Java, I think 
up-to Java 1.6 String pool is located in permgen area of heap, but in Java 1.7 updates its 
moved to main heap area. Earlier since it was in PermGen space, it was always a risk to 
create too many String object, because its a very limited space, default size 64 MB and used
 to store classmetadata e.g. .class files. Creating too many String literals can cause
 java.lang.OutOfMemory: permgen space. Now because String pool is moved to a much 
larger memory space, it's much more safe. By the way, don't misuse memory
here, always try to minimize temporary String object e.g. "a", "b" and then "ab".
Always use StringBuilder to deal with temporary String object.

Difference between String literal and String object

At high level both are String object, but main difference comes from the point that new()
operator always creates a new String object. Also when you create String using literal they 
are interned. This will be much more clear when you compare two String objects created 
using String literal and new operator, as shown in below example :

String a = "Java";
String b = "Java";
System.out.println(a == b);  // True

Here two different objects are created and they have different references:

String c = new String("Java");
String d = new String("Java");
System.out.println(c == d);  // False

Similarly when you compare a String literal with an String object created using new()
 operator using ==operator, it will return false, as shown below :

String e = "JDK";
String f =  new String("JDK");
System.out.println(e == f);  // False

In general you should use the string literal notation when possible. It is easier to read and
it gives  the compiler a chance to optimize your code. By the way any answer to this 
question is incomplete until you explain what is String interning, so let's see that in
 next section.

String interning using inter() method

Java by default doesn't put all String object into String pool, instead they gives you
flexibility to explicitly store any arbitrary object in String pool. You can put any object
to String pool by calling intern()method of java.lang.String class. Though,
 when you create using String  literal notation of Java, it automatically call intern()
 method to put that object into String pool, provided it was not present in the pool 
already. This is another difference between string literal and  new string, because in 
case of new, interning doesn't happen automatically, until you call intern() method
 on that object. Also don't forget to use StringBuffer and StringBuilder for
string concatenation, they will reduce number


That's all about this question, what is difference between String literal and String object 
in Java. Always remember that literal Strings are returned from string pool and Java put
them in pool if not stored already. This difference is most obvious, when you compare two 
String objects using equality operator (==). That's why it's suggested as always compare 
two String object using equals() method and never compare them using == operator,
 because you never know which one is coming from pool and which one is created using
 new() operator. If you know the difference between string object and string literal, you
 can also solve questions from Java written test, which also test this concept. It's something, 
every Java programmer should know.  of temporary
String object in heap space.