String, StringBuilder and StringBuffer

String

• Strings are objects like other objects.
• They can create instances and they can have constructors and methods.
• String is final class, so they can't be extended and overridden.
• Strings are immutable objects.
• They can be assigned to String variables.
• They can be used as parameters in methods and constructors.

Ways of create Strings

First way: String s = "apple";

•     In this situation String object is 'apple'. 's' is the reference variable to refer 'apple'. 
•        Created one object and one reference variable "apple" will be placed at the heap area(pool) and "s" refers it.

Second way: String s = new String ("apple");

•         In this way it creates two objects. Because we have used new keyword.

•     But both are not placed at the pool. "apple" will be placed at the pool while another object(new String) stay in the normal memory. 

String Literal Pool 

•    String literal pool is a section of heap in the memory. This is pool of Strings which is maintained by JVM(Java Virtual Machine) to store Strings.

•      Once you create a String, JVM checks it already exists in the pool.

o   If it exist, a reference to the instance in the pool returns.

o   If it not exists a new String object will be created

 Advantages of immutability 

• It helps to reduce memory usage(Allowing identical values to be combined together and referenced from various locations in your codes)
• It simplifies multi-threaded programming(Once you initialized, the value cannot be changed)
• Safe to work(There is no problem with alternatives)  
• It simplifies the design and implementation(Re-usability of a String )

Disadvantages of immutability 

•        Less efficient (It creates a heavy garbage collection)

String	StringBuilder	StringBuffer
immutable	mutable	mutable
Synchronized	Not synchronized	Synchronized
Thread safe	Not thread safe	Thread safe
-	Faster than string buffer	Slower than string builder
Java.lang.String	java.lang.StringBuilder	java.lang.StringBuffer
Use more memory	use a memory in efficient way	use a memory in efficient way
charAt(), concat(), equalsIgnoreCase(), length(), replace(), substring(), toLowerCase(), toString(), toUpperCase(), trim()	append(),replace(), insert(), delete(), reverse()	Same methods in string builder