String, String Buffer and String Builder

[atif aslam]

What all do we actually need to know about String, String Buffer and String Builder API's for SCJP 6.0. Can somebody explain the differences in detail?

[Amit Singla]

I am not sure about in context to the SCJP but following are the major difference

String are Immutable objects which means they are final instances and value can not be changed.

StringBuffer and StringBuilder are better choises to String objects if the you have to make a string on the run. i.e. you have to concatenate
various string objects dynamically.

StringBuffer is thread safe but StringBuilder is not thread safe. So if you have a single thread environment, StringBuilder will give you better
performance over StringBuffer.

[Kieren Dixon]

The exam objectives are listed here
Discuss the differences between the String, StringBuilder, and StringBuffer classes.

As said above, you'll have to understand the concept of mutability/immutability. StringBuilder and StringBuffer are pretty much exactly the same exception StringBuilder's methods are synchronized.

Understand the basic API methods. That you can not compare string using == and need to use .equals(). The concept of the String pool.

[Amit Singla]

exception StringBuilder's methods are synchronized.

It's StringBuffer whose methods are synchronized.

[Pankaj Upadhyay]

I would like to suggest to take care of String Pool memory. That will help you in understanding how JVM utilizes String objects and you will be able to know when String objects are created.

When you will be able to understand String pool memory, understanding StringBuffer and StringBuilder is very easy.

Hope I am clear.

[Stephen Davies]

Indeed StringBuffer is Thread Safe in that its methods are synchronized. However remember using Thread Safe classes is all good and well, but don't rely on them alone for thread safety, make your methods using them synchronized if they can alter the state of your Objects! Further synchronization causes a little more overhead, as the threads are locking and blocking, so if you need to keep your memory footprints down and speed up, consider if you need StringBuffer, or could you use other threading techniques.

[Ravikanth kolli]

one of the most common question that can asked is related to the number of objects at a particular time in a given piece of code.

[Stephen Davies]

StringBuilder and Buffer are identical, However, performance-wise StringBuilder is faster as it it's methods are not synchronized. Also, in comparisson with string literals and String objects, the destructive methods do not require the creation of a new String object on the heap:

For example

String mystring = "My String" 

/*creates a new String object
* on the heap with the value 
*My String and assigns it to 
*the ref mystring.
*/

/*-----------------------------------------------*/
mystring.concat(" is great")

/*creates a new String object 
*on the heap with the value 
*"My String is great" 
*but its lost to gc as there is no 
*reference to it*/

System.out.println(mystring); //Output: My String. Two objects created 1 reference


/*------------------------------------------------*/

-- However if we now look at the StringBuffer / StringBuilder class --

StringBuffer sb = new StringBuffer("abc");
sb.append("def");
System.out.println(sb); // outputs abcdef. One object created one reference!

String pooling is used in the creation of String literals, so if I state:

String a = new String("Hello"); // creates a new string Object with the value "Hello" and asigns it to reference a.
String b = a; // simply assigns a new reference to a:
String c = "Hello"; //goes to the string constant pool and if it finds literal "Hello" if not a new String object is made, assigns it to variable c.
String d = new String("Hello); creates a new String Object with the value Hello and assigned to variable d;

From this we see three String Objects, and 4 references.