Why is StringBuffer needed in java? i am totally new to java, and was studying about operations on Strings and String methods when i saw another set of methods, which were similar somewhat similar, in section of StringBuffer. I studied about i, but was not able to grasp a clear idea about it.
In practice everybody should always use StringBuilder (and never use StringBuffer; often you don't really need StringBuilder because the compiler takes care of adding a helper to do this for you). The only reason StringBuffer is still around is for fear that taking it away would break some code somewhere. Java takes backward compatibility really seriously.
The difference between StringBuffer and StringBuilder is that the methods of StringBuffer are synchronized. That means each thread trying to call a synchronized method has to acquire the lock on the object, this prevents problems where two threads calling methods on the same object could result in the object's state becoming corrupted. The backing data store within the object is a dynamically-growing array, and if two threads are messing with it at the same time and one changes it out from under the other, bad things happen: exceptions get thrown, or worse, data gets stomped on silently.
It turns out that there is not a real life need for a string-building object that concatenates strings received concurrently from multiple threads, because nobody wants a string constructed in a way that makes it unpredictable what order the substrings show up in. It's overwhelmingly likely that when you want to construct a string (and have determined you do need a builder) that builder will be thread-confined (meaning only one thread can access the builder), so no synchronization is needed.