Java.lang.StringBuffer.reverse() is a built-in method for reversing the characters in StringBuffer. This method causes this character sequence to be replaced by the reverse of the sequence.

In Java, the StringBuffer class provides a convenient way to manipulate strings. One of the methods provided by the StringBuffer class is the reverse() method, which is used to reverse the characters in a StringBuffer object. This method returns the reversed version of the current StringBuffer object, making it very useful for applications that require string reversal.

The reverse() method works by swapping the characters in the StringBuffer object. The first character is swapped with the last character, the second character is swapped with the second-to-last character, and so on. This process continues until all the characters have been swapped and the entire string has been reversed.

To use the reverse() method, you simply call it on the StringBuffer object you want to reverse, like this: 

StringBuffer str = new StringBuffer("hello world");
str.reverse();

After this code is executed, the StringBuffer object str will contain the string "dlrow olleh", which is the reversed version of the original string "hello world".

It is worth noting that the reverse() method is only available for the StringBuffer class, and not for the String class. This is because String objects in Java are immutable, meaning that their contents cannot be changed once they are created. StringBuffer objects, on the other hand, are mutable, meaning that their contents can be modified after they are created.