The CharSequence interface in Java is used to represent a sequence of characters. It is defined in the java.lang package and is implemented by several classes, including String, StringBuffer, and StringBuilder.

The CharSequence interface defines the following methods:

  1. charAt(int index): returns the character at the specified index in the sequence.
  2. length(): returns the length of the sequence.
  3. subSequence(int start, int end): returns a new CharSequence that is a subsequence of the original sequence, starting at the specified start index and ending at the specified end index.
  4. toString(): returns the sequence as a String.

Here is an example of using the CharSequence interface with a StringBuilder object:

StringBuilder sb = new StringBuilder("Hello, World!");
CharSequence cs = sb;

System.out.println(cs.charAt(0));   // prints 'H'
System.out.println(cs.length());    // prints 13
System.out.println(cs.subSequence(0, 5));  // prints 'Hello'
System.out.println(cs.toString()); // prints 'Hello, World!'

In the example above, we create a StringBuilder object with the initial value of "Hello, World!". We then assign the StringBuilder object to a CharSequence variable named cs. We can then use the CharSequence methods to access the characters in the sequence, get the length of the sequence, get a subsequence of the sequence, and get the sequence as a String.