StringReader is a class in Java that allows you to read character streams from a string. It is a subclass of the Reader class and is often used when you need to read data from a string as if it were a character stream.

Here's an example of how to use StringReader to read a string as a character stream:

String data = "Hello, world!";
StringReader reader = new StringReader(data);

int character;
while ((character = reader.read()) != -1) {
    System.out.print((char) character);
}

reader.close();

In this example, we create a new StringReader object and pass it the string "Hello, world!". We then use a while loop and the read() method to read each character from the string and print it to the console. Note that we cast the character to a char before printing it. Finally, we close the StringReader using the close() method.

The StringReader class provides several methods that can be used to read from the character stream:

1. read(): This method reads a single character from the stream and returns it as an integer value.

2. read(char[] cbuf, int off, int len): This method reads characters from the stream into an array, starting at the specified off offset and reading up to len characters.

3. skip(long n): This method skips n characters in the stream and returns the number of characters actually skipped.

4. ready(): This method returns true if the stream is ready to be read, or false otherwise.

5. mark(int readAheadLimit): This method marks the current position in the stream, so that it can be returned to later using the reset() method.

6. reset(): This method returns the stream to the position that was marked using the mark() method.

7. close(): This method closes the stream and releases any system resources associated with it.

Note that many of these methods can throw an IOException if there is an error while reading from the stream. Therefore, it is important to handle exceptions appropriately when using these methods.