java PushbackInputStream和PushbackReader用法

import java.io.PushbackInputStream;
import java.io.PushbackReader;
import java.io.InputStreamReader;

public class tmp
{
        public static void main(String args[]) throws Exception
        {
                PushbackInputStream in = new PushbackInputStream(System.in);
                in.unread('a');
                int ch = in.read();
                System.out.println((char) ch);
                in.close();

                PushbackReader reader = new PushbackReader(new InputStreamReader(System.in));
                reader.unread('b');
                ch = reader.read();
                System.out.println((char) ch);
                reader.close();
        }
}