java StringWriter使用方法

import java.io.StringWriter;
import java.io.Reader;
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class tmp
{
        public static void main(String args[]) throws Exception
        {
                BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
                StringWriter writer = new StringWriter();

                while (true)
                {
                        String str = reader.readLine();
                        if (str == null)
                                break;
                        else
                                writer.write(str);
                }

                String result = writer.toString();
                System.out.println(result);

                reader.close();
                writer.close();
        }
}