关于Java缓冲流的一点误解

public class BufferedTest {
    public static void main(String[] args) {
        InputStream in = null;
        OutputStream out = null;
        try  {
            // 此处为演示效果 将in和out都定义在try块内 防止自动关闭
            in = new BufferedInputStream(new FileInputStream("collect.txt"));
            int count = in.available();
            byte[] bytes = new byte[count]; // 通常大家都认为这是为缓冲流定义的缓冲字节数组
            if (count > 0)
                in.read(bytes);
            
            // 正常输出，好像也没什么问题，完美的缓冲机制
            for (byte b : bytes)
                System.out.print((char)b);
            System.out.println();
            
            // 再来看一下输出流
            out = new BufferedOutputStream(new FileOutputStream("test.txt"));
            out.write(bytes);  // 奇怪。。test.txt文件是空的
            
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public BufferedInputStream(InputStream in, int size) {
        super(in);
        if (size <= 0) {
            throw new IllegalArgumentException("Buffer size <= 0");
        }
        buf = new byte[size];
    }