IO流简单示意图：
java.io.* 下面重点掌握这几个
FileInputStream
FileOutputStream
FileReader
FileWriter
BufferedReader
BufferedWriter
BufferedInputStream
BufferedOutputStream
DataInputStream
DataOutputStream
ObjectInputStream
ObjectOutputStream
InputStreamReader 转换流
OutputStreamWriter 转换流
PrintWriter
PrintStream
java语言中的流分为：四大家族（InputStream, OutputStream, Reader, Writer）