> Filter streams are streams that filter bytes for some purpose. The basic byte input stream provides a read method that can only be used for reading bytes. If you want to read integers, doubles, or strings, you need a filter class to wrap the byte input stream. Using a filter class enables you to read integers, doubles, and strings instead of bytes and characters. FilterInputStream and FilterOutputStream are the base classes for filtering data. When you need to process primitive numeric types, use DataInputStream and DataOutputStream to filter bytes.
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> 这个类很奇怪，书上说如果需要做不同数据类型的读写，需要该类型。其实情况要复杂得多，有兴趣可以研究其源代码。奇怪的是，这个类型是非抽象类型，但是却不能使用 new 方法创建（看源码可以分析出来）。
