看文档就很清楚了 in是System类的一个静态域 已经打开并且准备提供输入数据in public static final InputStream in The "standard" input stream. This stream is already open and ready to supply input data. Typically this stream corresponds to keyboard input or another input source specified by the host environment or user.
System.in是java的标准输入流,System.out是java的标准输出流
public final static InputStream in = nullInputStream();
System.in是标准的输入流~
in是System类的一个静态域 已经打开并且准备提供输入数据in
public static final InputStream in
The "standard" input stream. This stream is already open and ready to supply input data. Typically this stream corresponds to keyboard input or another input source specified by the host environment or user.