Java IO课件.pdf

The Java I/O System Qiuyan Huo Software Engineering Institute qyhuo@mail.xidian.edu.cn Java (SSXDU 2009) Java Input/Output  I/O libraries are hard to design, and everyone can find something to complain about.  Java ’s I/O library is extensive, and it seems like you need to know 100 things before you can start using it.  Today, let ’s learn some things and still do something useful. 2 Java (SSXDU 2009) #1: Why Is Java I/O Hard?  Java is intended to be used on many very different machines, having – different character encodings (ASCII, EBCDIC, 7- 8- or 16- bit…) – different internal numerical representations – different file systems, so different filename pathname conventions – different arrangements for EOL, EOF, etc.  The Java I/O classes have to “stand between”your code and all these different machines and conventions. 3 Java (SSXDU 2009) #2: Java’s Internal Characters  Unicode. 16-bit. Good idea.  So, the primitive type char is 16-bit.  Reading from a file using 8-bit ASCII characters (for example) requires conversion.  Same for writing.  But binary files (e.g., graphics) are “byte-sized”, so there is a primitive type byte.  So Java has two systems to handle the two different requirements.  Both are in java.io, so import this always ! 4 Java (SSXDU