File Input - Introduduction to Jave Programming - Lecture Slides, Slides of Java Programming

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File Input and Output

Highlights

• text file output
• text file input

Streams

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“Opening” a file

File output is very similar to terminal output, except we have to open and close files To create a stream between a variable name and file name: Type Variable name PrintWriter constructor Another constructor File nameDocsity.com

Writing to a file

After you have opened a file (stream), you can then write to it This is done in an almost identical manner with println() (or print() or printf()) Terminal: File:

File output imports

You need to import PrintWriter and FileOutputStream from java.io You can also simply import everything from java.io by doing: (See: HelloWorldFile.java)

File I/O exceptions

Whenever you open a file (for write or read), a checked exception is thrown named java.io.FileNotFoundException This means you have to catch it! (See: HelloWorldFileCorrected.java)

Appending to files

What happens if I run HelloWorldFile multiple times? Open file and override: Open file and append: (See: HelloWorldFileAppend.java) Docsity.com

File writing overview

• You need to open a file before writng to it
• You should close the file when you are done
• You need to catch FileNotFoundException
• You can either override or append to files
• You cannot go backwards and “replace” or “undo”
• You cannot “preppend” to a file (must either append from end or override)

Scanner setup

Open: Read: Close: FileInputStream not FileOutputStream (See: HelloWorldReader.java)

End of file (EOF)

What would happen if I tried to have two nextLine() calls in HelloWorldReader? This would in fact crash the program, very similar to if you used nextInt() on a String There is no way for Scanner to tell if it reached the end of the file (EOF)

BufferedReader

BufferedReader can also read from a file To open a file: FileReader not FileInputStream However, BufferedReader only has 2 methods: read() - reads a single character readLine() - reads a line, like nextLine()

EOF in BufferedReader

If the end of file is reached in BufferedReader, then read() will return - (readLine() will return null) Since read() only reads a single character, you have to put together Strings/ints manually (See: BufferedReaderMaxBad) (See: BufferedReaderMaxGood)