cs 200 - programming i: file input / output · cs200-programmingi:fileinput/output marcrenault...

CS 200 - Programming I: File Input / Output

Marc Renault

Department of Computer SciencesUniversity of Wisconsin – Madison

Fall 2019TopHat Sec 3 (1:20 PM) Join Code: 682357TopHat Sec 4 (3:30 PM) Join Code: 296444

Output Formatting Streams Paths File I/O

Output Formatting


Output / String Formatting

String.format(String format, Object... args)System.out.format(String format, Object... args)System.out.printf(String format, Object... args)

String formatThe format string.A template string with placeholders (format specifiers) forthe arguments.

Object... argsVarargs – Variadic or variable arity method.Allows any number of objects.

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Format SpecifiersFormat Conversion Specification

%[arg_idx$][flags][width][.precision]type

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arg_idx$The position of the argument in the argument list (1-based).

flags- Left-align the output.+ Prepend a “+” for positive numeric types.

(space) prepend a space for positive numeric types.0 Prepend with zeros (requires the width option).( Negative numbers enclosed in parentheses., Locale-specific grouping separators.

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widthMinimum number of characters.

.precisionFor floats: number of digits to the right of the decimal.For strings: maximum number of characters to output.Rest: Not applicable, causes exception.

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typeb,B A boolean.s,S A string.c,C A char.d integer as a decimal integer.o integer as a octal integer.

x,X integer as a hexadecimal integer.

Note: Upper-case conversion converts argument to upper-case.

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typef float in decimal notation: [-]ddd.ddd.

e,E float in scientific notation: [-]d.ddde±dd.a,A float in hexadecimal notation.g,G float in f or e (resp. E), depending on magnitude.% Prints a literal “%”.n Platform specific line separator.

Note: Upper-case conversion converts argument to upper-case.

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TopHat Question 1

What is the output?

System.out.printf("%B %d 0x%08x",true , 1, 15);

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TopHat Question 2

What is the output?

System.out.printf("%4$c %3$C %2c %1$C",’a’, ’b’, ’c’, ’d’);

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TopHat Question 3

What is the output?

System.out.printf("%4$C %c %3$C %c %2$C %c %1$C %c",’a’, ’b’, ’c’, ’d’);

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Streams


Standard Output StreamSystem.out.print("Hello\nWorld!");

Buffer:

Console:

Console:HelloConsole:HelloWorld!

System.outReference to PrintStream object.Default behaviour is to output to console.Buffered output:

Print methods adds data to buffer.Buffered data is output periodically.

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H e l l o \n W o r l d !

Buffer:

Console:

Console:HelloConsole:HelloWorld!



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W o r l d !

Buffer:

Console:

Console:Hello

Console:HelloWorld!



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Buffer:

Console:Console:Hello

Console:HelloWorld!



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Standard Input StreamScanner sc = new Scanner(System.in);String s = sc.next ();

Buffer:

Console:

Console:Hello World!

System.inReference to InputStream object.Default behaviour is to input from the console.Buffered input:

Input enters a buffer.Buffered data is read periodically.

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H e l l o W o r l d !

Buffer:

Console:




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W o r l d !

Buffer:

Console:




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Paths


Navigating the File System/

homerenault

workspace

Current Working Directory (CWD)Current location in the file system.Denoted by: .The directory from which the jvmwas launched.

Relative PathA path relative to the CWD.To move up: ..

Absolute PathA path that starts at the root of thefile system.Root: ’/’

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Navigating the File System/home/renault/workspace




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CS200ExbinData

InOut

srcPalindrome

binsrc

OrdinalExbinsrc

CmdLinebinsrc




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CS200ExbinData

InOut

srcPalindrome

binsrc

OrdinalExbinsrc

CmdLinebinsrc



Absolute PathA path that starts at the root of thefile system.Root: ’/’ 8/22



CS200ExbinData

InOut

srcPalindrome

binsrc

OrdinalExbinsrc

CmdLinebinsrc

TopHat Question 4Is “./Palindrome/src” an absolute or arelative path?

TopHat Question 5Is “/home/renault/workspace/CmdLine”an absolute or a relative path?

TopHat Question 6If the CWD is/home/renault/workspace/CS200Ex/bin,what is the (shortest) relative path to In?

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File I/O


File Classjava.io.File

ConstructorFile f = new File(pathToFile)

Creates a File object, but not an actual file on the disc.pathToFile – Path to file.All the instance methods are base on this path.

Permissionsboolean canRead() – Can read the contents?boolean canWrite() – Can change the contents?boolean canExecute() – Can execute the path?There are also methods to set the permissions.

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File Testsboolean exists() – Does the file exist?boolean isFile() – Is it a normal file?boolean isDirectory() – Is it a directory?boolean isHidden() – Is it a directory?

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Other File Propertieslong lastModified() – Date last modified.long length() – Number of bytes.String getPath() – Returns the path.String getAbsolutePath() – Returns the absolute path.String getParent() – Returns the parent from the path.

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Basic Operationsboolean createNewFile() – Create an empty file.boolean renameTo(File dest) – Move to dest.boolean delete() – Delete the path.void deleteOnExit() – Delete the path when the JVMterminates.

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File Class

java.io.File



Directory Operationsboolean mkdir() – Create director based on path.boolean mkdirs() – Create all the directories in the path.String[] list() – List files contained in directory.

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Eclipse CWDTopHat Question 7

What is the CWD in this case?This code is in an Eclipse project called CS200Example:import java.io.File;

public class FileInfo {

public static void main(String [] arg){File f = new File(".");System.out.println(f.getPath ());System.out.println(f.getAbsolutePath ());

}}

When run, the output is:./home/renault/workspace/CS200Examples/.

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Creating FilesTopHat Question 8

What is the output, assuming proper permissions andthat the files don’t yet exist?import java.io.File;import java.io.IOException;

public class CreateFile{public static void main(String [] arg){

File f = new File("tmp.txt");File f2 = new File("/home/renault/tmp2.txt");File f3 = new File("/home/renault/tmp2.txt");try{

if(!f.createNewFile ()){System.out.print("f exists.");

}if(!f2.createNewFile ()){

System.out.print("f2 exists.");}if(!f3.createNewFile ()){

System.out.print("f3 exists.");}

}catch(IOException e){

System.out.print("Exception!");}

}}

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Deleting FilesTopHat Question 9

What is the output, assuming proper permissions andthat the files don’t yet exist?import java.io.File;

public class DeleteFile {

public static void main(String [] args){File f = new File("tmp.txt");File f2 = new File("t.txt");try{

f.createNewFile ();if(f.delete ()){

System.out.println("f deleted.");}if(f2.delete ()){

System.out.println("f2 deleted.");}

}catch(Exception e){

System.out.println("Exception!");}

}}

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Listing FilesTopHat Question 10

How would you run the following code from thecommand line so that it prints out the contents of thecurrent directory?

import java.io.File;

public class FileListing {

public static void main(String [] args) {File f = new File(args [0]);for (String s: f.list ()) {

System.out.println(s);}

}

}

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File InputIn Java, there are many ways to read a file. You can...

Start with...java.io.File

java.io.FileReader

java.io.FileInputStream

Wrap it in...java.util.Scanner

java.io.Readerwrapped in a java.io.BufferedReader

Exercise: Write a simple cat.Write a program that prints out every line from the filenamepassed in on the command line.

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Simple cat (one file) with Scanner

import java.io.File;import java.io.FileNotFoundException;import java.util.Scanner;

public class CatScannerFile {

public static void main(String [] args) {if(args.length >= 1) {

Scanner s = null;try {

File f = new File(args [0]);s = new Scanner(f);while(s.hasNextLine ()) {

System.out.println(s.nextLine ());}

}catch(FileNotFoundException e) {

System.out.println("File not found: " + args [0]);}finally {

if(s != null)s.close ();

}}

}}

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Simple cat (one file) with BufferedReader

import java.io.FileReader;import java.io.FileNotFoundException;import java.io.BufferedReader;import java.io.IOException;

public class CatBuffReadFile {

public static void main(String [] args) throws IOException {if(args.length >= 1) {

BufferedReader br = null;FileReader fr = null;try {

fr = new FileReader(args [0]);br = new BufferedReader(fr);while(br.ready ()) {

System.out.print(br.readLine ());}


System.out.println("File not found: " + args [0]);}finally {

if(br != null)br.close ();

}}

}}

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Basic try with Resources

try ( SomeCloseable1 a = new SomeCloseable1 (...);SomeCloseable2 b = new SomeCloseable2 (...) ) {

tryStmtBlock;}

try-with-resourcesNew in Java 7.try statement in which you declare the resources.The declared resources are closed automatically in theopposite order of their creation.The resources must implement AutoCloseable.

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try-with-resources-catch-finallyTopHat Question 11

If an exception is thrown in the try statement block whathappens in what order?

try ( SomeCloseable1 a = new SomeCloseable1 (...);SomeCloseable2 b = new SomeCloseable2 (...) ) {

tryStmtBlock;}catch (Exception e) {

catchStmtBlock;}finally {

finallyStmtBlock;}

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Simple cat (one file) with Scanner andtry-with-resources

import java.io.File;import java.io.FileNotFoundException;import java.util.Scanner;

public class CatScannerFileTryRes {

public static void main(String [] args) {if(args.length >= 1) {

try (Scanner s = new Scanner(new File(args [0]))) {while(s.hasNextLine ()) {

System.out.println(s.nextLine ());}


System.out.println("File not found: " + args [0]);}

}}

}

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File OutputIn Java, there are many ways to write to a file. You can...


java.io.FileWriter

java.io.FileOutputStream

Wrap it in...java.io.PrintWriter

java.io.Writerwrapped in a java.io.BufferedWriter

Some ExampleszyBooks has examples of FileOutputStreamwrapped in aPrintWriter.

Let’s consider File wrapped in a PrintWriter.

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File OutputIn Java, there are many ways to write to a file. You can...


java.io.FileWriter

java.io.FileOutputStream

Wrap it in...java.io.PrintWriter

java.io.Writerwrapped in a java.io.BufferedWriter

Some ExampleszyBooks has examples of FileOutputStreamwrapped in aPrintWriter.Let’s consider File wrapped in a PrintWriter.

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Output ExampleTopHat Question 12

How many lines are written to test.txt when the code isexecuted with java FileOutput 10 test.txt 10000?import java.util.Random;import java.io.File;import java.io.PrintWriter;import java.io.FileNotFoundException;

public class FileOutput {public static void main(String [] args) {

if(args.length >= 3) {int numVals = Integer.parseInt(args [0]);int modCols = numVals % 4;numVals = numVals + (modCols != 0 ? 4 - modCols : 0);try (PrintWriter pout = new PrintWriter(new File(args [1]))) {

Random rand = new Random ();for(int i = 0; i < numVals; i += 4)

pout.printf("%8d\t%8d\t%8d\t%8d\n",rand.nextInt(Integer.parseInt(args [2])),rand.nextInt(Integer.parseInt(args [2])),rand.nextInt(Integer.parseInt(args [2])),rand.nextInt(Integer.parseInt(args [2])));


System.out.println("Unable to open output file: " + args [1]);}

}}

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Further Reading

COMP SCI 200: Programming IzyBooks.com, 2015.zyBook code:WISCCOMPSCI200Fall2019

Chapter 11. File Input / Output

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Appendix References

Appendix

Appendix References

References

Appendix References

Image Sources I

https://brand.wisc.edu/web/logos/

http://www.zybooks.com/

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https://brand.wisc.edu/web/logos/

http://www.zybooks.com/

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