Java Files & I/O

To perform input and output(I/O) operations in java we use java.io package which contains classes for these operations. Most application needs to read input from a source, process it and generate output depending on input.

InputStream: InPutStream is used when you want to read data from a source.

OutPutStream: OutPutStream is used when you want to write data at some destination.

In following example we are reading a text file and displaying its contents on screen using java.io package classes.

import java.io.*;

public class JavaReadTextFile {

public static void main(String args[]) throws IOException

{

FileInputStream ins = null;

try {

ins = new FileInputStream(“textfile.txt”);

BufferedReader br = new BufferedReader(new InputStreamReader(ins));

String c = null;

while ((c = br.readLine()) != null) {

System.out.println(c);

}

if (br != null) {

br.close();

}

} finally {

if (ins != null) {

ins.close();

}

}

}

}

Output:

The contents of textfile.txt will be displayed on screen line by line.

Java Byte Stream

Byte stream in java is used to perform input and output operations of 8 bit bytes. The following example will copy the contents of a text file to another text file using FileInputStream and FileOutputStream.

import java.io.*;

public class JavaCopyFileContents {

public static void main(String args[]) throws IOException

{

FileInputStream fin = null;

FileOutputStream fout = null;

int ch;

try {

fin = new FileInputStream(“inputtextfile.txt”);

fout = new FileOutputStream(“outputtextfile.txt”);

while ((ch = fin.read()) != -1) {

fout.write(ch);

}

fin.close();

fout.close();

} catch (IOException e){

System.out.println(“Something went wrong”);

}

}

}

Output:

A new file “outputtextfile.txt” will be created with “inputtextfile.txt” contents.

Standard Streams

All programming languages are required to have standard I/O streams so a program can take input form user’s keyboard and display output on user’s screen after processing the input. Java has following standard I/O streams.

System.in: standard input stream

System.out: standard output stream

System.err: standard error stream

Java Directories

A directory is a type of file which can contain other directories and files. In java File object is used to create new directories and list the files and sub-directories. The mkdir() and mkdirs() methods are used to create directories. If parent directories don’t exists the mkdir() method will return false. mkdirs() methods create both the directory and all parent directories.

Creating Directory

import java.io.File;

public class JavaCreateDirectory {

public static void main(String args[]) {

String dirPath = “C:/javaexamples/files”;

File f = new File(dirPath);

f.mkdirs();

}

}

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