Reading an image, File Read/Write, Reading all bytes and Copying a file

Reading an image from a file

To load a properties file bundled with your application:

import java.awt.Image;
import javax.imageio.ImageIO;
...
try {
	Image img = ImageIO.read(new File("~/Desktop/cat.png"));
} catch (IOException e) {
	e.printStackTrace();
}

File Read/Write Using FileInputStream/FileOutputStream

Write to a file examplefile.txt:

import java.io.FileOutputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
 
public class CustomFileWriting {
    public static void main(String[] args) {
        String customFilePath = "C:\\examplefile.txt";
        FileOutputStream customFos = null;
 
        try {
            customFos = new FileOutputStream(customFilePath);
            byte[] customBuffer = "This is a custom file content.".getBytes();
            customFos.write(customBuffer, 0, customBuffer.length);
            System.out.println("Content has been written to: " + customFilePath);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (customFos != null)
                    customFos.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

Read from file examplefile.txt:

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
 
public class CustomFileReading {
    public static void main(String[] args) {
        String customFilePath = "C:\\examplefile.txt";
        FileInputStream customFis = null;
 
        try {
            customFis = new FileInputStream(customFilePath);
            int customLength = (int) new File(customFilePath).length();
            byte[] customBuffer = new byte[customLength];
            customFis.read(customBuffer, 0, customLength);
            System.out.println("Content read from " + customFilePath + ":\n" + new String(customBuffer));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (customFis != null)
                    customFis.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

Note, that since Java 1.7 the try-with-resources statement was introduced what made implementation of reading\writing operation much simpler:

Write to a file examplefile.txt:

import java.io.File;
import java.io.FileOutputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
 
public class CustomFileWriting {
    public static void main(String[] args) {
        String customFilePath = "C:\\examplefile.txt";
 
        try (FileOutputStream customFos = new FileOutputStream(customFilePath)) {
            byte[] customBuffer = "This will be written in examplefile.txt".getBytes();
            customFos.write(customBuffer, 0, customBuffer.length);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Read from file examplefile.txt:

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
 
public class CustomFileReading {
    public static void main(String[] args) {
        String customFilePath = "C:\\examplefile.txt";
 
        try (FileInputStream customFis = new FileInputStream(customFilePath)) {
            int customLength = (int) new File(customFilePath).length();
            byte[] customBuffer = new byte[customLength];
            customFis.read(customBuffer, 0, customLength);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Reading all bytes to a byte[]

Java 7 introduced the very useful Files class

Version ≥ Java SE 7
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.Path;
 
Path path = Paths.get("path/to/file");
 
try {
	byte[] data = Files.readAllBytes(path);
} catch(IOException e) {
	e.printStackTrace();
}

Copying a file using Channel

We can use Channel to copy file content faster. To do so, we can use transferTo() method of FileChannel .

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.channels.FileChannel;
 
public class CustomFileCopier {
 
    public static void main(String[] args) {
        File customSourceFile = new File("sourceFile.txt");
        File customDestFile = new File("destinationFile.txt");
        performCopy(customSourceFile, customDestFile);
    }
 
    public static void performCopy(File sourceFile, File destFile) {
        if (!sourceFile.exists() || !destFile.exists()) {
            System.out.println("Source or destination file doesn't exist");
            return;
        }
 
        try (FileChannel sourceChannel = new FileInputStream(sourceFile).getChannel();
             FileChannel destChannel = new FileOutputStream(destFile).getChannel()) {
            sourceChannel.transferTo(0, sourceChannel.size(), destChannel);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Basic Programs

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