Useful File java.io Mini API

Java io is a very extensive library. There are numerous ways and combinations to write/read data on a file. This article summaries some very useful classes in java.io package.

Here is a table from OCP Java SE 8 Programmer II Exam Guide (Exam 1Z0-809) that summarizes the basic classes in file io.

java.io Class	Extends From	Key Constructor(s)	Key Methods
File	Object	File, String String String, String	createNewFile() delete() exists() isDirectory() isFile() list() mkdir() renameTo()
FileWriter	Writer	File String	close() flush() write()
BufferedWriter	Writer	Writer	close() flush() newLine() write()
PrintWriter	Writer	File String OuputStream Writer	close() flush() format() printf() print() println() write()
FileOutputStream	OutputStream	File String	close() write()
FileReader	Reader	File String	read()
BufferedReader	Reader	Reader	read() readLine()
FileInputStream	InputStream	File String	read() close()

Examples

import java.io.*;

class Writer1 {
	public static void main(String... args) {
		try {
			boolean newFile = false;
			File file = new File("fileWrite1.txt");
			System.out.println(file.exists());
			newFile = file.createNewFile();
			System.out.println(newFile);
			System.out.println(file.exists());
		} catch(IOException e) {}

	}
}

class Writer2 {
	public static void main(String... args) {
		char[] in = new char[50];
		int size = 0;
		try {
			File file = new File("fileWrite2.txt");
			FileWriter fw = new FileWriter(file);
			fw.write("howdy\nfolks\n");
			fw.flush();
			fw.close();
			FileReader fr = new FileReader(file);
			size = fr.read(in);
			System.out.print(size + " ");
			for(char c: in) {
				System.out.print(c);
			}
			fr.close();
		} catch(IOException e) { }
	}
}

class Writer3 {
	public static void main(String... args) {
		byte[] in = new byte[50];
		int size = 0;
		File file = new File("fileWrite3.txt");
		try(FileOutputStream outStream = new FileOutputStream(file);
			FileInputStream inStream = new FileInputStream(file)) {
			String s = "howdy\nfolks\n";
			outStream.write(s.getBytes());
            outStream.flush();

			size = inStream.read(in);
			System.out.print(size + " ");
			for(byte b: in) {
				System.out.print((char) b);
			}
		} catch(IOException e) {}
	}
}

class Writer4 {
    public static void main(String... args) {
        File file = new File("fileWrite4.txt");
        try(PrintWriter pw = new PrintWriter(file);
            BufferedReader br = new BufferedReader(new FileReader(file))) {
            pw.println("howdy");
            pw.println("folks");
            pw.flush();
            
            String line = null;
            while(true) {
                line = br.readLine();
                if (line == null) break;
                System.out.println(line);
            }

        } catch(IOException e) {}    
    }
}