c_8_1_understandingFilesAndDirectories

🧠 8.1 Understanding Files and Directories

• This chapter focuses on java.io API to interact with files and streams

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🟥 8.1.1 Conceptuazlizing the File System

• A file is a record within a file system which stores data
• A directory is also a record, but it contains files and directories.
• We shall refer to directories as a file record for the rest of this chapter.
• The file system is in charge of reading and writing data within a compute.
• Different OS's have different file systems, we shall see how to connect automatically to thesde
• A path is the string representation of a file or directory
• In most file systems, the hierarchy is represented using strings concatenated with forward slashes

🟥 8.1.2 Introducing the File Class

• The java.io.File class is used to read information about existing files and directories, list contents, create/delete files/directories
• An instance of the File class represents the pathname of a particular file or directory on the file system.
• The File class cannot read or write data within the file directly, but it can be a passed as a reference to stream class to read and write data.

🟡 Creating a File Object

• The File object is often initialised using either the absolute or relative path.
• The seperate character varies from OS's, we can obtain the seperator character using either:

System.getProperty("file.seperator");
System.out.println(java.io.File.separator);
// both print \ on my windows machine

• The following code creates a File object and determines if the path it references exists within the file system:

public class FileSample {
    public static void main(String[] args) {
		File file = new File("\\home\\zoo.txt");
		System.out.println(file.exists()); // false
    }
}

• This example uses an absolute path (which does not existt on my pc)
• So that I can verify that this method does indeed work, I find some code which gets the directory of my OCP project:

System.out.println(System.getProperty("user.dir"));
// C:\Users\shiv.kumar\Documents\Github\OCP-Java-Examples

• I then create a file in the src folder of my project:

• I then created a parent and child class for this file, and ran:

File parent = new File(System.getProperty("user.dir")+"\\src");
File child = new File(parent, "home\\zoo.txt");
System.out.println(child.exists()); // true

🟡 Commonly Used java.io.File Methods

1. exists()
2. getName() - returns name of the file or directory

File parent = new File(System.getProperty("user.dir")+"\\src");
File child = new File(parent, "home\\zoo.txt");
System.out.println(child.getName()); // zoo.txt

3. getAbsolutePath()

System.out.println(child.getAbsolutePath());
// C:\Users\shiv.kumar\Documents\Github\OCP-Java-Examples\src\home\zoo.txt

4. isDirectory() - returns true if file is a directory

System.out.println(child.isDirectory()); // false
System.out.println(parent.isDirectory()); // true

5. isFile() - returns if the file exists

System.out.println(child.isFile()); // true
System.out.println(child.isFile()); // true
File madeup = new File("random.txt");
System.out.println(madeup.isFile()); // false

6. length() - returns number of bytes in the file
7. lastModified() - returns epoch time
8. delete() - deletes the file, can delete directory if empty
9. renameTo(File file)
10. mkdir() - creates directory, returns true if operation is successfully

File newDirectory = new File(parent,"new_directory");
if (!newDirectory.isDirectory())
    newDirectory.mkdir();
System.out.println(newDirectory.exists()); // true
System.out.println(newDirectory.mkdir()); // false as already exists
System.out.println(newDirectory.delete()); // true

11. mkdirs() - creates directory, and parent structures as needed.
12. getParent() - returns path name of the parent directory

System.out.println(child.getParent());
// C:\Users\shiv.kumar\Documents\Github\OCP-Java-Examples\src\home
System.out.println(parent.getParent()); 
// C:\Users\shiv.kumar\Documents\Github\OCP-Java-Examples

12. listFiles() - returns File[] denoting files in the directory

File[] files = child.listFiles();
System.out.println(Arrays.toString(files)); // null
File[] files2 = parent.listFiles();
System.out.println(Arrays.toString(files2)); 
// [C:\Users\shiv.kumar\Documents\Github\OCP-Java-Examples\src\.classpath, ...]