Question

Write a Java program to accept one parameter on the command line. If there are no command line arguments entered, the program should print the error message and exit. The program should check whether the input is a directory. And also display the names of files in the directory.

Short answer

The program checks if a command line argument is given, verifies it's a directory, and lists its files.

Step-by-step solution

Import Required Classes

Begin by importing necessary Java classes to help in file handling. Use `java.io.File` to check the directory and display file names.

Main Method Definition

Define the `main` method which takes `String[] args` as a parameter to handle command line arguments.

Check Command Line Arguments

In the `main` method, check if `args.length` is zero. If true, print an error message like "No command line arguments provided." and use `System.exit(1)` to exit the program.

Accept and Validate Input Parameter

Declare a `File` object using `args[0]`. Use `File.isDirectory()` method to check if the input is a valid directory. If it's not a directory, print "Not a valid directory" and exit.

Retrieve and Display File Names

If the directory is valid, use `listFiles()` method from the `File` class to get an array of `File` objects. Loop through this array and print each file name.

Implement Error Handling

Wrap file handling logic within a try-catch block to handle any potential `SecurityException` that might occur when accessing files.

Key concepts

Command Line Arguments

Command line arguments are inputs given to a program when it is executed. In Java, these inputs are passed as a `String` array to the `main` method. For example, `public static void main(String[] args)`. Each element in the `args` array is a command line argument.
This is a very flexible system of input as it allows you to change input each time you run your program without modifying the code.
• To check if any arguments are provided, use `args.length`. If it is zero, this indicates no arguments were given.
• The first argument can be accessed with `args[0]`, the second with `args[1]`, and so on.
It's important to handle cases where the expected arguments aren't provided, to prevent your program from crashing or misbehaving.

File Handling

In Java, file handling involves reading from and writing to files using the API provided by the `java.io` package. The `File` class is central to this process.
To create a `File` object, you initialize it with a path string: `File file = new File("path/to/directory")`. This does not create a new file or directory; it simply represents a path in the file system.

• The `isDirectory()` method checks if the path is a directory.


• `listFiles()` returns an array of `File` objects in the directory. You can then iterate over this array to read file names or access file details.

File handling is a basic yet powerful feature allowing for directory traversal, file creation, deletion, and more advanced file operations.

Directory Validation

Before performing any operations on directories, validating them is crucial to avoid errors. You need to ensure the provided path actually exists and is recognized as a directory.
Using the Java `File` class, you can perform these checks efficiently:

• `File.exists()` verifies whether the path exists in the file system.


• `File.isDirectory()` confirms the path is a directory, not a file.

If these checks fail, it's best to print an error message and terminate the program or request a valid input. This proactive approach prevents exceptions and maintains program stability.

Error Handling

Robust error handling is indispensable to creating resilient applications. In Java, the `try-catch` construct is used extensively for handling exceptions that may occur during program execution.
• Use a `try` block to encapsulate the code segment where exceptions might be thrown.
• Follow it with `catch` blocks to handle specific exceptions like `SecurityException` or `IOException`.
For our directory and file handling task, incorporating error handling ensures that the application doesn't fail abruptly due to unexpected issues. Additionally, providing meaningful error messages guides users to correct the inputs or handle problems effectively.