String args

Understanding String[] args

• String[] args is an array of String objects that stores command-line arguments passed to the program when it is executed.

• When you run a Java program from the command line, you can provide additional information (arguments) right after the program name. These arguments are passed to the main method as the args array.

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Example of using String[] args

public class UserInput {
    public static void main(String[] args) {
        if (args.length > 0) {
            System.out.println("Hello " + args[0]);
        } else {
            System.out.println("No command-line arguments provided.");
        }
    }
}

• In this example, if you run the program with a command-line argument like java UserInput Alice, args[0] will contain the value “Alice”, and the output will be Hello Alice.

• If you don’t provide any command-line arguments, the program will output No command-line arguments provided.

Breaking it Down

1. String[] args

   • String[] args is an array of strings that stores the command-line arguments provided when the program is run.
   • If you don’t pass any arguments, this array will be empty (args.length will be 0).

2. args[0]

   • args[0] accesses the first element in the args array.
   • Arrays in Java are zero-indexed, meaning the first element is at index 0, the second at index 1, and so on.

Let’s say you have a Java program named UserInput.java, and it includes the above code. and then run the program with a command-line argument like this : java UserInput Alice

Here’s what happens:

• args array: The args array is populated with the command-line arguments. In this case, args will contain one element: “Alice”.

• args[0]: The value of args[0] is “Alice”. So, System.out.println("Hello " + args[0]); will output: Hello Alice

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Why String[] args is Necessary

• Standard Signature: The Java runtime looks for a method with the signature public static void main(String[] args) to start the program. If this method is not present, the program will not run and will throw an error.
• Command-Line Arguments: The args array allows the program to receive command-line arguments if provided. Even if your program doesn’t need command-line arguments, the signature must remain unchanged.