Accessing arguments in Java - Mini Project: Build & Apply

Accessing arguments

📖 Scenario: You are creating a simple Java program that takes input values when it starts. These inputs are called arguments. They are like notes you pass to a friend before they begin a task.

🎯 Goal: Build a Java program that reads the first two arguments given when running the program and prints them out.

📋 What You'll Learn

Create a Java class named ArgumentPrinter

Access the first two command-line arguments using args[0] and args[1]

Print the two arguments on separate lines

💡 Why This Matters

🌍 Real World

Many Java programs take input from users or other programs when they start. These inputs help customize what the program does.

💼 Career

Understanding how to access and use command-line arguments is important for writing flexible Java applications and tools.

1

Create the Java class and main method

Write a Java class called ArgumentPrinter with a main method that accepts a String[] args parameter.

Java

public class ArgumentPrinter {
    public static void main(String[] args) {
        // Your code here
    }
}

Hint

Remember, the main method is the entry point of a Java program and must be public static void main(String[] args).

2

Access the first two arguments

Inside the main method, create two String variables named firstArg and secondArg. Assign them the values of args[0] and args[1] respectively.

Java

public class ArgumentPrinter {
    public static void main(String[] args) {
        // Your code here
    }
}

Hint

Use args[0] for the first argument and args[1] for the second.

3

Print the two arguments

Use System.out.println to print firstArg and secondArg on separate lines inside the main method.

Java

public class ArgumentPrinter {
    public static void main(String[] args) {
        String firstArg = args[0];
        String secondArg = args[1];
        // Your code here
    }
}

Hint

Use System.out.println twice, once for each argument.

4

Run and display the output

Run the program with arguments Hello and World and print the output.

Java

public class ArgumentPrinter {
    public static void main(String[] args) {
        String firstArg = args[0];
        String secondArg = args[1];
        System.out.println(firstArg);
        System.out.println(secondArg);
    }
}

// Run the program with arguments "Hello" and "World"

Hint

When running the program, pass Hello and World as arguments to see them printed.

Practice

1. What is the correct way to access the first command-line argument in a Java program inside the main method?

easy

A. args.first()

B. args[0]

C. args(0)

D. args.get(0)

Solution

1. Step 1: Understand the type of args

   The args parameter in main is an array of strings.

2. Step 2: Access array elements in Java

   Array elements are accessed using square brackets with the index inside, starting at 0.

3. Final Answer:

   args[0] -> Option B

4. Quick Check:

   Array access uses square brackets [OK]

Hint: Use square brackets to access array elements in Java [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Trying to call methods like first() on array
• Using get() which is for lists, not arrays

2. Which of the following is the correct signature of the main method to access command-line arguments in Java?

easy

A. public void main(String[] args)

B. public static int main(String[] args)

C. static public void main(String args)

D. public static void main(String args[])

Solution

1. Step 1: Recall the standard main method signature

   The main method must be public, static, return void, and accept a String array parameter.

2. Step 2: Check each option

   public static void main(String args[]) matches the correct signature. public void main(String[] args) is missing static. static public void main(String args) has wrong parameter type. public static int main(String[] args) returns int which is invalid.

3. Final Answer:

   public static void main(String args[]) -> Option D

4. Quick Check:

   Main method must be public static void with String array [OK]

Hint: Main method must be public static void with String array [OK]

Common Mistakes:

• Omitting static keyword
• Wrong return type
• Using single String instead of String array

3. What will be the output of the following Java program if run with arguments java Program Hello World?

public class Program {
    public static void main(String[] args) {
        System.out.println(args.length);
        System.out.println(args[1]);
    }
}

medium

A. 3
World

B. 2
Hello

C. 2
World

D. 3
Hello

Solution

1. Step 1: Count the command-line arguments

   The program is run with two arguments: "Hello" and "World", so args.length is 2.

2. Step 2: Access the second argument

   args[1] is the second argument, which is "World".

3. Final Answer:

   2 World -> Option C

4. Quick Check:

   args.length=2, args[1]=World [OK]

Hint: args.length counts arguments; args[1] is second argument [OK]

Common Mistakes:

• Counting program name as argument
• Using args[0] as second argument
• Confusing args.length with number of arguments plus one

4. Identify the error in the following Java program that tries to print the first command-line argument:

public class Test {
    public static void main(String[] args) {
        System.out.println(args(0));
    }
}

medium

A. Using parentheses instead of square brackets to access array element

B. Missing static keyword in main method

C. Incorrect class name

D. Missing return statement in main

Solution

1. Step 1: Check how array elements are accessed

   In Java, array elements are accessed using square brackets, not parentheses.

2. Step 2: Identify the error in the code

   The code uses args(0) which is invalid syntax and causes a compile error.

3. Final Answer:

   Using parentheses instead of square brackets to access array element -> Option A

4. Quick Check:

   Array access uses square brackets [OK]

Hint: Use square brackets [] for arrays, not parentheses () [OK]

Common Mistakes:

• Using parentheses for array access
• Confusing method call syntax with array access
• Ignoring compile errors due to wrong syntax

5. You want to write a Java program that prints all command-line arguments separated by commas. Which code snippet correctly does this inside main?

hard

A. for(int i=0; i

B. for(String arg : args) {
System.out.print(arg + ", ");
}

C. int i = 0;
while(i <= args.length) {
System.out.print(args[i] + ", ");
i++;
}

D. for(int i=1; i<=args.length; i++) {
System.out.print(args[i] + ", ");
}

Solution

1. Step 1: Understand the goal

   We want to print all arguments separated by commas without trailing comma.

2. Step 2: Analyze each option

   for(int i=0; i

3. Final Answer:

   for(int i=0; i<args.length; i++) { System.out.print(args[i] + (i < args.length - 1 ? ", " : "")); } -> Option A

4. Quick Check:

   Loop with index and conditional comma [OK]

Hint: Use index loop and add comma only if not last element [OK]

Common Mistakes:

• Adding comma after last argument
• Off-by-one errors in loops
• Accessing array out of bounds