Syntax for command line arguments in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java program with command line arguments?

Consider the following Java program. What will it print if run with arguments apple banana?

Java

public class Main {
    public static void main(String[] args) {
        System.out.println(args[0] + " and " + args[1]);
    }
}

A apple and banana

B args[0] and args[1]

C applebanana

D ArrayIndexOutOfBoundsException

💡 Hint

Remember that args holds the command line arguments as strings in order.

The args array contains the command line arguments. args[0] is "apple" and args[1] is "banana". The program prints them joined by " and ".

❓ Predict Output

intermediate

What happens if no command line arguments are passed?

What will this Java program print if run without any command line arguments?

Java

public class Main {
    public static void main(String[] args) {
        if (args.length == 0) {
            System.out.println("No arguments");
        } else {
            System.out.println(args[0]);
        }
    }
}

A No arguments

B null

C ArrayIndexOutOfBoundsException

D args[0]

💡 Hint

Check the length of args before accessing elements.

If no arguments are passed, args.length is 0, so the program prints "No arguments".

❓ Predict Output

advanced

What is the output when printing all command line arguments?

What will this Java program print if run with arguments one two three?

Java

public class Main {
    public static void main(String[] args) {
        for (String arg : args) {
            System.out.print(arg + "-");
        }
    }
}

A -eerht-owt-eno

B one-two-three-

C one two three

D one-two-three

💡 Hint

Notice the use of System.out.print inside the loop.

The loop prints each argument followed by a dash without a newline. So the output is "one-two-three-" all on one line.

❓ Predict Output

advanced

What error occurs when accessing args without checking length?

What error will this Java program throw if run without any command line arguments?

Java

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

A Compilation error

B NullPointerException

C No output, program ends silently

D ArrayIndexOutOfBoundsException

💡 Hint

Accessing an array element that does not exist causes an exception.

Since args is empty, accessing args[0] causes an ArrayIndexOutOfBoundsException.

🧠 Conceptual

expert

How to correctly parse integer command line arguments in Java?

You want to read two integer numbers from command line arguments and add them. Which code snippet correctly does this?

A int a = Integer.valueOf(args[0]); int b = Integer.valueOf(args[1]); System.out.println(a + b);

B int a = (int) args[0]; int b = (int) args[1]; System.out.println(a + b);

C int a = Integer.parseInt(args[0]); int b = Integer.parseInt(args[1]); System.out.println(a + b);

D int a = args[0]; int b = args[1]; System.out.println(a + b);

💡 Hint

Remember to convert strings to integers properly.

Integer.parseInt converts a string to a primitive int. Casting strings directly or assigning strings to int variables causes errors. Integer.valueOf returns an Integer object, not int, so option C is invalid without unboxing.

Practice

1. What is the correct way to declare the main method to accept command line arguments in Java?

easy

A. public void main(String args)

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

C. static void main(String args[])

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

Solution

1. Step 1: Understand 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 exactly. Others either miss static, have wrong return type, or wrong parameter type.

3. Final Answer:

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

4. Quick Check:

   Main method signature = public static void main(String[] args) [OK]

Hint: Main method always: public static void main(String[] args) [OK]

Common Mistakes:

• Omitting static keyword
• Using wrong parameter type
• Changing return type from void

2. Which of the following is the correct way to access the first command line argument inside the main method?

easy

A. args[0]

B. args.first()

C. args.get(0)

D. args(0)

Solution

1. Step 1: Recall how arrays are accessed in Java

   Java arrays use square brackets [] with zero-based indexing to access elements.

2. Step 2: Evaluate options

   Only args[0] uses correct syntax. args(0) and args.get(0) are invalid for arrays, and args.first() is not a method for arrays.

3. Final Answer:

   args[0] -> Option A

4. Quick Check:

   Array access uses square brackets = args[0] [OK]

Hint: Use square brackets to access array elements: args[0] [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Trying to call methods on array
• Confusing with List syntax

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

public class Program {
    public static void main(String[] args) {
        System.out.println(args.length);
        System.out.println(args[0]);
        System.out.println(args[1]);
    }
}

medium

A. 2
Hello
World

B. 3
Hello
World

C. 2
World
Hello

D. 3
World
Hello

Solution

1. Step 1: Count command line arguments

   Arguments are "Hello" and "World", so args.length is 2.

2. Step 2: Check printed values

   args[0] is "Hello", args[1] is "World". So output lines are 2, Hello, World.

3. Final Answer:

   2 Hello World -> Option A

4. Quick Check:

   args.length=2, args[0]=Hello, args[1]=World [OK]

Hint: args.length counts arguments; args[0] is first argument [OK]

Common Mistakes:

• Counting program name as argument
• Swapping args[0] and args[1]
• Assuming args.length includes program name

4. Identify the error in this Java program that tries to print all command line arguments:

public class Test {
    public static void main(String[] args) {
        for(int i=1; i<=args.length; i++) {
            System.out.println(args[i]);
        }
    }
}

medium

A. System.out.println should use args(i) instead of args[i]

B. args should be declared as int[] not String[]

C. Loop starts at 1 and uses <= causing ArrayIndexOutOfBoundsException

D. main method must return int, not void

Solution

1. Step 1: Analyze loop indexing

   The loop starts at i=1 and runs while i <= args.length. Array indices go from 0 to args.length-1.

2. Step 2: Identify error cause

   Accessing args[args.length] causes ArrayIndexOutOfBoundsException. Also, skipping args[0] misses first argument.

3. Final Answer:

   Loop starts at 1 and uses <= causing ArrayIndexOutOfBoundsException -> Option C

4. Quick Check:

   Array indices 0 to length-1, loop must use i=0 to i<length [OK]

Hint: Loop from 0 to args.length-1 to avoid index errors [OK]

Common Mistakes:

• Starting loop at 1 instead of 0
• Using <= instead of < in loop condition
• Using parentheses instead of brackets for array access

5. You want to write a Java program that sums two integers passed as command line arguments and prints the result. Which code snippet correctly parses and sums the arguments?

hard

A. int sum = Integer.parseInt(args[0]) + args[1].parseInt(); System.out.println(sum);

B. int sum = args[0] + args[1]; System.out.println(sum);

C. int sum = Integer.valueOf(args[0]) + args[1]; System.out.println(sum);

D. int sum = Integer.parseInt(args[0]) + Integer.parseInt(args[1]); System.out.println(sum);

Solution

1. Step 1: Convert String arguments to integers

   Use Integer.parseInt() to convert each String argument to int.

2. Step 2: Sum the parsed integers and print

   Sum the two parsed ints and print the result using System.out.println.

3. Final Answer:

   int sum = Integer.parseInt(args[0]) + Integer.parseInt(args[1]); System.out.println(sum); -> Option D

4. Quick Check:

   Parse strings to int with Integer.parseInt() before adding [OK]

Hint: Parse args with Integer.parseInt() before arithmetic [OK]

Common Mistakes:

• Adding strings directly without parsing
• Using wrong method to parse integers
• Calling parseInt() on string variable incorrectly