Syntax for command line arguments in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the syntax to access command line arguments in a Java program?

Command line arguments are accessed through the String[] args parameter in the main method, like public static void main(String[] args).

beginner

How do you run a Java program with command line arguments?

Use the command java ClassName arg1 arg2 where arg1, arg2 are the arguments passed to the program.

beginner

How can you access the first command line argument inside the Java program?

Use args[0] to get the first argument passed from the command line.

intermediate

What happens if you try to access a command line argument that was not provided?

Accessing an argument index that does not exist causes an ArrayIndexOutOfBoundsException error.

beginner

Why is the command line argument array declared as String[] args?

Because all command line arguments are passed as strings, even if they represent numbers or other data types.

What is the correct signature of the main method to receive command line arguments in Java?

A public static int main(String[] args)

B public void main(String args)

C static void main(String args[])

D public static void main(String[] args)

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

How do you run a Java program named MyApp with two arguments: hello and 123?

A java MyApp "hello 123"

B java MyApp hello 123

C java MyApp -hello -123

D java MyApp args hello 123

Arguments are passed separated by spaces after the class name.

What type are command line arguments inside the Java program?

A String

B int

C Object

D char

All command line arguments are passed as strings.

What happens if you try to access args[2] but only two arguments were passed?

A Program runs normally

B NullPointerException

C ArrayIndexOutOfBoundsException

D Compilation error

Accessing an index outside the array length throws ArrayIndexOutOfBoundsException.

Which of these is NOT a valid way to declare the main method to accept command line arguments?

A public static void main(String args)

B public static void main(String args[])

C public static void main(String... args)

D public static void main(String[] args)

The parameter must be an array or varargs of String, not a single String.

Explain how command line arguments are passed to and accessed in a Java program.

Think about the main method and how you get the arguments inside your code.

Describe what happens if you try to access a command line argument index that was not provided.

What error occurs when you go beyond the array size?

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