Command line execution in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Java program with command line arguments

What is the output of this Java program when run with arguments hello world?

Java

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

A

0

B

1
hello world 

C

2
hello
world

D

2
hello world 

💡 Hint

Remember that command line arguments are passed as an array of strings to main.

The program prints the number of arguments, which is 2, then prints each argument separated by a space.

🧠 Conceptual

intermediate

Understanding Java command line argument indexing

If a Java program is run with the command java Program one two three, what is the value of args[1] inside main?

A IndexOutOfBoundsException

B "two"

C "three"

D "one"

💡 Hint

Array indexing in Java starts at zero.

args[0] is "one", args[1] is "two", args[2] is "three".

🔧 Debug

advanced

Identify the error when running Java program with no arguments

What error occurs when running this Java program with no command line arguments?

Java

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

A NoSuchElementException

B NullPointerException

C ArrayIndexOutOfBoundsException

D Compilation error

💡 Hint

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

args is an empty array when no arguments are passed, so args[0] access throws ArrayIndexOutOfBoundsException.

📝 Syntax

advanced

Correct command to compile and run Java program with arguments

Which command correctly compiles and runs a Java program named App.java with command line arguments foo bar?

A javac App.java && java App foo bar

B java App.java foo bar

C javac App.java foo bar && java App

D java App foo bar && javac App.java

💡 Hint

Remember that javac compiles and java runs the class without the .java extension.

Option A compiles first, then runs the class with arguments. Others misuse commands or order.

🚀 Application

expert

Determine output of Java program processing command line arguments

What is the output of this Java program when run with arguments 3 5 2?

Java

public class SumArgs {
    public static void main(String[] args) {
        int sum = 0;
        for (String arg : args) {
            sum += Integer.parseInt(arg);
        }
        System.out.println(sum);
    }
}

A 10

B 352

C 0

D NumberFormatException

💡 Hint

The program converts each argument to a number and adds them.

3 + 5 + 2 equals 10, so the program prints 10.

Practice

1. What command do you use to compile a Java file named Example.java from the command line?

easy

A. javac Example.java

B. java Example.java

C. compile Example.java

D. run Example.java

Solution

1. Step 1: Understand Java compilation

   Java source files (.java) must be compiled into bytecode (.class) before running.

2. Step 2: Use the correct command

   The javac command compiles Java files, so javac Example.java compiles the file.

3. Final Answer:

   javac Example.java -> Option A

4. Quick Check:

   Compile Java file = javac [OK]

Hint: Use 'javac' to compile Java files before running [OK]

Common Mistakes:

• Using 'java' to compile instead of run
• Typing 'compile' or 'run' commands which don't exist
• Forgetting to compile before running

2. Which command correctly runs a compiled Java program with the main class named App?

easy

A. java App

B. javac App

C. run App

D. java App.class

Solution

1. Step 1: Identify how to run Java bytecode

   Compiled Java classes (.class) are run using the java command followed by the class name without extension.

2. Step 2: Choose the correct syntax

   Running java App executes the App.class file. Adding '.class' or using 'javac' is incorrect here.

3. Final Answer:

   java App -> Option A

4. Quick Check:

   Run Java class = java ClassName [OK]

Hint: Run with 'java' plus class name, no file extension [OK]

Common Mistakes:

• Adding '.class' extension when running
• Using 'javac' to run instead of compile
• Typing 'run' command which is invalid

3. Given the commands executed in order:
javac HelloWorld.java
java HelloWorld
What will be the output if HelloWorld.java contains:

public class HelloWorld {
  public static void main(String[] args) {
    System.out.println("Hello, Java!");
  }
}

medium

A. No output

B. HelloWorld

C. Compilation Error

D. Hello, Java!

Solution

1. Step 1: Compile the Java file

   The javac HelloWorld.java command compiles the source code into HelloWorld.class without errors.

2. Step 2: Run the compiled class

   The java HelloWorld command runs the main method, printing the string inside System.out.println.

3. Final Answer:

   Hello, Java! -> Option D

4. Quick Check:

   Print statement output = Hello, Java! [OK]

Hint: Compile first, then run to see printed output [OK]

Common Mistakes:

• Trying to run without compiling first
• Expecting class name printed instead of message
• Confusing compile errors with runtime output

4. You try to run java MyProgram but get the error:
Error: Could not find or load main class MyProgram
What is the most likely cause?

medium

A. You ran java MyProgram.java instead of java MyProgram

B. You forgot to compile with javac MyProgram.java

C. The class name inside the file is different from the file name

D. You used the wrong Java version

Solution

1. Step 1: Understand the error meaning

   This error means Java cannot find the compiled .class file for MyProgram.

2. Step 2: Check compilation step

   If you did not run javac MyProgram.java first, the .class file does not exist, causing this error.

3. Final Answer:

   You forgot to compile with javac MyProgram.java -> Option B

4. Quick Check:

   Compile before run to avoid class not found [OK]

Hint: Always compile before running to avoid class not found errors [OK]

Common Mistakes:

• Running with .java extension in 'java' command
• Ignoring class name and file name mismatch
• Blaming Java version without checking compilation

5. You have two Java files:

File1.java:
public class File1 {
  public static void main(String[] args) {
    System.out.println("File1 running");
  }
}

File2.java:
public class File2 {
  public static void main(String[] args) {
    System.out.println("File2 running");
  }
}

Which sequence of commands correctly compiles and runs File2 from the command line?

hard

A. java File2javac File2.java

B. javac File2.javajava File2.java

C. javac File2.javajava File2

D. javac File1.java File2.javajava File2.java

Solution

1. Step 1: Compile the correct file

   To run File2, you must compile File2.java first using javac File2.java.

2. Step 2: Run the compiled class properly

   Run the program with java File2 (without .java extension). Running before compiling or adding extensions causes errors.

3. Final Answer:

   javac File2.java java File2 -> Option C

4. Quick Check:

   Compile then run target class without extension [OK]

Hint: Compile target file, then run class name without extension [OK]

Common Mistakes:

• Running with .java extension
• Running before compiling
• Compiling unrelated files unnecessarily