Command line execution in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the command to compile a Java program named HelloWorld.java?

Use javac HelloWorld.java to compile the Java source file into bytecode.

beginner

How do you run a compiled Java program named HelloWorld from the command line?

Use <code>java HelloWorld</code> to run the compiled Java class file.

beginner

What does the java command expect as input after the command itself?

It expects the name of the class containing the <code>main</code> method to start execution.

intermediate

How can you pass arguments to a Java program via the command line?

Add the arguments after the class name in the <code>java</code> command, e.g., <code>java HelloWorld arg1 arg2</code>. These are accessible in the program via the <code>String[] args</code> parameter of <code>main</code>.

beginner

What is the difference between javac and java commands?

<code>javac</code> compiles Java source code into bytecode (.class files). <code>java</code> runs the compiled bytecode on the Java Virtual Machine.

Which command compiles a Java source file named App.java?

A compile App.java

B java App.java

C run App.java

D javac App.java

The javac command compiles Java source files.

How do you execute a compiled Java class named App?

A java App

B javac App

C run App

D execute App.java

Use java App to run the compiled class.

Where do command line arguments appear in a Java program?

A In the <code>Scanner</code> class automatically

B In the <code>System.out</code> stream

C In the <code>String[] args</code> parameter of <code>main</code>

D In the <code>java.util</code> package

Arguments passed on the command line are received in main(String[] args).

What file extension does the Java compiler produce?

A .class

B .java

C .exe

D .jar

The compiler produces bytecode files with the .class extension.

If you run java HelloWorld but get an error, what might be a common cause?

A You forgot to write <code>javac</code> before <code>java</code>

B The <code>HelloWorld.class</code> file is missing or not in the current directory

C You need to add <code>.java</code> extension when running

D Java programs cannot be run from the command line

The java command needs the compiled .class file in the current directory or classpath.

Explain the steps to compile and run a Java program from the command line.

Think about what commands you type and in what order.

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

Focus on the main method signature and command line syntax.

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