Java compilation and execution flow - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the first step in the Java compilation process?

The first step is writing the Java source code in a file with a .java extension.

beginner

What does the Java compiler (javac) do?

It converts the Java source code (.java) into bytecode, which is saved in a .class file.

beginner

What is bytecode in Java?

Bytecode is a set of instructions that the Java Virtual Machine (JVM) understands and can execute. It is platform-independent.

beginner

What role does the Java Virtual Machine (JVM) play in execution?

The JVM reads the bytecode from the .class file and executes it on the host machine, making Java programs platform-independent.

beginner

What is the full flow from writing Java code to running a program?

1. Write Java source code in a .java file.
2. Compile the source code using javac to create bytecode .class files.
3. Run the bytecode using the JVM with the java command.

What file extension does Java source code use?

A .java

B .class

C .exe

D .jar

Java source code is saved in files ending with .java.

What does the javac command do?

A Starts the JVM

B Runs Java bytecode

C Packages Java files into a JAR

D Compiles Java source code into bytecode

javac compiles .java files into .class bytecode files.

Which component executes Java bytecode?

A Java Virtual Machine (JVM)

B Java compiler

C Java source code

D Java Development Kit (JDK)

The JVM reads and runs the bytecode on the computer.

Why is Java considered platform-independent?

A Because source code runs directly on all machines

B Because bytecode runs on JVM which is available on many platforms

C Because Java programs are compiled to machine code

D Because Java uses .exe files

Bytecode runs on the JVM, which is available for many operating systems, making Java platform-independent.

What command runs a compiled Java program?

A javac

B javadoc

C java

D jar

The java command runs the Java program by starting the JVM and executing bytecode.

Explain the steps from writing Java code to running it on your computer.

Think about what happens before and after compiling.

Why does Java use bytecode and the JVM instead of compiling directly to machine code?

Consider how Java runs on different computers.

Practice

1. What is the first step when running a Java program?

easy

A. Compile the Java source code into bytecode using javac

B. Run the bytecode directly using the java command

C. Convert bytecode into machine code manually

D. Write the Java source code in a text editor

Solution

1. Step 1: Understand Java program execution

   Java source code must first be compiled into bytecode before running.

2. Step 2: Identify the compilation command

   The javac command compiles source code into bytecode.

3. Final Answer:

   Compile the Java source code into bytecode using javac -> Option A

4. Quick Check:

   Compilation = javac [OK]

Hint: Remember: javac compiles, java runs bytecode [OK]

Common Mistakes:

• Thinking java command compiles code
• Confusing source code with bytecode
• Skipping compilation step

2. Which command correctly runs a compiled Java program named HelloWorld?

easy

A. javac HelloWorld.java

B. java HelloWorld

C. java HelloWorld.java

D. javac HelloWorld

Solution

1. Step 1: Identify the command to run bytecode

   The java command runs compiled bytecode, not source files.

2. Step 2: Match the correct syntax

   Use java HelloWorld without the .java extension to run the program.

3. Final Answer:

   java HelloWorld -> Option B

4. Quick Check:

   Run bytecode = java ClassName [OK]

Hint: Run with java ClassName, no .java extension [OK]

Common Mistakes:

• Trying to run source file with java command
• Using javac to run program
• Including .java extension when running

3. Given the commands below, what will be the output?

javac Test.java
java Test

Assuming Test.java contains:

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

medium

A. Compilation error

B. Runtime error

C. Hello Java

D. No output

Solution

1. Step 1: Compile the source code

   javac Test.java compiles the code into bytecode without errors.

2. Step 2: Run the compiled bytecode

   java Test runs the program and prints "Hello Java".

3. Final Answer:

   Hello Java -> Option C

4. Quick Check:

   javac + java = program output [OK]

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

Common Mistakes:

• Expecting output without compiling
• Confusing compilation and runtime errors
• Not including main method

4. What is wrong with this sequence of commands?

java HelloWorld
javac HelloWorld.java

medium

A. Running before compiling causes an error

B. javac should be run after java

C. Commands are correct and will work

D. HelloWorld.java file is missing

Solution

1. Step 1: Understand command order

   You must compile source code before running it.

2. Step 2: Identify error cause

   Running java HelloWorld before compiling causes a runtime error because bytecode is missing.

3. Final Answer:

   Running before compiling causes an error -> Option A

4. Quick Check:

   Compile first, then run [OK]

Hint: Always compile before running Java programs [OK]

Common Mistakes:

• Running java command before javac
• Assuming java compiles automatically
• Ignoring missing .class files

5. You have a Java source file App.java that depends on another class Helper.java. Which sequence correctly compiles and runs the program?

hard

A. javac App.java
java Helper
java App

B. java App
javac Helper.java
javac App.java

C. javac App.java Helper.java
java Helper

D. javac Helper.java
javac App.java
java App

Solution

1. Step 1: Compile dependencies first

   Compile Helper.java before App.java because App depends on Helper.

2. Step 2: Run the main program

   After compiling both, run java App to execute the program.

3. Final Answer:

   javac Helper.java javac App.java java App -> Option D

4. Quick Check:

   Compile dependencies first, then run main [OK]

Hint: Compile dependencies before main class, then run main [OK]

Common Mistakes:

• Running before compiling
• Compiling only main class
• Running wrong class with java command