Java compilation and execution flow - Mini Project: Build & Apply

Java Compilation and Execution Flow

📖 Scenario: You are learning how Java programs are compiled and run. Understanding this flow helps you know what happens when you write and run Java code.

🎯 Goal: Build a simple Java program step-by-step to see how compilation and execution work together.

📋 What You'll Learn

Create a Java class with a main method

Declare a variable inside the main method

Print the variable value to the console

Understand the order of compilation and execution

💡 Why This Matters

🌍 Real World

Java programs are everywhere, from mobile apps to large servers. Knowing how to write and run Java code is the first step to building these applications.

💼 Career

Understanding Java compilation and execution is essential for software developers working with Java, enabling them to write, debug, and run programs effectively.

1

Create a Java class with a main method

Write a Java class named FlowDemo with a public static void main(String[] args) method. Leave the method body empty for now.

Java

public class FlowDemo {
    public static void main(String[] args) {
        // Your code here
    }
}

Hint

The main method is the entry point for Java programs. It must be public static void main(String[] args).

2

Declare a variable inside the main method

Inside the main method, declare an integer variable named number and set it to 10.

Java

public class FlowDemo {
    public static void main(String[] args) {
        // Declare int number = 10 here
    }
}

Hint

Use int number = 10; inside the main method to declare the variable.

3

Print the variable value to the console

Add a line inside the main method to print the value of number using System.out.println.

Java

public class FlowDemo {
    public static void main(String[] args) {
        int number = 10;
        // Print number here
    }
}

Hint

Use System.out.println(number); to print the value to the console.

4

Run the program and see the output

Compile and run the FlowDemo class. The program should print 10 to the console.

Java

// Run the program to see the output

Hint

Use javac FlowDemo.java to compile and java FlowDemo to run the program in your terminal or command prompt.

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