Java compilation and execution flow - Step-by-Step Execution

Concept Flow - Java compilation and execution flow

Write Java Source Code (.java)

↓

Compile with javac

↓

Generate Bytecode (.class)

↓

Run JVM with java command

↓

JVM Loads Bytecode

↓

JVM Executes Bytecode

↓

Program Output or Result

This flow shows how Java source code is written, compiled into bytecode, and then executed by the JVM to produce output.

Execution Sample

Java

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

This simple Java program prints 'Hello, Java!' to the console.

Execution Table

Step	Action	Input	Output/Result
1	Write source code	Hello.java file with code	Source code saved
2	Compile source	javac Hello.java	Hello.class bytecode file created
3	Run JVM	java Hello	JVM starts and loads Hello.class
4	Execute main method	Bytecode instructions	Prints 'Hello, Java!' to console
5	Program ends	Main method completes	JVM exits

💡 Program ends after main method completes execution

Variable Tracker

Variable	Start	After Compilation	After JVM Load	After Execution	Final
Source Code	Hello.java (text)	N/A	N/A	N/A	N/A
Bytecode	N/A	Hello.class (binary)	Loaded in JVM	Executed	N/A
Output	N/A	N/A	N/A	"Hello, Java!" printed	"Hello, Java!" printed

Key Moments - 3 Insights

Why do we need to compile Java code before running it?

Java source code (.java) is human-readable but not understood by the computer directly. Compilation converts it into bytecode (.class), which the JVM can execute. See execution_table step 2.

What role does the JVM play in running a Java program?

The JVM loads the compiled bytecode and executes it step-by-step, managing memory and system resources. This is shown in execution_table steps 3 and 4.

Why can't we run the .java file directly?

The computer hardware cannot understand Java source code directly. It needs the JVM to interpret the compiled bytecode. This is why step 2 (compilation) and step 3 (JVM run) are both necessary.

Visual Quiz - 3 Questions

Look at the execution_table, what is created after compiling Hello.java?

A JVM executable

B Hello.java source code

C Hello.class bytecode file

D Program output

💡 Hint

Check execution_table step 2 for the output of compilation

At which step does the JVM start executing the program?

A Step 4

B Step 1

C Step 3

D Step 5

💡 Hint

See execution_table where main method execution and output printing happen

If the source code has a syntax error, what happens in the flow?

A JVM runs the code anyway

B Compilation fails, no bytecode generated

C Program prints error at runtime

D Output is printed partially

💡 Hint

Compilation step must succeed to create bytecode (execution_table step 2)

Concept Snapshot

Java code is written in .java files
Compiled by javac into .class bytecode
Bytecode runs on JVM, which executes it
JVM prints output and manages program flow
Compilation errors stop the process before running

Full Transcript

Java programs start as source code files with .java extension. This code is human-readable but cannot run directly on a computer. The javac compiler translates the source code into bytecode files with .class extension. Bytecode is a special format understood by the Java Virtual Machine (JVM). When you run the program using the java command, the JVM loads the bytecode and executes it step-by-step. The JVM handles memory and system resources, and the program produces output such as printing text to the console. If there are errors in the source code, compilation fails and no bytecode is created, so the program cannot run. This flow ensures Java programs are portable and secure across different systems.

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