Deep Explanation of How Java Works Step by Step

Below diagram visually explains the steps involved when writing, compiling, and executing a Java program using Notepad and CMD.


Below is detailed explanation:

• Step 1: Writing the Java Program
  • We write our Java program in a plain text editor like Notepad.
  • The program is saved with a .java extension, e.g., MainApp.java.

    MainApp.java

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

  • The .java file contains our Java source code, including a class with a main method (the entry point of the program).

• Step 2: Compilation
  • In compilation phase, we open CMD, navigate to the directory where our .java file is located, and run the javac command:
    javac MainApp.java
  • What happens in compilation phase:
    • The Java Compiler (javac) reads the .java file.
    • It checks the code for syntax errors.
    • If there are no errors, it compiles the code into bytecode, a platform-independent intermediate representation.
    • The bytecode is saved in a .class file (e.g., MainApp.class).

• Step 3: Bytecode (.class file)
  • The .class file contains the compiled bytecode, which can be executed on any system with a Java Virtual Machine (JVM).
  • Bytecode ensures Java's "Write Once, Run Anywhere" principle because it is not tied to a specific machine.

• Step 4: Execution
  • To run the program, we execute the java command in CMD: java MainApp
    Do not include the .class extension in this command.
  • What happens during execution phase:
    • JVM (Java Virtual Machine):
      • The JVM reads the bytecode in the .class file.
      • It converts the bytecode into machine code that the operating system understands.
      • It executes the machine code line by line.
    • The main method is the entry point for execution.

• Step 5: Output
  • If the program contains a print statement, such as:
    System.out.println("Hello Deepak");
  • The JVM executes it, and the output is displayed in the Command Prompt:
    Hello Deepak