Program Internal

You write a .java file in plain text. javac compiles it into a .class file. That file holds bytecode, not a Windows .exe. Bytecode is for the JVM, not for one operating system.

When you run the program, ClassLoader brings the .class into the JVM. A verifier checks it is not unsafe. Then JVM starts at main. Often JIT makes hot parts faster after they run many times.

Remember the chain and say it out loud: .java → javac → .class → JVM → output. Same .class can run on another OS if JVM is installed.

Let's take this on the board with one tiny Main class — no extra files. Program Internal — reads JVM properties — proves your runtime is available. Start from main, go line by line, and stop at each print. That output is the proof for Program Internal.

Diagram

Main.java
      │ javac
      ▼
  Main.class  (bytecode)
      │ ClassLoader + verifier
      ▼
     JVM  →  main()
      │ JIT (hot code faster)
      ▼
   output

Exam tip

Say the chain: source → bytecode → JVM → run. Then add: not an .exe.

Example

// JVM / runtime info
public class Main {
  public static void main(String[] args) {
    System.out.println("java.version = " + System.getProperty("java.version"));
    System.out.println("java.home = " + System.getProperty("java.home"));
    System.out.println("os.name = " + System.getProperty("os.name"));
  }
}

Program Internal — reads JVM properties — proves your runtime is available.

Short notes

DefYou write .java. javac makes .class bytecode. JVM runs it.
Flow.java → javac → .class → JVM → output.
Remember.class is not a Windows .exe. Same file can run on another OS.
RuleClassLoader loads the class. Verifier checks it. Then main runs.

Program Internal

You write a .java file in plain text. javac compiles it into a .class file. That file holds bytecode, not a Windows .exe. Bytecode is for the JVM, not for one operating system.

When you run the program, ClassLoader brings the .class into the JVM. A verifier checks it is not unsafe. Then JVM starts at main. Often JIT makes hot parts faster after they run many times.

Remember the chain and say it out loud: .java → javac → .class → JVM → output. Same .class can run on another OS if JVM is installed.

Diagram

Main.java
      │ javac
      ▼
  Main.class  (bytecode)
      │ ClassLoader + verifier
      ▼
     JVM  →  main()
      │ JIT (hot code faster)
      ▼
   output

Exam tip

Say the chain: source → bytecode → JVM → run. Then add: not an .exe.

Example

// JVM / runtime info
public class Main {
  public static void main(String[] args) {
    System.out.println("java.version = " + System.getProperty("java.version"));
    System.out.println("java.home = " + System.getProperty("java.home"));
    System.out.println("os.name = " + System.getProperty("os.name"));
  }
}

Program Internal — reads JVM properties — proves your runtime is available.

Short notes

DefYou write .java. javac makes .class bytecode. JVM runs it.
Flow.java → javac → .class → JVM → output.
Remember.class is not a Windows .exe. Same file can run on another OS.
RuleClassLoader loads the class. Verifier checks it. Then main runs.

Program Internal

Example

Short notes

Questions

Program Internal

Example

Short notes

Questions