Parts of Java

Parts of Java

1. Beginner Big Picture

Java = Language + Platform. Language is syntax you type; platform (JVM + API) is what runs it.

Part	What it is	Example
Java Language	keywords, syntax, OOP rules	class, if, for
JDK (tools)	compiler + runner + debugger	javac, java, javadoc
JRE (runtime)	JVM + standard libraries to run apps	install JRE to run, JDK to develop
JVM	engine executing bytecode	HotSpot JVM
Java API (libraries)	ready-made classes	String, ArrayList, Scanner
IDE / Build	helpers (not part of language)	IntelliJ, Eclipse, Maven

2. Minimal Program Anatomy (every word explained)

public class Hello {           // file Hello.java, class = blueprint
    public static void main(String[] args) { // entry: JVM calls this first
        System.out.println("Hi"); // API call: print line
    }
}

• public → visible to JVM. class → container. static → no object needed. void → returns nothing. String[] args → command-line inputs.

3. Compile-Run Chain

Hello.java --javac--> Hello.class --java Hello--> output Hi

4. Beginner Mistakes

• File name must match public class (Hello ↔ Hello.java), case-sensitive.
• No main → Main method not found error.
• One public class per file (beginner rule).

5. Exam Points

• Java language vs platform, JDK vs JRE vs JVM — 5 marks, very frequent.
• Explain each word of main — 3-5 marks, frequent.