Introduction to Java

Java is a statically typed, object-oriented language designed to run anywhere through the Java Virtual Machine (JVM): "write once, run anywhere".

JDK, JRE and JVM​

Compilation model​

Hello.java --javac--> Hello.class (bytecode) --JVM--> machine code

Hello, Java​

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

javac Hello.java
java Hello

Program structure​

• Every class lives in a file with the same name (for public classes).
• main is the entry point: public static void main(String[] args).
• Packages organise classes (package com.prepverse.app;).
• Statements end with ; and blocks use { }.

Primitive types​

Everything else is a reference type (objects, arrays, String). String is immutable.

Memory​

• Stack: method frames and local variables.
• Heap: objects, managed by the garbage collector.
• You never free memory manually, but you can still leak it by holding references.