What Java is and how it runs

What Java is and how it runs

Java is a programming language, but it is also a system for running programs almost anywhere. You write human-readable instructions called source code. A tool called the compiler translates them into bytecode, a compact intermediate format. The Java Virtual Machine (JVM) — a program on your computer — reads that bytecode and runs it. Because a JVM exists for Windows, Mac, Linux, and more, the same compiled Java program runs on all of them. That is Java's famous promise: write once, run anywhere.

Your first Java program

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

Save it as Hello.java. In a terminal, run javac Hello.java to compile, then java Hello to run. The text Hello, Java! appears on screen.

Every Java statement — one instruction — ends with a semicolon. Code lives inside classes (named containers for code), and the JVM always looks for a method named main to start your program.

Data: types, variables, and strings

Java knows the kind of data each piece of information holds. A primitive type is a simple built-in kind, such as int (whole numbers), double (decimals), and boolean (true or false). A variable is a named box that stores a value. Java converts types automatically only when no information is lost, like int into double; otherwise you must convert explicitly.

int count = 3;
double total = count * 2.5;   // int becomes double safely
boolean done = total > 7.0;
String greeting = "Hi, " + "Java";  // joining text is easy
System.out.println(done);     // prints true

A String is a piece of text, and + glues strings together.

Decisions, repetition, arrays, and methods

Programs make choices and repeat work:

int[] scores = {90, 75, 88};   // an array: numbered list of values

for (int i = 0; i < scores.length; i++) {
    if (scores[i] >= 80) {
        System.out.println(scores[i] + " passed");
    } else {
        System.out.println(scores[i] + " retry");
    }
}

An array is a fixed-size list where each item has a number, starting at 0. An if statement chooses between paths; a for loop repeats.

A method is a named, reusable block of code you can call with different inputs:

static String describe(int score) {
    return score >= 80 ? "passed" : "retry";
}