Learn Java in 60 minutes

Java is a programming language — a set of instructions a computer can follow, written in a way that's stricter than English but still readable. The name comes from coffee (the developers were drinking a lot of it), and the logo is a coffee cup.

Think of a Java program like a recipe: it's a list of exact steps, done in order, with no guessing allowed. If a recipe says "add 2 eggs," you can't add "some eggs" — Java is the same way. It wants exact instructions.

Every Java program lives inside a class, and every program starts running from a special method called main. Here's the smallest complete Java program:

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

Let's break that down line by line:

• public class HelloWorld — every file needs a class, and the class name must match the filename (HelloWorld.java).
• public static void main(String[] args) — this is the "start here" signal. Java always looks for main first.
• System.out.println("...") — prints text to the screen, then moves to a new line.
• The curly braces { } mark where a block of code begins and ends. The semicolon ; ends an instruction, like a period ends a sentence.

A variable is a labeled box that stores a value. In Java, every box has to say up front what type of thing it will hold — that's what makes Java "strict."

int level = 1;
double health = 100.0;
boolean hasSword = false;
String playerName = "Ash";

System.out.println(playerName + " is level " + level);

Operators let you do math and ask questions about values.

int score = 85;
int bonus = 10;
int total = score + bonus;
boolean passed = total >= 90;

System.out.println("Total: " + total);
System.out.println("Passed: " + passed);

Programs need to make choices. if lets your code do different things depending on whether something is true.

int score = 85;

if (score >= 90) {
    System.out.println("Grade: A");
} else if (score >= 80) {
    System.out.println("Grade: B");
} else {
    System.out.println("Keep going!");
}

Java checks each condition top to bottom and runs the first one that's true. If none match, it falls into else.

Loops repeat a block of code so you don't have to copy-paste it. The two you'll use most: for (when you know how many times) and while (when you don't).

// for loop: runs a known number of times
for (int i = 1; i <= 5; i++) {
    System.out.println("Round " + i);
}

// while loop: runs until a condition becomes false
int lives = 3;
while (lives > 0) {
    System.out.println("Lives left: " + lives);
    lives--;
}

An array is a row of boxes that all hold the same type, so you can group related values under one name instead of making a new variable for each one.

int[] scores = {90, 85, 77, 92};

System.out.println(scores[0]);      // first item
System.out.println(scores.length); // how many items

for (int i = 0; i < scores.length; i++) {
    System.out.println("Score " + i + ": " + scores[i]);
}

A method is a named block of code you can run whenever you need it, instead of retyping the same steps. You've already used one: main.

public static int square(int number) {
    return number * number;
}

public static void main(String[] args) {
    int result = square(5);
    System.out.println("5 squared is " + result);
}

Break down a method's parts:

• int before the name — the type of value it hands back (its return type).
• square — the method's name, chosen by you.
• (int number) — the parameter: input the method needs to do its job.
• return number * number; — the answer sent back to whoever called it.

A class is a blueprint. An object is something built from that blueprint. One cookie cutter (class) can stamp out many cookies (objects) — each cookie can be decorated differently, but they all share the same shape.

public class Dog {
    String name;
    int age;

    public void bark() {
        System.out.println(name + " says Woof!");
    }
}

Dog rex = new Dog();
rex.name = "Rex";
rex.age = 3;
rex.bark();

Dog milo = new Dog();
milo.name = "Milo";
milo.bark();

name and age are fields — data the object stores. bark() is a method that belongs to every Dog. new Dog() stamps out a brand-new object from the blueprint.