A statically typed, object-oriented language that compiles to bytecode run on the JVM, the same language and runtime behind Minecraft Java plugins and JDA Discord bots.

Setup

java -version
javac -version

Real projects use a build tool rather than calling javac by hand, Maven or Gradle.

Core syntax

public class Main {
    public static void main(String[] args) {
        String name = "Ada";
        int count = 0;

        System.out.println("Hello, " + name + "!");

        var nums = List.of(1, 2, 3);
        var doubled = nums.stream().map(n -> n * 2).toList();
    }
}

Classes, records, and interfaces

public class Animal {
    private final String name;

    public Animal(String name) { this.name = name; }

    public String speak() { return name + " makes a sound."; }
}

// records (Java 16+), immutable data carriers, no boilerplate getters/equals/hashCode
public record Point(int x, int y) {}

// interfaces
public interface Shape {
    double area();
}

public class Circle implements Shape {
    private final double radius;
    public Circle(double radius) { this.radius = radius; }
    @Override public double area() { return Math.PI * radius * radius; }
}

Streams (functional-style collection processing)

List<String> names = List.of("Ada", "Bob", "Cy");

List<String> upper = names.stream()
    .filter(n -> n.length() > 2)
    .map(String::toUpperCase)
    .sorted()
    .toList();

Pattern matching (modern Java)

Object value = 42;

String description = switch (value) {
    case Integer i when i > 100 -> "big int";
    case Integer i -> "int: " + i;
    case String s -> "string: " + s;
    default -> "unknown";
};

Exceptions

try {
    int result = Integer.parseInt(input);
} catch (NumberFormatException e) {
    System.err.println("Invalid number: " + e.getMessage());
} finally {
    cleanup();
}

Checked exceptions (throws IOException in a method signature) must be declared or caught, a distinctly Java design choice most other languages here don't share.

Popular frameworks

Common gotchas

• == compares object references, not content, use .equals() for strings/objects ("a".equals(str) avoids a NullPointerException if str is null).
• Autoboxing: Integer caches small values (-128 to 127), so == on boxed integers appears to work for small numbers and silently breaks outside that range, another reason to default to .equals().
• A missing @Override typo (wrong method signature) silently creates a new overload instead of overriding, always annotate overrides so the compiler catches the mismatch.