Types and Declarations

Classes

Basic Class Declaration

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
}

Inheritance

public class Employee extends Person {
    private String department;

    public Employee(String name, int age, String department) {
        super(name, age);
        this.department = department;
    }

    public String getDepartment() {
        return department;
    }
}

Abstract Classes

public abstract class Shape {
    protected String color;

    public abstract double area();

    public String getColor() {
        return color;
    }
}

Interfaces

Basic Interface

public interface Drawable {
    void draw();

    default void print() {
        System.out.println("Printing...");
    }

    static void info() {
        System.out.println("Drawable interface");
    }
}

Implementing Interfaces

public class Circle implements Drawable, Comparable<Circle> {
    private double radius;

    public void draw() {
        System.out.println("Drawing circle");
    }

    public int compareTo(Circle other) {
        return Double.compare(this.radius, other.radius);
    }
}

Enums

Basic Enum

public enum Day {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

Enum with Fields and Methods

public enum Planet {
    MERCURY(3.303e23, 2.4397e6),
    VENUS(4.869e24, 6.0518e6),
    EARTH(5.976e24, 6.37814e6);

    private final double mass;
    private final double radius;

    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }

    public double getMass() { return mass; }
    public double getRadius() { return radius; }
}

Records (Java 14+)

Basic Record

public record Point(int x, int y) {}

Records automatically generate:

• Constructor with all components
• Accessor methods (x(), y())
• equals(), hashCode(), toString()

Record with Custom Methods

public record Rectangle(int width, int height) {
    public int area() {
        return width * height;
    }

    public Rectangle {
        if (width < 0 || height < 0) {
            throw new IllegalArgumentException("Negative dimensions");
        }
    }
}

Sealed Classes (Java 17+)

public sealed class Shape permits Circle, Rectangle, Triangle {
    public abstract double area();
}

public final class Circle extends Shape {
    private double radius;
    public double area() { return Math.PI * radius * radius; }
}

public non-sealed class Rectangle extends Shape {
    private double width, height;
    public double area() { return width * height; }
}

Generics

Generic Class

public class Box<T> {
    private T content;

    public void set(T content) {
        this.content = content;
    }

    public T get() {
        return content;
    }
}

Bounded Type Parameters

public class NumberBox<T extends Number> {
    private T value;

    public double doubleValue() {
        return value.doubleValue();
    }
}

public class ComparableBox<T extends Comparable<T>> {
    public int compare(T a, T b) {
        return a.compareTo(b);
    }
}

Generic Methods

public <T> void printArray(T[] array) {
    for (T element : array) {
        System.out.println(element);
    }
}

public <T extends Comparable<T>> T max(T a, T b) {
    return a.compareTo(b) > 0 ? a : b;
}

Wildcards

// Unbounded wildcard
public void printList(List<?> list) { }

// Upper bounded wildcard
public double sum(List<? extends Number> list) { }

// Lower bounded wildcard
public void addIntegers(List<? super Integer> list) { }

Arrays

Array Declaration and Initialization

int[] numbers = new int[5];
int[] primes = {2, 3, 5, 7, 11};
String[] names = new String[]{"Alice", "Bob"};

// Multi-dimensional arrays
int[][] matrix = new int[3][3];
int[][] grid = {{1, 2}, {3, 4}, {5, 6}};

Array Operations

int length = numbers.length;
int first = numbers[0];
numbers[0] = 42;

// Enhanced for loop
for (int n : numbers) {
    System.out.println(n);
}

Type Inference with var

var number = 42;              // int
var message = "Hello";        // String
var list = new ArrayList<String>();  // ArrayList<String>
var map = new HashMap<String, Integer>();

// Works in for loops
for (var item : list) {
    System.out.println(item);
}

Nested Classes

Inner Class

public class Outer {
    private int value;

    public class Inner {
        public void show() {
            System.out.println(value);
        }
    }
}

Static Nested Class

public class Outer {
    private static int staticValue;

    public static class Nested {
        public void show() {
            System.out.println(staticValue);
        }
    }
}