Classes and Objects

Defining a Class in Java

A class is a blueprint for creating objects. It defines the properties (fields) and behaviors (methods) that objects of that class will have.

Basic Class Structure

public class Car {
    // Fields (instance variables)
    String make;
    String model;
    int year;
    double speed;

    // Methods
    public void accelerate() {
        speed += 10;
    }

    public void brake() {
        speed = Math.max(0, speed - 10);
    }

    public String getInfo() {
        return year + " " + make + " " + model;
    }
}

Access Modifiers

public class Person {
    public String name;      // Accessible everywhere
    protected int age;       // Accessible in same package and subclasses
    String email;            // Package-private (default)
    private String password; // Accessible only within this class

    public void publicMethod() { }
    protected void protectedMethod() { }
    void defaultMethod() { }
    private void privateMethod() { }
}

Static vs Instance Members

public class Counter {
    // Instance variable - each object has its own
    int count;

    // Static variable - shared among all objects
    static int totalCount;

    // Instance method - called on an object
    public void increment() {
        count++;
        totalCount++;
    }

    // Static method - called on the class
    public static int getTotalCount() {
        return totalCount;
    }
}

Getters and Setters

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

    // Getter
    public String getName() {
        return name;
    }

    // Setter
    public void setName(String name) {
        this.name = name;
    }

    // Getter with validation
    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        if (age >= 0 && age <= 150) {
            this.age = age;
        }
    }
}

Object Creation

Using the new Keyword

// Create object using new
Car myCar = new Car();

// Initialize fields
myCar.make = "Toyota";
myCar.model = "Camry";
myCar.year = 2024;
myCar.speed = 0;

// Call methods
myCar.accelerate();
System.out.println(myCar.speed);  // 10

Object Reference

// Object variable is a reference
Car car1 = new Car();
Car car2 = car1;  // car2 references same object as car1

car1.make = "Toyota";
System.out.println(car2.make);  // "Toyota" (same object!)

// Creating new object
Car car3 = new Car();
car3.make = "Honda";
System.out.println(car1.make);  // "Toyota" (different object)

Multiple Objects

// Each new creates a separate object
Car car1 = new Car();
Car car2 = new Car();

// car1 and car2 are different objects
car1.make = "Toyota";
car2.make = "Honda";

System.out.println(car1.make);  // "Toyota"
System.out.println(car2.make);  // "Honda"

Object Methods

// Calling methods
Car myCar = new Car();
myCar.make = "Toyota";
myCar.model = "Camry";
myCar.year = 2024;

// Pass objects as parameters
public void printCarInfo(Car car) {
    System.out.println(car.year + " " + car.make + " " + car.model);
}

// Return objects from methods
public Car createCar(String make, String model) {
    Car car = new Car();
    car.make = make;
    car.model = model;
    return car;
}

Garbage Collection

// Objects are automatically garbage collected
public void createObjects() {
    Car car = new Car();
    car.make = "Toyota";
    // car goes out of scope here
    // Object is eligible for garbage collection
}

// Explicitly making object eligible
Car car = new Car();
car = null;  // Now eligible for GC

Null References

Car car = null;  // Reference points to nothing
// car.make = "Toyota";  // NullPointerException!

if (car != null) {
    car.make = "Toyota";  // Safe
}

Instance Variables

Instance variables are fields that belong to each object (instance) of a class.

Declaration and Initialization

public class Employee {
    // Instance variables
    String name;
    int age;
    double salary;
    boolean active;

    // With default values
    String department = "Engineering";
    int yearsOfService = 0;
}

Default Values

public class DefaultValues {
    // Primitive types
    int i;          // 0
    double d;       // 0.0
    boolean b;      // false
    char c;         // '\u0000'
    long l;         // 0L
    float f;        // 0.0f
    byte bt;        // 0
    short s;        // 0

    // Object references
    String str;     // null
    Object obj;     // null
    int[] arr;      // null
}

Instance vs Local Variables

public class Comparison {
    int instanceVar = 10;  // Instance variable

    public void method() {
        int localVar = 20;  // Local variable

        // Instance variable accessible here
        System.out.println(instanceVar);  // 10

        // Local variable accessible here
        System.out.println(localVar);  // 20
    }

    public void anotherMethod() {
        // Cannot access local variable from other method
        // System.out.println(localVar);  // COMPILE ERROR!

        // Can access instance variable
        System.out.println(instanceVar);  // 10
    }
}

Scope and Lifetime

public class ScopeExample {
    int instanceVar;  // Lives as long as object exists

    public void method() {
        int localVar;  // Lives only during method execution

        for (int i = 0; i < 10; i++) {
            int loopVar;  // Lives only during loop iteration
        }
        // loopVar not accessible here
    }
    // localVar not accessible here
}
// instanceVar still accessible as long as object exists

Initialization Order

public class InitOrder {
    // 1. Instance variables initialized (in declaration order)
    int a = 10;
    String s = "hello";

    // 2. Instance initializer block
    {
        System.out.println("Instance initializer");
    }

    // 3. Constructor runs
    public InitOrder() {
        System.out.println("Constructor");
    }
}

The this Keyword

this is a reference to the current object - the object whose method or constructor is being called.

Disambiguating Variables

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

    public Person(String name, int age) {
        // this.name = instance variable
        // name = parameter
        this.name = name;
        this.age = age;
    }
}

Returning Current Object

public class Builder {
    private String name;
    private int value;

    public Builder setName(String name) {
        this.name = name;
        return this;  // Returns current object
    }

    public Builder setValue(int value) {
        this.value = value;
        return this;
    }
}

// Method chaining
Builder builder = new Builder()
    .setName("test")
    .setValue(42);

Passing this as Parameter

public class EventListener {
    public void register(EventSource source) {
        source.addListener(this);  // Pass current object
    }
}

public class EventSource {
    private List<EventListener> listeners = new ArrayList<>();

    public void addListener(EventListener listener) {
        listeners.add(listener);
    }
}

this() for Constructor Chaining

public class Rectangle {
    private int width;
    private int height;
    private String color;

    public Rectangle() {
        this(10, 10);  // Call constructor with parameters
    }

    public Rectangle(int width, int height) {
        this(width, height, "black");
    }

    public Rectangle(int width, int height, String color) {
        this.width = width;
        this.height = height;
        this.color = color;
    }
}

this in Inner Classes

public class Outer {
    private int outerVar = 10;

    public class Inner {
        private int outerVar = 20;  // Shadows outer's outerVar

        public void printVars() {
            System.out.println(outerVar);        // 20 (inner's)
            System.out.println(this.outerVar);    // 20 (inner's)
            System.out.println(Outer.this.outerVar); // 10 (outer's)
        }
    }
}