Inheritance, Package, Method, Polymorphism

Classes, objects, methods, packages, constructors, static members, inheritance and polymorphism.

Session 1 covers classes as blueprints, object creation, access modifiers, variables, constants and packages via import. Session 2 explains constructors, their rules, and static members. Session 3 covers encapsulation with private fields and public getters, inheritance via extends in single, multi-level and hierarchical forms, the super reference, and polymorphism via overriding and overloading; multiple inheritance only through interfaces.

Example

class Pizza extends FoodItem {
  Pizza(int price) {
    super("Pizza", price);
  }
}

Code Snippets 4

Single Inheritance

Basic

Single Inheritance

package vs.oop.inheritence;
class FoodItem {
  private String name;
  protected int price;
  public FoodItem(String name, int price) {
    this.name = name;
    this.price = price;
  }
  public int billamount(int qty) {
    return(qty * price);
  }
}
class Pizza extends FoodItem {
  public Pizza(int price) {
    super("Pizza", price);
  }
}
public class SingleInheritence {
  public static void main(String args[]) {
    Pizza p = new Pizza(275);
    System.out.println("Bill amount: " + p.billamount(4));
  }
}

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Multi Level Inheritance

Basic

Multi Level Inheritance

package vs.oop.inheritence;
class FoodItem2 {
  private String name;
  protected int price;
  public FoodItem2(String name, int price) {
    this.name = name;
    this.price = price;
  }
  public int billamount(int qty) {
    return(qty * price);
  }
}
class Pizza2 extends FoodItem {
  public Pizza2(String n, int price) {
    super(n, price);
  }
}
class VegPizza extends Pizza2 {
  String type;
  VegPizza(String t, int price) {
    super(t, price);
  }
}
public class MultiLevelInheritence {
  public static void main(String args[]) {
    Pizza2 p = new Pizza2("Pizza", 275);
    System.out.println("Bill amount: " + p.billamount(4));
    VegPizza v = new VegPizza("Mushroom", 300);
    System.out.println("Bill amount: " + v.billamount(2));
  }
}

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Method overriding - Object class toString() and equals()

Intermediate

Method overriding - Object class toString() and equals()

class Product {
  private int id;
  private String name;
  Product(int id, String name) {
    this.id = id;
    this.name = name;
  }
  @Override public String toString() {
    // overriding Object.toString()
    return "Product[id=" + id + ", name=" + name + "]";
  }
  @Override public boolean equals(Object obj) {
    // overriding Object.equals()
    if(this == obj) return true;
    if(!(obj instanceof Product)) return false;
    Product other =(Product) obj;
    return this.id == other.id&& this.name.equals(other.name);
  }
}
class ObjectClassDemo {
  public static void main(String[] args) {
    Product p1 = new Product(1, "Laptop");
    Product p2 = new Product(1, "Laptop");
    Product p3 = p1;
    System.out.println(p1);
    // uses toString()
    System.out.println("p1.equals(p2) = " + p1.equals(p2));
    // true
    System.out.println("p1 == p3      = " +(p1 == p3));
    // true (aliases)
  }
}

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Overloading vs Overriding comparison

Intermediate

Overloading vs Overriding comparison

class MathCalc {
  // OVERLOADING - same name, different parameter lists
  int multiply(int a, int b) {
    return a * b;
  }
  int multiply(int a, int b, int c) {
    return a *b * c;
  }
  double multiply(double a, double b) {
    return a * b;
  }
}
class OverloadVsOverride {
  public static void main(String[] args) {
    MathCalc mc = new MathCalc();
    System.out.println(mc.multiply(3, 4));
    // 12
    System.out.println(mc.multiply(3,4, 5));
    // 60
    System.out.println(mc.multiply(2.5, 2.0));
    // 5.0
  }
}

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