InShortView

• Java has been evolving since 1991 with different editions
• According to the TIOBE Programming Community Index, it has been one of the top 5 programming languages for the past several years
• As of 2018, it is being used by 10 million developers worldwide
• More than 15 billion devices are powered by Java technology
• More than 125 million Java-based TV devices have been deployed
• 97% of enterprise desktops run Java
• The VOIP at your desk and the editor that you use to write your programs are powered by Java

• Java standard edition (J2SE): provides a platform for writing programs which can run on desktops and servers.
  Example applications: popular editors for programming like eclipse and netbeans.
• Java enterprise edition (J2EE): provides a platform for building and deploying software for an enterprise.
  Example applications: sparsh, icicibank.com.

• A single .java file can have multiple classes, but they are compiled into their own separate .class files.
• It is a good practice to have a single Java class in a .java file.
• The class containing the main method must be public.
• The .java file should have the same name as that of the public class.
• There can be only one public class in a .java file.

// Problem 1: Find the monthly average sales and the number of months which have sales more than the average

class AverageSales {
    public static void main(String[] args) {
        int sum = 0, count = 0;
        int[] sales = { 6, 9, 7, 10, 11, 9, 7, 12, 14, 15, 13, 11 };
        for (int index = 0; index < sales.length; index++) {
            sum += sales[index];
        }
        float average = (float) sum / sales.length;
        for (int sale : sales) {
            if (sale > average) count++;
        }
        System.out.println("Average sale: " + average);
        System.out.println("Sales above average: " + count);
    }
}


Java

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Execute

• Code comments in Java are given using // Comment  OR  /* Comment */
• Code blocks in Java are given inside curly braces { }

// Problem 1: Find the monthly average sales and the number of months which have sales more than the average

class AverageSales {
    public static void main(String[] args) {
        int sum = 0, count = 0;
        int[] sales = { 6, 9, 7, 10, 11, 9, 7, 12, 14, 15, 13, 11 };
        for (int index = 0; index < sales.length; index++) {
            sum += sales[index];
        }
        float average = (float) sum / sales.length;
        for (int sale : sales) {
            if (sale > average) count++;
        }
        System.out.println("Average sale: " + average);
        System.out.println("Sales above average: " + count);
    }
}

// Problem 1: Find the monthly average sales and the number of months which have sales more than the average

class AverageSales {
    public static void main(String[] args) {
        int sum = 0, count = 0;
        int[] sales = { 6, 9, 7, 10, 11, 9, 7, 12, 14, 15, 13, 11 };
        for (int index = 0; index < sales.length; index++) {
            sum += sales[index];
        }
        float average = (float) sum / sales.length;
        for (int sale : sales) {
            if (sale > average) count++;
        }
        System.out.println("Average sale: " + average);
        System.out.println("Sales above average: " + count);
    }
}

int sum = 0;

// Problem 1: Find the monthly average sales and the number of months which have sales more than the average

class AverageSales {
    public static void main(String[] args) {
        int sum = 0, count = 0;
        int[] sales = { 6, 9, 7, 10, 11, 9, 7, 12, 14, 15, 13, 11 };
        for (int index = 0; index < sales.length; index++) {
            sum += sales[index];
        }
        float average = (float) sum / sales.length;
        for (int sale : sales) {
            if (sale > average) count++;
        }
        System.out.println("Average sale: " + average);
        System.out.println("Sales above average: " + count);
    }
}

• Arithmetic Operators (+, -, *, /, %, ++, --)
• Bitwise Logical Operators (&, |, ^, ~, <<, >>, >>>)
• Relational Operators (==, !=, <, <=, >, >=)
• Assignment Operators (=, +=, -=, *=, /=, %=)
• Short-Circuit Logical Operators (&&, ||, !)
• Ternary Operator: It is a short form of if-then-else statement
  Syntax:   ? : ;

// Problem 1: Find the monthly average sales and the number of months which have sales more than the average

class AverageSales {
    public static void main(String[] args) {
        int sum = 0, count = 0;
        int[] sales = { 6, 9, 7, 10, 11, 9, 7, 12, 14, 15, 13, 11 };
        for (int index = 0; index < sales.length; index++) {
            sum += sales[index];
        }
        float average = (float) sum / sales.length;
        for (int sale : sales) {
            if (sale > average) count++;
        }
        System.out.println("Average sale: " + average);
        System.out.println("Sales above average: " + count);
    }
}

long regId = 4567L;
int iRegId = (int)regId;    // Explicit Type Casting

float percentage = ((float)totalMarks/800) * 100;    // Explicit Type Casting

• In narrowing conversions, char data type can be converted to byte or short
• boolean cannot be type casted to any other data type

int intValue = 1002;
long longVariable = intValue;

// Problem 1: Find the monthly average sales and the number of months which have sales more than the average

class AverageSales {
    public static void main(String[] args) {
        int sum = 0, count = 0;
        int[] sales = { 6, 9, 7, 10, 11, 9, 7, 12, 14, 15, 13, 11 };
        for (int index = 0; index < sales.length; index++) {
            sum += sales[index];
        }
        float average = (float) sum / sales.length;
        for (int sale : sales) {
            if (sale > average) count++;
        }
        System.out.println("Average sale: " + average);
        System.out.println("Sales above average: " + count);
    }
}

if () {              // Curly braces are not required if there is only one statement inside the block
    ;
}

if () {
    ;
}
else {
    ;
}

if () {
    ;
}
else if () {
    ;
}
else {
    ;
}

if () {
    if () {
        ;
    }
    else {
        ;
    }
}

• It is generally used as a replacement for if-else ladder as it improves readability
• Value can be a byte, short, char, int or String

switch (variable) {                             // The program flow jumps to the case which matches this variable's value
    case value1: ;
                 break;                         // breaks out of the switch block. Placed at the end of a case
    case value2: ;
                 break;
    default: ;                      // Executes when none of the cases match
}

// Problem 1: Find the monthly average sales and the number of months which have sales more than the average

class AverageSales {
    public static void main(String[] args) {
        int sum = 0, count = 0;
        int[] sales = { 6, 9, 7, 10, 11, 9, 7, 12, 14, 15, 13, 11 };
        for (int index = 0; index < sales.length; index++) {
            sum += sales[index];
        }
        float average = (float) sum / sales.length;
        for (int sale : sales) {
            if (sale > average) count++;
        }
        System.out.println("Average sale: " + average);
        System.out.println("Sales above average: " + count);
    }
}

• Can be used to store data of primitive as well as reference data type
• Holds a fixed number of data, determined at the time of array declaration
• It is an object in Java and is created dynamically

int[] marks = new int[3];    // Creating an array of size 3
marks[0] = 98;               // Setting element at index 0
marks[1] = 86;
marks[2] = 92; 

• Array index always starts from zero
• The length attribute of an array can be used to get its size
• Once initialized, the size of an array cannot be changed

int[][] dayWiseTemperature = new int[7][3];    // The row size is 7, and the column size (optional) is 3

// Another way of creating a 2-D array
int[][] dayWiseTemperature = new int[][] {
    {30, 35, 28},
    {29, 32, 0},
    .
    .
    .
};

// To set or change individual values
dayWiseTemperature[1][2] = 28;

for (int index = 0; index < sales.length; index++) {
    sum += sales[index];                                   // Accessing element at position index
}

for (int sale : sales) {                                   // For each int value sale in the sales array
    if (sale > average) count++;
}

public static void main(String args[]) {
    // Code
}

// Problem 1.1: Find the monthly average sales of the first half of 2016-2017

class AverageSales  {
 public static void main(String[] args) {
        int sum = 0;
        int[] sales = { 6, 9, 7, 10, 11, 9, 7, 12, 14, 15, 13, 11 };
        for (int index = 0; index <= 6; index++) {
            sum += sales[index];
        }
        float average = (float) sum / 6;
        System.out.println("Average sale for first half of the year: " + average);
    }
}

• It has its own perspective to help in debugging
• Allows using breakpoints and watchpoints, and updating variable values at runtime
• Provides modes like Step Into and Step Over to have greater control over the execution

1. It removes all terminated programs from the debug view
2. It resumes the execution of the program until the next breakpoint
3. It suspends the execution
4. It terminates the current program
5. Step Into - It executes the current line and goes to the next line in the program. If the current line is a method call, then the debugger will take you to the called method
6. Step Over - If the current line is a method call, then the debugger will execute the method without stepping into the called method
7. Use Step Filters - If selected, then debug will skip from stepping into any of the packages/ classes mentioned in the Java Debug Step Filtering preferences

• Products should have details like id, name, price and stock, along with functionalities to display details, calculate discounted price, etc.
• Customers should have details like id, password, name, phone, and address. They should also have functionalities to display details, order products, etc.
• There are two types of customers: Regular and Premium. Regular customers get some discount, while the premium customers get discount as well as membership benefits. Membership benefits currently include a standard membership card which can be used for acquiring and redeeming points while shopping at E-Mart
• Customers should be informed if they try to buy a product which is not in stock

class Product {

    private int productId;
    private String name;
    private float price;
    private int stock;
    
    public float getPriceAfterDiscount(int discountPercentage) {
        float discountedPrice = price - (price * discountPercentage / 100);
        return discountedPrice;
    }

    public void displayDetails() {
        System.out.println("Product Id: " + productId);
        System.out.println("Product Name: " + name);
        System.out.println("Product Price: " + price);
        System.out.println("Stock: " + stock);
    }
    
    public void displayDetails(int discountPercentage) {
        System.out.println("Product Name: " + name);
        System.out.println("Discounted price: " + getPriceAfterDiscount(discountPercentage));
        if(stock > 0) System.out.println("In stock");
        else System.out.println("Out of stock!");
    }

    public int getProductId() {
        return productId;
    }
    public void setProductId(int productId) {
        this.productId = productId;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public float getPrice() {
        return price;
    }
    public void setPrice(float price) {
        this.price = price;
    }
    public int getStock() {
        return stock;
    }
    public void setStock(int stock) {
        this.stock = stock;
    }
}

• It starts with the keyword "class" followed by a name
• It specifies attributes (characteristics) and methods (behaviors)
• Attributes are the elements (instance variables) which hold the values of a particular entity.
  Here, productId, name and price are the attributes
• Methods are the set of instructions which define the behaviors of the entity.
  Here, display() is a method
• Access specifiers help in limiting access to the members of a class.
  private members are accessible only inside their class, but public members are accessible in other classes as well.
  More about access specifiers will be discussed later

• The first line of the method contains the method signature, which includes the name of the method and the arguments (parameters) it takes
• Input arguments of a method are optional (but not the parenthesis). The arguments specified in the method definition are called formal arguments or parameters. Here, discountPercentage is the formal argument
• Variables declared inside method blocks (including arguments) are called local variables.
  Here, discountPercentage and discountedPrice are local variables

• The return keyword is used to return a value
• It also sends back the flow of execution from a method to the calling method
• If a method does not return anything, the return type should be specified as void 

// Calling getPriceAfterDiscount() method in the same class
float finalPrice = getPriceAfterDiscount(12);        // Here 12 is the actual argument

Product prodObj = new Product();                               // Creating a product object

prodObj.setProductId(101);                                     // Setting attribute values
prodObj.setName("T-Shirt");
prodObj.setPrice(799f);
String name = prodObj.getName();                               // Getting attribute value
float finalPrice = prodObj.getPriceAfterDiscount(12);

• It allows us to use the attributes and methods specified in the class
• A class can have any number of objects
• An object holds data related to an instance of a class

• Instance variables are automatically initialized to their default values during object creation.
  For example, before setting values to the product object, productId will be 0, name will be null, price will be 0.0f
• Local variables (declared in methods) have to be initialized before they can be used
• Variables referring to objects are called reference variables.
  In the above code, prodObj is a reference variable

class Product  {
    // Attributes and methods

     public void displayDetails() {
        System.out.println("Product Id: " + productId);
        System.out.println("Product Name: " + name);
        System.out.println("Product Price: " + price);
        System.out.println("Stock: " + stock);
    }
    
    public void displayDetails(int discountPercentage) {
     System.out.println("Product Name: " + name);
     System.out.println("Product discounted price: " + getPriceAfterDiscount(discountPercentage));
     if(stock > 0) System.out.println("In stock");
     else System.out.println("Out of stock!");     
    }
}

product.displayDetails();         // Displaying all the product details for the employees
product.displayDetails(5);        // Displaying the discounted product details for customers

Product prodObj = new Product();                                 // Creating product object
prodObj.setName("Jeans");                                        // Setting value to attribute

class Product {
    private int productId;
    private String name;
    private float price;
    private int stock;  
    public Product(int id, String name, float price, int stock) {
        this.productId = id;
        this.name = name;
        this.price = price;
        this.stock = stock;
        System.out.println("Product created!");
    }  
    // Other methods
}

Product prodObj = new Product(101, "T-Shirt", 799, 25);

• Default constructor
  • A constructor which does not take any parameter

    public Product() { }
    

  • If the programmer doesn't provide a default constructor, it will be provided by the compiler
  • It is used to initialize attributes to their default values

    Product prodObj = new Product();        // Here 'Product()' indicates default constructor invocation 

• Parameterized constructor
  • A constructor which accepts parameters (arguments)

    public Product(int id, String name, float price, int stock) { }
    

  • If the programmer provides a parameterized constructor then the default constructor is not provided by the compiler

Product {
    // Attributes and methods

    Product() { }
    Product(int id, String name) { }
    Product(int id, String name, float price, int stock) { }
}

Product p1 = new Product();
p1.setProductId(101);

public void setProductId(int pId) {
    productId = pId;              // Assigning argument to instance variable
}

public void setProductId(int productId) {
    productId = productId;             // What happens here?
}

public void setProductId(int productId) {
    this.productId = productId;                       // Assigning argument to instance variable
}

class Product {
    // Fields and methods
    
    public Product(int productId, String name) {
        this.productId = productId;
        this.name = name;
    }
    
    public Product(int productId, String name, float price, int stock) {
        this(productId, name);                                              // Invoking overloaded constructor
        this.price = price;
        this.stock = stock;
    }
}

class Car {
    private float speed;                                   // private prevents direct access

    public void accelerate(float speedToAdd) {             // Method to increase speed
        this.speed += speedToAdd;
        if(this.speed > 120) this.speed = 120;             // Limiting speed to 120 kmph
    }

    // Other fields and methods
}

class Product {
    private int productId;
    private String name;
    private float price;
    private int stock;
    
    public int getProductId() {                         // Getter method for productId
        return productId;
    }
    public void setProductId(int productId) {           // Setter method for productId
        this.productId = productId;
    }

    // Other methods
}

• The data members (attributes) are usually private to prevent direct access to them
• Methods (behaviors) are usually public to be accessible from outside the class

public class ProductTester {
    public static void main(String[] args) {
        Product prodObj = new Product("T-Shirt", 799);
        float finalPrice = prodObj.getPriceAfterDiscount(15);
        System.out.println("Final price of " + prodObj.getName() + " is " + finalPrice);
    }
}

• Behaviors can be invoked without knowing how they work
• What to invoke is relevant, but how it works is not

Product p1 = new Product();
p1.setProductId(1001);

class Product {
    private int productId;
    private String name;
    private float price;
    private int stock;
    
    private static int idCounter = 100;
    
    public Product(String name, float price, int stock) {
        this.productId = ++idCounter;
        this.name = name;
        this.price = price;
        this.stock = stock;
    }
    // Other methods
}

Product.getLastId();              // Calling a static method

Product p1;
p1.getLastId();                   // Calling static method using reference

private static int idCounter;
static {                                                // Static block
    int randomNumber = 31;
    idCounter = (int) Math.pow(randomNumber, 3);        // Initializing counter
}

public class Product {

    private int productId;
    private String name;
    private float price;
    private int stock;
    
    private static int idCounter = 1000;

    public static void generateProductId() {
        this.productId = ++idCounter;                       // Which object's productId!?
    }
}

• Static variables and methods can be accessed without creating an object of a class
• Static blocks execute only once

class Product {
 /* Instance variables or attributes */
    private int productId;
    private String name;
    private float price;
    private int stock;
 
 /* static variable for autogenerating productId */
 private static int idCounter = 100;
 
 /* Parameterized constructor to initialize attributes during instantiation */
 public Product(String name, float price, int stock) {
        this.productId = ++idCounter;
        this.name = name;
        this.price = price;
        this.stock = stock;
    }
    
    public float getPriceAfterDiscount(int discountPercentage) {
        float discountedPrice = price - (price * discountPercentage / 100);
        return discountedPrice;
    }

 /* Overloaded methods to display product details for employees and customers respectively */
    public void displayDetails() {
        System.out.println("Product Id: " + productId);
        System.out.println("Product Name: " + name);
        System.out.println("Product Price: " + price);
        System.out.println("Stock: " + stock);
    }
    
    public void displayDetails(int discountPercentage) {
        System.out.println("Product Name: " + name);
        System.out.println("Discounted price: " + getPriceAfterDiscount(discountPercentage));
        if(stock > 0) System.out.println("In stock");
        else System.out.println("Out of stock!");
    }

 /* Getters and setters */
    public int getProductId() {
        return productId;
    }
    public void setProductId(int productId) {
        this.productId = productId;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public float getPrice() {
        return price;
    }
    public void setPrice(float price) {
        this.price = price;
    }
    public int getStock() {
        return stock;
    }
    public void setStock(int stock) {
        this.stock = stock;
    }
}

abstract class Customer {
    
    private int customerId;
    private String password;
    private String name;
    private String phone;
    private Address address;
    
    public float orderProducts(Product[] products) {
        BillCalculator billCalc = new BillCalculator();
        billCalc.processOrder(products);
        return billCalc.getAmount();
    }
    
    public abstract void displayDetails();

    // Getters and setters
}

class Address {
    private String street;
    private String city;
    private int zipCode;

    public Address(String street, String city, int zipCode) {
        this.street = street;
        this.city = city;
        this.zipCode = zipCode;
    }

    // Getters and setters
}

public class BillCalculator {
    private int billId;
    private float amount;

    public void processOrder(Product[] products) {
        for (int i = 0; i < products.length; i++) {
            if (products[i].getStock() <= 0) {
                System.out.println(products[i].getName() + " out of stock"); break;
            }
            else {
                this.amount += products[i].getPrice();
                products[i].setStock(products[i].getStock() - 1);
            }
        }
    }
    
    // Getters and setters
}

interface MembershipCardHolder {
    int DEFAULT_POINTS = 100;
    
    float redeemPoints(int pointsToRedeem);
    void addPoints(float money);
}

final class RegularCustomer extends Customer {

    public RegularCustomer(int id, String name, String password, String phone, Address address) {
        this.setCustomerId(id);
        this.setName(name);
        this.setPassword(password);
        this.setPhone(phone);
        this.setAddress(address);
    }

    @Override
    public void displayDetails() {
        System.out.println("Customer Id: " + this.getCustomerId());
        System.out.println("Customer Name: " + this.getName());
        System.out.println("Customer phone: " + this.getPhone());
    }
    
    @Override
    public float orderProducts(Product[] products) {
        float billAmount = super.orderProducts(products);
        return billAmount * 0.95f;                                    // Applying 5% discount
    }
}


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final class PremiumCustomer extends Customer implements MembershipCardHolder {
    
    private int points = DEFAULT_POINTS;

    public PremiumCustomer(int id, String name, String password, String phone, Address address) {
        this.setCustomerId(id);
        this.setName(name);
        this.setPassword(password);
        this.setPhone(phone);
        this.setAddress(address);
    }
    
    @Override
    public void displayDetails() {
        System.out.println("Customer Id: " + this.getCustomerId());
        System.out.println("Customer Name: " + this.getName());
        System.out.println("Customer phone: " + this.getPhone());
        System.out.println("Points remaining: " + this.points);
    }
    
    @Override
    public float redeemPoints(int pointsToRedeem) {
        pointsToRedeem = pointsToRedeem > this.points ? this.points : pointsToRedeem;
        float amountToRedeem = pointsToRedeem * 0.5f;                 // Point value is Rs. 0.5
        this.points = this.points - pointsToRedeem;
        return amountToRedeem;
    }
    
    @Override
    public void addPoints(float money) {
        this.points += (int) money / 100;                             // Point cost is Rs. 100
    }
    
    @Override
    public float orderProducts(Product[] products) {
        float billAmount = super.orderProducts(products);
        float finalAmount = billAmount * 0.90f;                       // Applying 10% discount
        addPoints(finalAmount);
        return finalAmount;
    }
    
    public float orderProducts(Product[] products, int pointsToRedeem) {
        float billAmount = super.orderProducts(products);
        float finalAmount = billAmount * 0.90f;                       // Applying 10% discount
        if(finalAmount != 0)
            finalAmount -= redeemPoints(pointsToRedeem);              // Redeeming points
        addPoints(finalAmount);
        return finalAmount;
    }
}