Low level design is a crucial aspect of software development, especially when it comes to building complex systems like a payment application. It involves breaking down the high-level requirements and problem statements into smaller components and designing the structure and behavior of each component.

In low level design, we focus on the implementation details of the system. It includes defining the entities and their relationships, creating class diagrams to visualize the class structure, and designing the database schema to store the data.

To start with low level design for a payment app, we first need to analyze the problem statements and identify the requirements. This helps in understanding the functionalities the app needs to perform and the data it needs to handle.

Once the requirements are clear, we can start creating a class diagram. A class diagram illustrates the relationships between classes, their attributes, and methods. It helps in understanding the overall structure of the application.

After designing the class diagram, we define the entities and their relationships in the system. Entities represent the real-world objects that the app interacts with, and their relationships define how these objects are connected.

Next, we move on to designing the database schema. The database schema defines the structure of the database, including tables, columns, and relationships between tables. It determines how the data is organized and accessed.

Throughout the low level design process, we also need to consider the design patterns that can be applied to improve the overall design of the application. Design patterns are proven solutions to common design problems, and they help in making the design more maintainable, scalable, and flexible.

Once the low level design is complete, we can proceed with the code implementation. In this course, we'll be implementing a payment app using low level design principles in Java. The implementation will include writing code to handle the functionalities defined during the low level design process.

Now that we have a high-level overview of low level design, let's dive deeper into each step and understand the concepts and techniques involved.

Understanding problem statements is a crucial step in the low-level design process. It involves analyzing the requirements presented in the problem statement and identifying the key functionalities and data that the payment app needs to handle.

As a senior engineer with a background in Java development, you are already familiar with the importance of understanding problem statements in software development. Just like you analyze user stories and break them down into tasks in Agile development, the same principle applies here.

To effectively analyze problem statements, consider the following steps:

1. Read and Understand the Problem Statement: Start by carefully reading and comprehending the problem statement provided. Pay attention to the context, requirements, and any constraints mentioned.

2. Identify Key Functionalities: Identify the main functionalities that the payment app must provide. These functionalities can include features like user authentication, processing payments, generating invoices, and managing transactions.

3. Define User Roles and Permissions: Determine the different user roles and their specific permissions within the payment app. For example, you may have roles like customer, merchant, and admin, each with their own set of permissions.

4. Analyze Data Requirements: Identify the types of data that the payment app needs to handle. This can include customer information, product details, transaction history, and payment-related data.

5. Consider External Integrations: If the payment app needs to integrate with external systems or APIs, identify those integration points. For example, the app may need to integrate with a third-party payment gateway for processing transactions.

By thoroughly analyzing problem statements and identifying requirements, you lay the foundation for the subsequent steps in the low-level design process, such as creating a class diagram and defining entities and relationships.

Now, let's practice analyzing a problem statement related to our payment app. Consider the following problem statement:

"Design a payment app that allows customers to make payments for purchases and merchants to receive payments. Customers should be able to add items to a cart, view their cart, and proceed to payment. Merchants should be able to manage products, view transaction history, and withdraw funds from their account. Ensure that the app is secure and supports multiple payment methods."

Based on this problem statement, take a moment to analyze and identify the key functionalities and data requirements. How would you approach designing the payment app to fulfill the stated requirements? Take into consideration your Java development experience and think about how you would handle user authentication, managing transactions, and integrating payment methods. Feel free to write some notes or code snippets in the box below:

1// Replace this comment with your code or notes

As a senior Java developer with a strong background in Spring Boot, MySQL, and AWS, you already have a good understanding of object-oriented programming concepts. Creating a class diagram is an essential part of the low-level design process as it helps visualize the relationships between classes and the overall structure of the system.

In Java, you can use various tools to create class diagrams, such as UML (Unified Modeling Language) tools like Visual Paradigm, Lucidchart, or PlantUML.

Let's consider an example of creating a class diagram for a payment app. Here are some key steps to follow:

1. Identify Classes: Start by identifying the main classes in your payment app. These can include classes like User, Payment, Transaction, Product, Cart, etc.

2. Define Class Relationships: Determine the relationships between these classes. For example, a User can have multiple Payment or Transaction instances, and a Cart can contain multiple Product instances. Use appropriate notations like associations, aggregations, compositions, etc., to represent these relationships.

3. Specify Class Attributes and Methods: Define the attributes and methods for each class. Attributes represent the data associated with a class, while methods represent the behavior or actions that the class can perform. For example, the User class may have attributes like name, email, and password, and methods like login, signup, etc.

4. Consider Inheritance and Interfaces: If your payment app has a hierarchical structure, utilize inheritance to represent the parent-child relationships between classes. Additionally, if you have any interfaces that define common behavior, represent them in the class diagram.

5. Validate and Refine: Review the initial class diagram for correctness and completeness. Make adjustments if needed and ensure that the diagram accurately represents the relationships and structure of your payment app.

Remember that a class diagram is a visual representation and should provide a high-level overview of the system's architecture and design. It should not include detailed implementation-level information or specific code snippets.

Once you have created the class diagram, it becomes a valuable reference that can be shared with other team members, stakeholders, or developers to provide a clear understanding of the system's structure and facilitate further discussions and development.

Now, it's time to put your Java skills and knowledge of low-level design concepts to practice by creating a class diagram for the payment app. You can use any UML tool or draw the diagram manually on paper or a whiteboard. Be sure to consider the mentioned steps and components of a class diagram.

1// Replace this comment with your class diagram

Great! You have successfully learned how to create a class diagram to visualize the relationships between classes.

In low-level design, entities represent the key objects or concepts within a system. These entities can include user accounts, products, orders, transactions, and more. Defining entities accurately is crucial for building a well-designed system.

To define entities and their relationships, you can follow these steps:

1. Identify the Entities: Begin by identifying the main entities in your system. Consider the different types of data that need to be stored and manipulated in your application. For example, in a payment app, you may have entities like 'User', 'Payment', 'Transaction', 'Product', and 'Cart'.

2. Establish Relationships: Once you have identified the entities, determine the relationships between them. Relationships can be one-to-one, one-to-many, or many-to-many. For example, a 'User' can have multiple 'Payments' or 'Transactions', and a 'Cart' can contain multiple 'Products'. Use appropriate notations like associations, aggregations, or compositions to represent these relationships in your design.

3. Define Attributes: For each entity, define the attributes or properties that it possesses. Attributes represent the data associated with an entity. For example, a 'User' entity may have attributes like 'name', 'email', and 'password'. Similarly, a 'Product' entity may have attributes like 'name', 'price', and 'quantity'.

4. Consider Cardinality and Multiplicity: Cardinality and multiplicity define the number of instances that can be associated with a relationship. For example, a 'User' can have one-to-many 'Transactions', meaning a single user can have multiple transactions, but each transaction is associated with only one user.

Once you have gone through these steps, you will have a clear understanding of the entities within your system and their relationships. This will serve as a foundation for further steps in the low-level design process, such as creating a class diagram and designing a database schema.

Let's put these concepts into practice with an example:

Consider a payment app where users can make payments and track their transactions. In this app, the main entities could be:

• User
• Payment
• Transaction
• Product

The relationships between these entities would be:

• A User can have multiple Payments and Transactions
• A Payment belongs to one User
• A Transaction belongs to one User
• A Transaction is linked to one Payment
• A Payment can have multiple Products
• A Product is associated with multiple Payments

The attributes for each entity would include:

• User: name, email, password, etc.
• Payment: amount, date, status, etc.
• Transaction: type, description, date, etc.
• Product: name, price, quantity, etc.

Keep in mind that these are just examples, and in real-world scenarios, the entities, relationships, and attributes may vary based on the specific requirements of your application.

Now that we have defined the entities and relationships for our payment app, we can proceed to the next steps in the low-level design process.

To design the database schema based on the entities and relationships we have identified, we can start by defining the table names and the columns for each table. Let's take a look at an example of how we can design the database schema for our payment app:

1public class DatabaseSchema {
2    // Define the table names
3    private static final String USERS_TABLE = "users";
4    private static final String PAYMENTS_TABLE = "payments";
5    private static final String TRANSACTIONS_TABLE = "transactions";
6    private static final String PRODUCTS_TABLE = "products";
7
8    // Define the columns for each table
9    private static final String USER_ID_COLUMN = "user_id";
10    private static final String NAME_COLUMN = "name";
11    private static final String EMAIL_COLUMN = "email";
12    private static final String PASSWORD_COLUMN = "password";
13
14    private static final String PAYMENT_ID_COLUMN = "payment_id";
15    private static final String AMOUNT_COLUMN = "amount";
16    private static final String DATE_COLUMN = "date";
17    private static final String STATUS_COLUMN = "status";
18
19    private static final String TRANSACTION_ID_COLUMN = "transaction_id";
20    private static final String TYPE_COLUMN = "type";
21    private static final String DESCRIPTION_COLUMN = "description";
22    private static final String TRANSACTION_DATE_COLUMN = "transaction_date";
23
24    private static final String PRODUCT_ID_COLUMN = "product_id";
25    private static final String PRODUCT_NAME_COLUMN = "product_name";
26    private static final String PRICE_COLUMN = "price";
27    private static final String QUANTITY_COLUMN = "quantity";
28
29    // Define the relationships using foreign keys
30    private static final String USER_PAYMENT_FK = "fk_user_payment";
31    private static final String USER_TRANSACTION_FK = "fk_user_transaction";
32    private static final String PAYMENT_TRANSACTION_FK = "fk_payment_transaction";
33    private static final String PAYMENT_PRODUCT_FK = "fk_payment_product";
34
35    // Define the create table statements for each table
36    private static final String CREATE_USERS_TABLE =
37        "CREATE TABLE IF NOT EXISTS " + USERS_TABLE + " ("
38        + USER_ID_COLUMN + " INT PRIMARY KEY AUTO_INCREMENT, "
39        + NAME_COLUMN + " VARCHAR(100) NOT NULL, "
40        + EMAIL_COLUMN + " VARCHAR(100) NOT NULL, "
41        + PASSWORD_COLUMN + " VARCHAR(100) NOT NULL)";
42
43    private static final String CREATE_PAYMENTS_TABLE =
44        "CREATE TABLE IF NOT EXISTS " + PAYMENTS_TABLE + " ("
45        + PAYMENT_ID_COLUMN + " INT PRIMARY KEY AUTO_INCREMENT, "
46        + USER_ID_COLUMN + " INT NOT NULL, "
47        + AMOUNT_COLUMN + " DECIMAL(10,2) NOT NULL, "
48        + DATE_COLUMN + " DATE NOT NULL, "
49        + STATUS_COLUMN + " VARCHAR(20) NOT NULL, "
50        + FOREIGN KEY ("
51        + USER_ID_COLUMN + ") REFERENCES "
52        + USERS_TABLE + "(" + USER_ID_COLUMN + "), "
53        + "FOREIGN KEY ("
54        + PAYMENT_ID_COLUMN + ") REFERENCES "
55        + TRANSACTIONS_TABLE + "(" + TRANSACTION_ID_COLUMN + "), "
56        + "FOREIGN KEY ("
57        + PAYMENT_ID_COLUMN + ") REFERENCES "
58        + PRODUCTS_TABLE + "(" + PRODUCT_ID_COLUMN + "))";
59
60    private static final String CREATE_TRANSACTIONS_TABLE =
61        "CREATE TABLE IF NOT EXISTS " + TRANSACTIONS_TABLE + " ("
62        + TRANSACTION_ID_COLUMN + " INT PRIMARY KEY AUTO_INCREMENT, "
63        + USER_ID_COLUMN + " INT NOT NULL, "
64        + TYPE_COLUMN + " VARCHAR(50) NOT NULL, "
65        + DESCRIPTION_COLUMN + " VARCHAR(200) NOT NULL, "
66        + TRANSACTION_DATE_COLUMN + " DATE NOT NULL, "
67        + "FOREIGN KEY ("
68        + USER_ID_COLUMN + ") REFERENCES "
69        + USERS_TABLE + "(" + USER_ID_COLUMN + "))";
70
71    private static final String CREATE_PRODUCTS_TABLE =
72        "CREATE TABLE IF NOT EXISTS " + PRODUCTS_TABLE + " ("
73        + PRODUCT_ID_COLUMN + " INT PRIMARY KEY AUTO_INCREMENT, "
74        + PRODUCT_NAME_COLUMN + " VARCHAR(100) NOT NULL, "
75        + PRICE_COLUMN + " DECIMAL(10,2) NOT NULL, "
76        + QUANTITY_COLUMN + " INT NOT NULL)";
77
78    public static void main(String[] args) {
79        // Execute the create table statements
80        executeCreateTableStatement(CREATE_USERS_TABLE);
81        executeCreateTableStatement(CREATE_PAYMENTS_TABLE);
82        executeCreateTableStatement(CREATE_TRANSACTIONS_TABLE);
83        executeCreateTableStatement(CREATE_PRODUCTS_TABLE);
84
85        System.out.println("Tables created successfully!");
86    }
87
88    private static void executeCreateTableStatement(String createTableStatement) {
89        try (Connection connection = DriverManager.getConnection("jdbc:mysql://localhost/payment_app", "username", "password");
90             Statement statement = connection.createStatement()) {
91
92            statement.execute(createTableStatement);
93
94        } catch (SQLException e) {
95            e.printStackTrace();
96        }
97    }
98}

xxxxxxxxxx

}

```java

public class DatabaseSchema {

    // Define the table names

    private static final String USERS_TABLE = "users";

    private static final String PAYMENTS_TABLE = "payments";

    private static final String TRANSACTIONS_TABLE = "transactions";

    private static final String PRODUCTS_TABLE = "products";

​

    // Define the columns for each table

    private static final String USER_ID_COLUMN = "user_id";

    private static final String NAME_COLUMN = "name";

    private static final String EMAIL_COLUMN = "email";

    private static final String PASSWORD_COLUMN = "password";

​

    private static final String PAYMENT_ID_COLUMN = "payment_id";

    private static final String AMOUNT_COLUMN = "amount";

    private static final String DATE_COLUMN = "date";

    private static final String STATUS_COLUMN = "status";

​

    private static final String TRANSACTION_ID_COLUMN = "transaction_id";

    private static final String TYPE_COLUMN = "type";

    private static final String DESCRIPTION_COLUMN = "description";

    private static final String TRANSACTION_DATE_COLUMN = "transaction_date";

​

    private static final String PRODUCT_ID_COLUMN = "product_id";

    private static final String PRODUCT_NAME_COLUMN = "product_name";

    private static final String PRICE_COLUMN = "price";

    private static final String QUANTITY_COLUMN = "quantity";

​

When it comes to low level design, understanding design patterns is crucial. Design patterns provide solutions to common problems that arise in software design. They offer proven and effective approaches to designing maintainable, flexible, and scalable systems.

In the context of the payment app we are developing, there are several design patterns that can be applied.

One commonly used design pattern in low level design is the Factory Method. The Factory Method pattern provides an interface for creating objects, but defers the actual instantiation to subclass implementations. This allows for flexibility in object creation and supports loose coupling between components.

Another design pattern that is frequently used is the Singleton pattern. The Singleton pattern ensures that only one instance of a class is created and provides a global point of access to that instance. This can be useful in scenarios where we need a single instance of a class throughout the system, such as for managing database connections.

Other design patterns that may be applicable in low level design include the Builder pattern, Adapter pattern, Observer pattern, and Strategy pattern.

Let's take a closer look at the Factory Method design pattern and how it can be applied in the context of our payment app.

1interface PaymentProcessor {
2  void processPayment(Payment payment);
3}
4
5class CreditCardPaymentProcessor implements PaymentProcessor {
6  public void processPayment(Payment payment) {
7    // Logic to process credit card payment
8  }
9}
10
11class PayPalPaymentProcessor implements PaymentProcessor {
12  public void processPayment(Payment payment) {
13    // Logic to process PayPal payment
14  }
15}
16
17class PaymentProcessorFactory {
18  public static PaymentProcessor createPaymentProcessor(String paymentMethod) {
19    if (paymentMethod.equals("Credit Card")) {
20      return new CreditCardPaymentProcessor();
21    } else if (paymentMethod.equals("PayPal")) {
22      return new PayPalPaymentProcessor();
23    }
24    return null;
25  }
26}

xxxxxxxxxx

class Main {

  public static void main(String[] args) {

    // Replace with relevant Java code here

  }

}

To implement the payment app, we can utilize the low level design principles we have learned so far. Let's start by creating a PaymentProcessor interface that defines the contract for processing payments.

1public interface PaymentProcessor {
2  void processPayment(Payment payment);
3}

The processPayment method accepts a Payment object as an argument and performs the necessary operations to process the payment.

Next, we can implement the PaymentProcessor interface for different payment methods. For example, let's create a CreditCardPaymentProcessor class and a PayPalPaymentProcessor class.

1class CreditCardPaymentProcessor implements PaymentProcessor {
2  public void processPayment(Payment payment) {
3    // Logic to process credit card payment
4    System.out.println("Processing credit card payment: " + payment.getAmount());
5  }
6}
7
8class PayPalPaymentProcessor implements PaymentProcessor {
9  public void processPayment(Payment payment) {
10    // Logic to process PayPal payment
11    System.out.println("Processing PayPal payment: " + payment.getAmount());
12  }
13}

Within the main method of our application, we can create an instance of the desired payment processor based on the selected payment method, and then process a payment by calling the processPayment method.

1String paymentMethod = "Credit Card";
2PaymentProcessor paymentProcessor = PaymentProcessorFactory.createPaymentProcessor(paymentMethod);
3
4Payment payment = new Payment();
5payment.setAmount(100);
6paymentProcessor.processPayment(payment);

This code snippet demonstrates how we can implement the payment app using low level design principles in Java. We create the necessary interfaces and classes to represent the payment processors, and then use the PaymentProcessorFactory to create an instance of the desired payment processor based on the selected payment method. Finally, we process a payment using the chosen payment processor.

Feel free to explore different payment methods and add more functionality to the payment app based on your requirements and low level design principles.

xxxxxxxxxx

String paymentMethod = "Credit Card";

PaymentProcessor paymentProcessor = PaymentProcessorFactory.createPaymentProcessor(paymentMethod);

​

Payment payment = new Payment();

payment.setAmount(100);

paymentProcessor.processPayment(payment);