In software development, low level design refers to the process of translating the high level design and architectural decisions into a detailed design that can be implemented. It focuses on the internal structure of the system, including the organization of modules, the relationships between them, and the algorithms and data structures used.

As a senior engineer with a strong background in Java development, Spring Boot, MySQL, and AWS, understanding low level design is crucial for efficiently building robust and scalable applications. Just like a solid foundation is essential for constructing a building, low level design provides the foundation for the successful implementation of software systems.

To understand the importance of low level design, let's consider an analogy with building a house. The architectural design of the house provides the overall layout and structure, similar to the high level design in software development. However, the low level design involves detailed decisions such as the specific materials to use for constructing the walls, the plumbing and electrical systems, and the interior design.

Similarly, in software development, low level design addresses the specifics of how the different components of a system will interact and function. It defines the modules, classes, methods, and variables required to implement the desired functionality.

Let's dive into an example to illustrate the concept of low level design. Imagine you are tasked with designing a payment app. You have already identified the high level requirements, such as the ability to process payments, generate invoices, and manage user accounts. Now, it's time to break down these requirements into a detailed design.

One aspect of the low level design for the payment app is the class diagram. This diagram depicts the classes and their relationships in the system. Just like architectural blueprints of a house, the class diagram provides a visual representation of how the different classes interact and collaborate to achieve the desired functionality. It helps ensure that the code is organized, maintainable, and extensible.

Let's take a look at a simplified class diagram for the payment app:

1class Payment {
2    private Account account;
3    private BigDecimal amount;
4
5    public Payment(Account account, BigDecimal amount) {
6        this.account = account;
7        this.amount = amount;
8    }
9
10    public void processPayment() {
11        // Logic to process the payment
12    }
13}
14
15class Account {
16    private String accountNumber;
17    private BigDecimal balance;
18
19    // Getters and setters
20}

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class Main {

  public static void main(String[] args) {

    // Replace with your Java logic here

    Payment payment = new Payment(new Account("1234567890"), new BigDecimal("100.00"));

    payment.processPayment();

  }

}

Understanding Problem Statements

To effectively design and implement a payment app, it is crucial to thoroughly understand the problem statements and identify key requirements. Analyzing problem statements helps us gain clarity on what the app needs to do and what features it should have.

A problem statement defines the problem that the app aims to solve. It describes the desired outcome and provides a clear understanding of the problem domain. For example:

1Implement a payment app that allows users to make payments and generate invoices.

Once we have the problem statement, we can then identify the key requirements of the app. These requirements define the functionalities and features that the app should have. Here are some example requirements for the payment app:

1. User should be able to create an account
2. User should be able to add funds to their account
3. User should be able to make payments to merchants
4. User should be able to generate invoices for payments made
5. User should be able to view transaction history

By analyzing the problem statements and identifying the key requirements, we can ensure that we have a clear understanding of what needs to be implemented. This step is crucial in the low level design process as it lays the foundation for the subsequent steps such as creating a class diagram, defining entities and relationships, and deciding on the appropriate design patterns.

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class Main {

  public static void main(String[] args) {

    // Replace this with your Java logic

    String problemStatement = "Implement a payment app that allows users to make payments and generate invoices.";

​

    String[] requirements = {

      "1. User should be able to create an account",

      "2. User should be able to add funds to their account",

      "3. User should be able to make payments to merchants",

      "4. User should be able to generate invoices for payments made",

      "5. User should be able to view transaction history"

    };

​

    // Print problem statement

    System.out.println("Problem Statement:");

    System.out.println(problemStatement);

​

    // Print requirements

    System.out.println("Requirements:");

    for (String requirement : requirements) {

      System.out.println(requirement);

    }

  }

}

Creating Class Diagram

In the low level design process, creating a class diagram is an important step. A class diagram provides a visual representation of the classes, their relationships, and the overall structure of the payment app.

A class diagram helps in organizing and understanding the various components of the app. It shows the classes, their attributes, methods, and the associations between classes.

To create a class diagram for the payment app, we need to identify the key entities and their relationships. For example, in the payment app, we may have entities such as User, Account, Payment, and Invoice. The relationships between these entities can be represented using different types of associations such as aggregation, composition, and inheritance.

Here is a simple example of a class diagram for the payment app:

1// Replace with your Java code
2
3// Payment Class
4public class Payment {
5  private double amount;
6  private String recipient;
7  
8  public double getAmount() {
9    return amount;
10  }
11  
12  public void setAmount(double amount) {
13    this.amount = amount;
14  }
15  
16  public String getRecipient() {
17    return recipient;
18  }
19  
20  public void setRecipient(String recipient) {
21    this.recipient = recipient;
22  }
23}
24
25// User Class
26public class User {
27  private String name;
28  private String email;
29  
30  public String getName() {
31    return name;
32  }
33  
34  public void setName(String name) {
35    this.name = name;
36  }
37  
38  public String getEmail() {
39    return email;
40  }
41  
42  public void setEmail(String email) {
43    this.email = email;
44  }
45}
46
47// Association between Payment and User
48class Association {
49  private User user;
50  private Payment payment;
51  
52  // Replace with relevant code
53}

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class Main {

  public static void main(String[] args) {

    // Replace with your Java logic here

    System.out.println("Creating Class Diagram for Payment App");

  }

}

Defining Entities and Relationships

In the process of low level design, one of the crucial steps is defining entities and their relationships. Entities are the objects or concepts of the system that play a significant role in the application.

For example, in a payment app, some entities could be User, Account, Payment, and Invoice. These entities represent real-world objects that have attributes and behaviors.

Entities are connected through relationships, which define how they interact with each other. Relationships can be one-to-one, one-to-many, or many-to-many. They help establish the logical connections between entities.

To define entities and relationships in the payment app, we need to analyze the problem statement and identify the important components. We can create a class diagram to visualize the entities and their relationships.

Here is an example of how entities and their relationships can be defined in Java:

1// User Class
2public class User {
3  private String name;
4  private String email;
5  
6  // Constructor, getters, setters, and other methods
7}
8
9// Account Class
10public class Account {
11  private double balance;
12  private List<Payment> payments;
13  
14  // Constructor, getters, setters, and other methods
15}
16
17// Payment Class
18public class Payment {
19  private double amount;
20  private String recipient;
21  
22  // Constructor, getters, setters, and other methods
23}
24
25// Invoice Class
26public class Invoice {
27  private double amount;
28  private String description;
29  
30  // Constructor, getters, setters, and other methods
31}

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class Main {

  public static void main(String[] args) {

    // Replace with your Java logic here

    String firstName = "John";

    String lastName = "Doe";

    int age = 30;

    double salary = 5000.50;

    System.out.println("Name: " + firstName + " " + lastName);

    System.out.println("Age: " + age);

    System.out.println("Salary: $" + salary);

  }

}

Design Patterns in Low Level Design

Design patterns provide proven solutions to common problems in software design. They are reusable solutions that can be applied to different scenarios. In low level design, design patterns help in structuring the code and making it more maintainable and flexible.

One commonly used design pattern in low level design is the Singleton pattern. This pattern ensures that only one instance of a class is created throughout the runtime of the application. It is useful in situations where a single instance of a class is required to be shared across different parts of the application.

Here is an example of implementing the Singleton pattern in Java:

1public class Singleton {
2  private static Singleton instance;
3
4  private Singleton() {}
5
6  public static Singleton getInstance() {
7    if (instance == null) {
8      instance = new Singleton();
9    }
10    return instance;
11  }
12
13  // Other methods
14}

In the above code, the getInstance() method ensures that only one instance of the Singleton class is created.

Another common design pattern in low level design is the Factory pattern. This pattern is used to create objects without exposing the instantiation logic to the client. It provides a way to create objects of various types based on a common interface.

Here is an example of implementing the Factory pattern in Java:

1public interface Shape {
2  void draw();
3}
4
5public class Circle implements Shape {
6  @Override
7  public void draw() {
8    System.out.println("Drawing a circle");
9  }
10}
11
12public class Rectangle implements Shape {
13  @Override
14  public void draw() {
15    System.out.println("Drawing a rectangle");
16  }
17}
18
19public class ShapeFactory {
20  public Shape createShape(String shapeType) {
21    if (shapeType.equals("circle")) {
22      return new Circle();
23    } else if (shapeType.equals("rectangle")) {
24      return new Rectangle();
25    }
26    return null;
27  }
28}

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class Main {

  public static void main(String[] args) {

    // Replace with your Java logic here

    System.out.println("Hello, world!");

  }

}

Applying Design Patterns to the Payment App

When it comes to designing and building a payment app, applying the right design patterns can greatly enhance the flexibility, scalability, and maintainability of the codebase. In this section, we will explore some design patterns that are commonly used in payment app development.

Factory Pattern

The Factory pattern is a creational design pattern that provides an interface for creating objects without specifying their exact classes. It encapsulates the object creation logic in a separate factory class, which allows the client code to be decoupled from the concrete implementation of the objects.

The Factory pattern can be particularly useful in scenarios where the payment app needs to support different types of payment methods, such as credit card, bank transfer, or mobile wallet. By using the Factory pattern, the payment app can dynamically create the appropriate payment method object based on the user's selection.

Here's an example implementation of the Factory pattern for creating payment methods in Java:

1public interface PaymentMethod {
2  void processPayment(double amount);
3}
4
5public class CreditCardPayment implements PaymentMethod {
6  @Override
7  public void processPayment(double amount) {
8    // Logic for processing credit card payment
9  }
10}
11
12public class BankTransferPayment implements PaymentMethod {
13  @Override
14  public void processPayment(double amount) {
15    // Logic for processing bank transfer payment
16  }
17}
18
19public class MobileWalletPayment implements PaymentMethod {
20  @Override
21  public void processPayment(double amount) {
22    // Logic for processing mobile wallet payment
23  }
24}
25
26public class PaymentMethodFactory {
27  public PaymentMethod createPaymentMethod(String paymentType) {
28    if (paymentType.equals("creditCard")) {
29      return new CreditCardPayment();
30    } else if (paymentType.equals("bankTransfer")) {
31      return new BankTransferPayment();
32    } else if (paymentType.equals("mobileWallet")) {
33      return new MobileWalletPayment();
34    }
35    throw new IllegalArgumentException("Invalid payment type: " + paymentType);
36  }
37}

In the above code, the PaymentMethodFactory class encapsulates the creation logic for different types of payment methods. The client code can use the factory to create the appropriate payment method object based on the payment type.

Singleton Pattern

The Singleton pattern is a creational design pattern that ensures the existence of only one instance of a class throughout the runtime of an application. It provides a global point of access to the instance, making it easy to share the same instance across different parts of the payment app.

The Singleton pattern can be useful in scenarios where there should be only one instance of certain classes, such as a transaction manager or a payment gateway. By using the Singleton pattern, the payment app can ensure that these critical components are instantiated only once and can be accessed globally.

Here's an example implementation of the Singleton pattern for a transaction manager in Java:

1public class TransactionManager {
2  private static TransactionManager instance;
3
4  private TransactionManager() {
5    // Private constructor to prevent instantiation
6  }
7
8  public static TransactionManager getInstance() {
9    if (instance == null) {
10      instance = new TransactionManager();
11    }
12    return instance;
13  }
14
15  // Other methods and properties
16}

In the above code, the TransactionManager class ensures that only one instance of itself is created using the getInstance() method.

Strategy Pattern

The Strategy pattern is a behavioral design pattern that allows the payment app to select an algorithm at runtime from a family of interchangeable algorithms. It decouples the algorithm implementation from the client code, making it easy to switch between different payment strategies without modifying the existing codebase.

The Strategy pattern can be useful in payment apps when there are different payment strategies or providers available, such as a flat fee, percentage-based fee, or third-party payment gateway. By using the Strategy pattern, the payment app can dynamically select the appropriate payment strategy at runtime based on various factors, such as the transaction amount or the selected payment method.

Here's an example implementation of the Strategy pattern for processing payments in Java:

1public interface PaymentStrategy {
2  double calculateFee(double transactionAmount);
3}
4
5public class FlatFeeStrategy implements PaymentStrategy {
6  @Override
7  public double calculateFee(double transactionAmount) {
8    // Logic for calculating flat fee
9    return 2.0;
10  }
11}
12
13public class PercentageFeeStrategy implements PaymentStrategy {
14  @Override
15  public double calculateFee(double transactionAmount) {
16    // Logic for calculating percentage-based fee
17    return transactionAmount * 0.05;
18  }
19}
20
21public class PaymentProcessor {
22  private PaymentStrategy paymentStrategy;
23
24  public void setPaymentStrategy(PaymentStrategy paymentStrategy) {
25    this.paymentStrategy = paymentStrategy;
26  }
27
28  public double processPayment(double transactionAmount) {
29    double fee = paymentStrategy.calculateFee(transactionAmount);
30    // Logic for processing payment with the selected strategy
31    return transactionAmount + fee;
32  }
33}

In the above code, the PaymentStrategy interface defines the contract for different payment strategies. The PaymentProcessor class can use any implementation of the PaymentStrategy interface to calculate the fee and process the payment.

By applying these design patterns, the payment app can benefit from improved flexibility, maintainability, and scalability. It is important to remember that the selection and application of design patterns should be based on the specific requirements and characteristics of the payment app.

1// Replace with relevant code for the payment app
2for(int i = 1; i <= 100; i++) {
3  if(i % 3 == 0 && i % 5 == 0) {
4    System.out.println("FizzBuzz");
5  } else if(i % 3 == 0) {
6    System.out.println("Fizz");
7  } else if(i % 5 == 0) {
8    System.out.println("Buzz");
9  } else {
10    System.out.println(i);
11  }
12}    

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class Main {

  public static void main(String[] args) {

    // Replace with relevant code for the payment app

  }

}

Implementing the Payment App in Java

To implement the payment app in Java, we'll start by creating the main class for our app:

1public class PaymentApp {
2  public static void main(String[] args) {
3    // Code for the payment app
4  }
5}

In the above code, we define the PaymentApp class with a main method as the entry point of our app.

Now let's create a class to represent the payment transaction:

1public class PaymentTransaction {
2  private String transactionId;
3  private double amount;
4  private PaymentMethod paymentMethod;
5
6  public PaymentTransaction(String transactionId, double amount, PaymentMethod paymentMethod) {
7    this.transactionId = transactionId;
8    this.amount = amount;
9    this.paymentMethod = paymentMethod;
10  }
11
12  public void processPayment() {
13    paymentMethod.processPayment(amount);
14  }
15
16  // Getters and setters
17}

In the above code, the PaymentTransaction class represents a single payment transaction with properties like transaction ID, amount, and the selected payment method. The processPayment method is responsible for processing the payment through the chosen payment method.

Next, let's create an interface for the payment methods:

1public interface PaymentMethod {
2  void processPayment(double amount);
3}

The PaymentMethod interface defines a contract for processing payments. Each payment method class will implement this interface and provide its own implementation logic.

Let's create a concrete implementation of the PaymentMethod interface for credit card payments:

1public class CreditCardPayment implements PaymentMethod {
2  private String cardNumber;
3  private String cvv;
4  private String expirationDate;
5
6  public CreditCardPayment(String cardNumber, String cvv, String expirationDate) {
7    this.cardNumber = cardNumber;
8    this.cvv = cvv;
9    this.expirationDate = expirationDate;
10  }
11
12  @Override
13  public void processPayment(double amount) {
14    // Logic for processing credit card payment
15    System.out.println("Processing credit card payment of $" + amount);
16  }
17
18  // Getters and setters
19}

The CreditCardPayment class is a concrete implementation of the PaymentMethod interface. It has additional properties like card number, CVV, and expiration date, and provides its own implementation logic for processing credit card payments.

Similarly, let's create classes for other payment methods like bank transfer and mobile wallet:

1public class BankTransferPayment implements PaymentMethod {
2  private String accountNumber;
3  private String routingNumber;
4
5  public BankTransferPayment(String accountNumber, String routingNumber) {
6    this.accountNumber = accountNumber;
7    this.routingNumber = routingNumber;
8  }
9
10  @Override
11  public void processPayment(double amount) {
12    // Logic for processing bank transfer payment
13    System.out.println("Processing bank transfer payment of $" + amount);
14  }
15
16  // Getters and setters
17}
18
19public class MobileWalletPayment implements PaymentMethod {
20  private String walletId;
21  private String passcode;
22
23  public MobileWalletPayment(String walletId, String passcode) {
24    this.walletId = walletId;
25    this.passcode = passcode;
26  }
27
28  @Override
29  public void processPayment(double amount) {
30    // Logic for processing mobile wallet payment
31    System.out.println("Processing mobile wallet payment of $" + amount);
32  }
33
34  // Getters and setters
35}

The BankTransferPayment and MobileWalletPayment classes provide their own implementation logic for processing bank transfer and mobile wallet payments, respectively.

Finally, let's put everything together in the main method of the PaymentApp class:

1public class PaymentApp {
2  public static void main(String[] args) {
3    // Create a payment transaction
4    PaymentTransaction transaction = new PaymentTransaction("123456", 100.0, new CreditCardPayment("1234567890", "123", "12/24"));
5
6    // Process the payment
7    transaction.processPayment();
8  }
9}

In the above code, we create a PaymentTransaction object with a transaction ID, amount, and a CreditCardPayment instance as the selected payment method. We then call the processPayment method to initiate the payment processing using the chosen payment method. Running this code will output the message "Processing credit card payment of $100.0" to the console.

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// Running this code will output the message "Processing credit card payment of $100.0" to the console.

// Implementing the Payment App in Java

​

// Let's start by creating the main class for our payment app:

​

public class PaymentApp {

  public static void main(String[] args) {

    // Code for the payment app

  }

}

​

// In the above code, we define the `PaymentApp` class with a `main` method as the entry point of our app.

// Now let's create a class to represent the payment transaction:

​

public class PaymentTransaction {

  private String transactionId;

  private double amount;

  private PaymentMethod paymentMethod;

​

  public PaymentTransaction(String transactionId, double amount, PaymentMethod paymentMethod) {

    this.transactionId = transactionId;

    this.amount = amount;

    this.paymentMethod = paymentMethod;

  }

​

  public void processPayment() {

    paymentMethod.processPayment(amount);

  }

​

  // Getters and setters