Establishing the Entity Relationship

In low-level design, establishing the entity relationship is an important step in defining the structure and interactions between objects. In our payment app, there are several main entities involved:

• PaymentApp: Represents the payment application itself. It contains the user, payment methods, and transaction history.
• User: Represents a user of the payment app. It contains the username and password.
• PaymentMethod: Represents a payment method, such as a credit card or digital wallet. It contains the type and details of the payment method.
• TransactionHistory: Represents the transaction history of a user. It contains a list of transactions.
• Transaction: Represents a single transaction. It contains the order ID, payment method ID, amount, and status.

To establish the entity relationship between these objects, we can define the following relationships:

• PaymentApp has a composition relationship with User, PaymentMethod, and TransactionHistory. This means that the existence of PaymentApp depends on these entities.
• User has an association relationship with PaymentApp. This represents the relationship between a user and the payment app.
• TransactionHistory has an aggregation relationship with Transaction. This means that a transaction history is made up of multiple transactions.

By establishing these entity relationships, we can better understand how the objects interact and collaborate within our payment app.

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// Establishing the entity relationship between the objects

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public class PaymentApp {

    private User user;

    private List<PaymentMethod> paymentMethods;

    private TransactionHistory transactionHistory;

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    // ... constructors, getters, and setters

}

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public class User {

    private String username;

    private String password;

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    // ... constructors, getters, and setters

}

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public class PaymentMethod {

    private String type;

    private String details;

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    // ... constructors, getters, and setters

}

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public class TransactionHistory {

    private List<Transaction> transactions;

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    // ... constructors, getters, and setters

}