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Customer Domain Model

A simple data class that represents a customer from the database. Uses a Java record to avoid writing boilerplate code.

Overview

A domain model is a class that represents data in your application. Instead of passing around raw database values, we create a Customer object that holds the id, first name, and last name together.

Why Use a Record?

A record is a special Java class (added in Java 14) that automatically generates code for you.

Without a record, you'd have to write:

public class Customer {
    private final long id;
    private final String firstName;
    private final String lastName;

    // Constructor
    public Customer(long id, String firstName, String lastName) {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
    }

    // Getters
    public long id() { return id; }
    public String firstName() { return firstName; }
    public String lastName() { return lastName; }

    // equals(), hashCode(), toString() - even more code!
}

With a record, one line does all of that:

public record Customer(long id, String firstName, String lastName) { }

What the record gives you automatically:

  • Constructor that takes all fields
  • Getter methods: id(), firstName(), lastName()
  • equals() - compares all fields
  • hashCode() - generates hash from all fields
  • toString() - prints all field values

@Override and toString()

What is @Override?

@Override is an annotation that tells the compiler: "I'm replacing a method that exists in the parent class."

@Override
public String toString() {
    // my custom version
}

Why use it?

  • The compiler checks that you're actually overriding something
  • If you make a typo (like tostring() instead of toString()), the compiler catches it
  • Makes code clearer - you can see this method replaces an inherited one

Why Override toString()?

Records give you a default toString() that looks like:

Customer[id=3, firstName=Josh, lastName=Bloch]

But the guide overrides it to use a custom format:

Customer[id=3, firstName='Josh', lastName='Bloch']

The custom version adds quotes around the string values. This is a style choice - you might want different formatting for logging or debugging.

String.format()

String.format() creates a string by filling in placeholders with values.

String.format("Customer[id=%d, firstName='%s', lastName='%s']", id, firstName, lastName)

The placeholders:

  • %d = decimal number (for long, int)
  • %s = string (for String and most objects)

How it works:

  1. First %d gets replaced with id
  2. First %s gets replaced with firstName
  3. Second %s gets replaced with lastName

Example:

String.format("Hello %s, you are %d years old", "Alice", 25)
// Result: "Hello Alice, you are 25 years old"

The Complete Customer Record

public record Customer(long id, String firstName, String lastName) {

    @Override
    public String toString() {
        return String.format(
                "Customer[id=%d, firstName='%s', lastName='%s']",
                id, firstName, lastName);
    }
}

Line by line:

  1. public record Customer(...) - defines a record with three fields
  2. @Override - we're replacing the default toString()
  3. public String toString() - method that returns a string representation
  4. String.format(...) - creates the formatted string
  5. The format string with %d and %s placeholders
  6. id, firstName, lastName - values that fill the placeholders

In This Module

Mapping to Database

Record Field DB Column Type
id id long
firstName first_name String
lastName last_name String

Note: Java uses camelCase (firstName), databases typically use snake_case (first_name). The RowMapper handles this translation.

RowMapper Lambda

(rs, rowNum) -> new Customer(
    rs.getLong("id"),
    rs.getString("first_name"),
    rs.getString("last_name")
)

This lambda converts each database row into a Customer object:

  • rs = ResultSet (the current row from the database)
  • rowNum = which row number (we don't use it here)
  • rs.getLong("id") = get the id column as a long
  • rs.getString("first_name") = get the first_name column as a String

Related Concepts

References