Builder

1// Product class with many fields
2public class User {
3    private final String firstName;    // Required
4    private final String lastName;     // Required
5    private final String email;        // Required
6    private final int age;             // Optional
7    private final String phone;        // Optional
8    private final String address;      // Optional
9    private final boolean isAdmin;     // Optional
10    
11    // Private constructor - only Builder can create
12    private User(UserBuilder builder) {
13        this.firstName = builder.firstName;
14        this.lastName = builder.lastName;
15        this.email = builder.email;
16        this.age = builder.age;
17        this.phone = builder.phone;
18        this.address = builder.address;
19        this.isAdmin = builder.isAdmin;
20    }
21    
22    // Static builder class
23    public static class UserBuilder {
24        // Required parameters
25        private final String firstName;
26        private final String lastName;
27        private final String email;
28        
29        // Optional parameters with default values
30        private int age = 0;
31        private String phone = "";
32        private String address = "";
33        private boolean isAdmin = false;
34        
35        // Builder constructor with required fields
36        public UserBuilder(String firstName, String lastName, String email) {
37            this.firstName = firstName;
38            this.lastName = lastName;
39            this.email = email;
40        }
41        
42        // Fluent setters return builder for chaining
43        public UserBuilder age(int age) {
44            this.age = age;
45            return this;
46        }
47        
48        public UserBuilder phone(String phone) {
49            this.phone = phone;
50            return this;
51        }
52        
53        public UserBuilder address(String address) {
54            this.address = address;
55            return this;
56        }
57        
58        public UserBuilder isAdmin(boolean isAdmin) {
59            this.isAdmin = isAdmin;
60            return this;
61        }
62        
63        // Build method creates the final object
64        public User build() {
65            return new User(this);
66        }
67    }
68}
69
70// Usage - clean and readable
71public class Main {
72    public static void main(String[] args) {
73        // Only required fields
74        User user1 = new User.UserBuilder("John", "Doe", "john@email.com")
75            .build();
76        
77        // With optional fields - method chaining
78        User user2 = new User.UserBuilder("Jane", "Smith", "jane@email.com")
79            .age(30)
80            .phone("123-456-7890")
81            .address("123 Main St")
82            .isAdmin(true)
83            .build();
84    }
85}