builder design pattern java

1public class BankAccount {
2    public static class Builder {
3        private long accountNumber; //This is important, so we'll pass it to the constructor.
4        private String owner;
5        private String branch;
6        private double balance;
7        private double interestRate;
8        public Builder(long accountNumber) {
9            this.accountNumber = accountNumber;
10        }
11        public Builder withOwner(String owner){
12            this.owner = owner;
13            return this;  //By returning the builder each time, we can create a fluent interface.
14        }
15        public Builder atBranch(String branch){
16            this.branch = branch;
17            return this;
18        }
19        public Builder openingBalance(double balance){
20            this.balance = balance;
21            return this;
22        }
23        public Builder atRate(double interestRate){
24            this.interestRate = interestRate;
25            return this;
26        }
27        public BankAccount build(){
28            //Here we create the actual bank account object, which is always in a fully initialised state when it's returned.
29            BankAccount account = new BankAccount();  //Since the builder is in the BankAccount class, we can invoke its private constructor.
30            account.accountNumber = this.accountNumber;
31            account.owner = this.owner;
32            account.branch = this.branch;
33            account.balance = this.balance;
34            account.interestRate = this.interestRate;
35            return account;
36        }
37    }
38    //Fields omitted for brevity.
39    private BankAccount() {
40        //Constructor is now private.
41    }
42    //Getters and setters omitted for brevity.
43}
44
45BankAccount account = new BankAccount.Builder(1234L)
46            .withOwner("Marge")
47            .atBranch("Springfield")
48            .openingBalance(100)
49            .atRate(2.5)
50            .build();
51BankAccount anotherAccount = new BankAccount.Builder(4567L)
52            .withOwner("Homer")
53            .atBranch("Springfield")
54            .openingBalance(100)
55            .atRate(2.5)
56            .build();

1public class User 
2{
3    //All final attributes
4    private final String firstName; // required
5    private final String lastName; // required
6    private final int age; // optional
7    private final String phone; // optional
8    private final String address; // optional
9 
10    private User(UserBuilder builder) {
11        this.firstName = builder.firstName;
12        this.lastName = builder.lastName;
13        this.age = builder.age;
14        this.phone = builder.phone;
15        this.address = builder.address;
16    }
17 
18    //All getter, and NO setter to provde immutability
19    public String getFirstName() {
20        return firstName;
21    }
22    public String getLastName() {
23        return lastName;
24    }
25    public int getAge() {
26        return age;
27    }
28    public String getPhone() {
29        return phone;
30    }
31    public String getAddress() {
32        return address;
33    }
34 
35    @Override
36    public String toString() {
37        return "User: "+this.firstName+", "+this.lastName+", "+this.age+", "+this.phone+", "+this.address;
38    }
39 
40    public static class UserBuilder 
41    {
42        private final String firstName;
43        private final String lastName;
44        private int age;
45        private String phone;
46        private String address;
47 
48        public UserBuilder(String firstName, String lastName) {
49            this.firstName = firstName;
50            this.lastName = lastName;
51        }
52        public UserBuilder age(int age) {
53            this.age = age;
54            return this;
55        }
56        public UserBuilder phone(String phone) {
57            this.phone = phone;
58            return this;
59        }
60        public UserBuilder address(String address) {
61            this.address = address;
62            return this;
63        }
64        //Return the finally consrcuted User object
65        public User build() {
66            User user =  new User(this);
67            validateUserObject(user);
68            return user;
69        }
70        private void validateUserObject(User user) {
71            //Do some basic validations to check 
72            //if user object does not break any assumption of system
73        }
74    }
75}
76