java builder pattern example

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();