Builder pattern aims to “Separate the construction of a complex object from its representation so that the same construction process can create multiple different representations.”

Lombok’s @Builder annotation is a useful technique to implement the builder pattern.

Example code

User.java

public class User
{
	//All final attributes
	private final String firstName; // required
	private final String lastName; // required
	private final int age; // optional
	private final String phone; // optional
	private final String address; // optional

	private User(UserBuilder builder) {
		this.firstName = builder.firstName;
		this.lastName = builder.lastName;
		this.age = builder.age;
		this.phone = builder.phone;
		this.address = builder.address;
	}

	//All getter, and NO setter to provde immutability
	public String getFirstName() {
		return firstName;
	}
	public String getLastName() {
		return lastName;
	}
	public int getAge() {
		return age;
	}
	public String getPhone() {
		return phone;
	}
	public String getAddress() {
		return address;
	}

	@Override
	public String toString() {
		return "User: "+this.firstName+", "+this.lastName+", "+this.age+", "+this.phone+", "+this.address;
	}

	public static class UserBuilder
	{
		private final String firstName;
		private final String lastName;
		private int age;
		private String phone;
		private String address;

		public UserBuilder(String firstName, String lastName) {
			this.firstName = firstName;
			this.lastName = lastName;
		}
		public UserBuilder age(int age) {
			this.age = age;
			return this;
		}
		public UserBuilder phone(String phone) {
			this.phone = phone;
			return this;
		}
		public UserBuilder address(String address) {
			this.address = address;
			return this;
		}
		//Return the finally consrcuted User object
		public User build() {
			User user =  new User(this);
			validateUserObject(user);
			return user;
		}
		private void validateUserObject(User user) {
			//Do some basic validations to check
			//if user object does not break any assumption of system
		}
	}
}

UserBuilder.java

public static void main(String[] args) 
{
	User user1 = new User.UserBuilder("Lokesh", "Gupta")
	.age(30)
	.phone("1234567")
	.address("Fake address 1234")
	.build();

	System.out.println(user1);

	User user2 = new User.UserBuilder("Jack", "Reacher")
	.age(40)
	.phone("5655")
	//no address
	.build();

	System.out.println(user2);

	User user3 = new User.UserBuilder("Super", "Man")
	//No age
	//No phone
	//no address
	.build();

	System.out.println(user3);
}

Existing Implementations in JDK

All implementations of java.lang.Appendable are infact good examples of the use of the Builder pattern in Java. e.g.

• java.lang.StringBuilder#append() [Unsynchronized class]
• java.lang.StringBuffer#append() [Synchronized class]
• java.nio.ByteBuffer#put() (also on CharBuffer, ShortBuffer, IntBuffer, LongBuffer, FloatBuffer and DoubleBuffer)
• Another use can be found in javax.swing.GroupLayout.Group#addComponent().

StringBuilder builder = new StringBuilder("Temp");

String data = builder.append(1)
		.append(true)
		.append("friend")
		.toString();