SeparateBuilderExample.java

PersonBuilder builder = new PersonBuilder();
Person bob = builder.firstName("Bob")
                    .lastName("Builder")
                    .age(33)
                    .description("Man, I love building stuff!")
                    .build();

Person.java

public class Person {

    private final String firstName;
    private final String lastName;
    private final String description;
    private final int age;

    public Person(String firstName, String lastName, String description, int age) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.description = description;
        this.age = age;
    }
}

PersonBuilder.java

public class PersonBuilder {
    private String firstName;
    private String lastName;
    private String description;
    private int age;

    public PersonBuilder() {
    }

    public PersonBuilder firstName(String val) {
        firstName = val;
        return this;
    }

    public PersonBuilder lastName(String val) {
        lastName = val;
        return this;
    }

    public PersonBuilder description(String val) {
        description = val;
        return this;
    }

    public PersonBuilder age(int val) {
        age = val;
        return this;
    }

    public Person build() {
        return new Person(firstName, lastName, description, age)
    }
}