Example of Builder as an inner static class. Enclosing class has private constructor, which prevents direct instantiation and allows creation only trough the uilder.

InnerBuilderExample.java

public class Person {

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

    private Person(Builder builder) {
        firstName = builder.firstName;
        lastName = builder.lastName;
        description = builder.description;
        age = builder.age;
    }

    public static final class Builder {
        private String firstName;
        private String lastName;
        private String description;
        private int age;

        public Builder() {
        }

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

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

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

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

        public Person build() {
            return new Person(this);
        }
    }
}