FunctionalBuilder.java

package foo;

import java.util.function.Consumer;

public class Foo {

    public static void main(String[] args) {

        Foo foo = new Foo($ -> $
                .firstName("John")
                .lastName("Doe")
                .age(42)
        );
        System.out.println(foo.getFirstName());

        // Will throw an exception (see Builder's constructor below)
        // Foo foo2 = new Foo();
        // Foo.Builder builder2 = foo2.new Builder();
        // builder2.age(10);
    }

    // ------------------------------------------------------------------------------------------

    private String firstName;
    private String lastName;
    private int age;

    public Foo() {
    }

    public Foo(Consumer<Builder> builder) {
        builder.accept(new Builder());
    }

    // Look ma, no setters here !

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public int getAge() {
        return age;
    }

    public class Builder {

        public Builder() {
            // Builder Will throw an exception if called outside Foo's constructor (optional but cleaner IMHO)
            Class<?> caller = StackWalker.getInstance(StackWalker.Option.RETAIN_CLASS_REFERENCE).getCallerClass();
            if (caller != Foo.class) {
                throw new IllegalStateException("Cannot call builder outside the constructor");
            }
        }

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

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

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