Builder with deep clone

[Builder with copy]

Hello all, 

Sometimes when working with immutable object you need to create a copy and changing a couple of filed.
What would be nice is to have a deep cloner in the builder constructor something like : 

Person twin1 = Person.builder()
    .firstname("twin 1")
    .lastName("last name")
    .age(10)
    .build();

Person twin2 = Person.builder(twin1)
    .firstName("twin 2")
    .build();

Let me know what you think of it. 

[alejandropg]

+1 I think this is a great idea because is a very common scenery.

[Martin Grajcar]

This already exists as toBuilder(). But the second snippet can instead be written using @Wither as

Person twin2 = twin1.withFirstName("twin 2");

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[alejandropg]

True! I hadn't noticed

A complete example:

package com.alex.example.lombok
import lombok.Builder;
import lombok.Data;
import lombok.NonNull;

@Data @Builder(toBuilder = true)
public class PostalCode {
    @NonNull private final String province;
    @NonNull private final String postalCode;
}

package com.alex.example.lombok;
import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class PostalCodeTest {
    @Test
    public void build_new_instance_in_the_same_package() {
        final PostalCode postalCode = new PostalCode("province", "pc");

        assertEquals("province", postalCode.getProvince());
        assertEquals("pc", postalCode.getPostalCode());
    }

    @Test
    public void build_with_builder() {
        final PostalCode postalCode = PostalCode.builder().province("province").postalCode("pc").build();

        assertEquals("province", postalCode.getProvince());
        assertEquals("pc", postalCode.getPostalCode());
    }

    @Test
    public void clone_with_builder() {
        final PostalCode postalCode = PostalCode.builder().province("province").postalCode("pc").build();

        final PostalCode cloned = postalCode.toBuilder().province("another province").build();

        assertEquals("another province", cloned.getProvince());
        assertEquals("pc", cloned.getPostalCode());
    }
}

The syntax is a little strange and not too common. Usually is in the form:

PostalCode.builder(original).province("another value").build()

or even

PostalCode.builder().clone(original).province("another value").build()

Anyway I don't understand why is necessary the @Wither if the builder exists.

Thanks!

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[Martin Grajcar]

Anyway I don't understand why is necessary the @Wither if the builder exists.

It's not. But it's handy when you need no builder or when you want to change a single property or two. Both

person.withFirstname("a").withLastname("b")

and

person.toBuilder().firstname("a").lastname("b").build()

create one throw-away object and I'd argue that the former is slightly more readable. For more than two changes, the builder creates less garbage.

[alejandropg]

I understand 👍

And why the toBuilder() implementation? there is any performance reason? Maybe in this way you don't need to copy all the original object?

[Martin Grajcar]

Having both is a bit redundant, but obviously a long "withX" chain is no good idea as you copy the object N times. For a single change, the builder is an overkill and produces garbage. So they both have a case when they perform better.

Moreover, @Wither predates @Builder and it used to be the only way how to keep sanity with many immutables. @Builder is much more complicated, and toBuilder() is IMHO a simple but powerful improvement.

Note that you need to ask for it explicitly using @Builder(toBuilder=true). You titled you question "deep clone", but all you get a shallow copy (which is just fine for immutables).

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[alejandropg]

Yep, I did not create the thread, and you're right that is not the best name.

For me this thread can be closed.

Thanks a lot!

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