Build an object from an existing one using lombok

问题

Lets say I have a lombok annotated class like

@Builder
class Band {
   String name;
   String type;
}

I know I can do:

Band rollingStones = Band.builder().name("Rolling Stones").type("Rock Band").build();

Is there an easy way to create an object of Foo using the existing object as a template and changing one of it's properties?

Something like:

Band nirvana = Band.builder(rollingStones).name("Nirvana");

I can't find this in the lombok documentation.

回答1:

You can use the toBuilder parameter to give your instances a toBuilder() method.

@Builder(toBuilder=true)
class Foo {
   int x;
   ...
}

Foo f0 = Foo.builder().build();
Foo f1 = f0.toBuilder().x(42).build();

From the documentation:

If using @Builder to generate builders to produce instances of your own class (this is always the case unless adding @Builder to a method that doesn't return your own type), you can use @Builder(toBuilder = true) to also generate an instance method in your class called toBuilder(); it creates a new builder that starts out with all the values of this instance.

Disclaimer: I am a lombok developer.

回答2:

Is there an easy way to create an object of Foo using the existing object as a template and changing one of it's properties? (emphasis mine)

If you really want to change a single property, then there's a nicer and more efficient way:

@Wither
class Band {
   String name;
   String type;
}

Band nirvana = rollingStones.withName("Nirvana");

The wither creates no garbage, but it can change just a single field. For changing many fields, you could use

withA(a).withB(b).withC(c)....

and produce tons of garbage (all intermediate results) but than toBuilder is more efficient and more natural.

来源：https://stackoverflow.com/questions/47069561/build-an-object-from-an-existing-one-using-lombok

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java

Lombok