How to create a semantically immutable object with many fields

Question

I have an object with 25 fields. It has no logic, just keep the values. It is built with an abstract builder. I do not want something to change this object after assembly. I want all fields to be final, but I don't want to create a 25-params constructor. What model should be used in this situation? Now I'm thinking of local setter packages, but this is worse than syntax checking for all the values ​​given in the final fields. I can’t pack these fields into 2-3 objects

Answer 1

I see three main options:

• Have a private class that only the builder knows about, and an open interface with only getters. The builder issues links using an interface, not a class.

• You have two classes: one modified (which is part of the Messenger class) and one that is immutable, which accepts the changed in its constructor and captures its fields.

• The class has reasonable default values ​​for all fields, and then there are setters that return a new instance of the class with this set of fields. The disadvantage here is that to create a 25-field instance, you create ~ 24 drop objects, depending on how many reasonable default values ​​you need to change .; -)

Answer 2

Assign the Builder class as a static nested class in the Entity class. Then the Builder class can set fields directly, and you don’t need the configuration methods or the n-arg constructor in Entity .

 public class Entity { // Just one attribute and getter here; could be 25. private int data; public int getData() { return data; } public static class Builder { // Just one attribute and getter here; could be 25. private int theData; public Entity build() { Entity entity = new Entity(); // Set private field(s) here. entity.data = theData; return entity; } public Builder setData(int someData) { theData = someData; return this; } } } 

Using:

 Entity entity = new Entity.Builder().setData(42).build();