Thread: Builder pattern

I am currently reading the book effective java and there is section where a class has a private constructor and an inner class assigns parameters to the outer class.WHY to do that and when i try to create setter, getter methods to retrieve data they don't work.Below is the code:

public class NutritionFacts {
    private final int servingSize;
    private final int servings;
    private final int calories;
    private final int fat;
    private final int sodium;
    private final int carbohydrate;
 
    public static class Builder {
        // Required parameters
        private final int servingSize;
        private final int servings;
 
        // Optional parameters - initialized to default values
        private int calories      = 0;
        private int fat           = 0;
        private int carbohydrate  = 0;
        private int sodium        = 0;
 
        public Builder(int servingSize, int servings) {
            this.servingSize = servingSize;
            this.servings    = servings;
        }
 
        public Builder calories(int val)
            { calories = val;      return this; }
        public Builder fat(int val)
            { fat = val;           return this; }
        public Builder carbohydrate(int val)
            { carbohydrate = val;  return this; }
        public Builder sodium(int val)
            { sodium = val;        return this; }
 
        public NutritionFacts build() {
            return new NutritionFacts(this);
        }
    }
 
    private NutritionFacts(Builder builder) {
        servingSize  = builder.servingSize;
        servings     = builder.servings;
        calories     = builder.calories;
        fat          = builder.fat;
        sodium       = builder.sodium;
        carbohydrate = builder.carbohydrate;
    }
}

HOW I WILL BE ABLE TO RETRIEVE DATA FROM THAT CLASS???WHY SOMEONE WANTS TO DO THAT?? i have learned with the public constructor i can pass the params i want and retrieve the data easily with getter methods, why to do it so complicate?

You're right, as it stands NutritionFacts is useless because doesn't expose any of its information. I'm guessing that's because it was simplified for the book's example, and the intent is to add more functionality, or at least field accessors where it can be passed to an algorithm to do things like calculate how much insulin should be taken. The point is, NutritionFacts has so many inputs (servingSize, servings, calories, carbs, fat, sodium) before it's state is ready, and some are optional, that creating constructors for all the possible combinations would be a trainwreck (and in this case impossible for all combinations because of int ambiguity). Maybe in the future a calculation must be ran before NutritionFacts is ready to be used. With this pattern, you don't get a NutritionFacts object without calling the build() method, which makes things a bit less error-prone.