How to Properly Implement the Builder Pattern in Java as Explained in Effective Java

Question

What are common errors when implementing the Builder pattern in Java?

public class NutritionalFacts {
    private int sodium;
    private int fat;
    private int carbo;

    public static class Builder {
        private int sodium;
        private int fat;
        private int carbo;

        public Builder(int s) {
            this.sodium = s;
        }

        public Builder fat(int f) {
            this.fat = f;
            return this;
        }

        public Builder carbo(int c) {
            this.carbo = c;
            return this;
        }

        public NutritionalFacts build() {
            return new NutritionalFacts(this);
        }
    }

    private NutritionalFacts(Builder b) {
        this.sodium = b.sodium;
        this.fat = b.fat;
        this.carbo = b.carbo;
    }
}

Answer

The Builder pattern is a creational design pattern used to simplify object creation, particularly for classes with multiple attributes. In your implementation, you have encountered a compilation error due to a scoping issue with the Builder class.

public class NutritionalFacts {
    private int sodium;
    private int fat;
    private int carbo;

    public static class Builder { // Make Builder static
        private int sodium;
        private int fat;
        private int carbo;

        public Builder(int s) {
            this.sodium = s;
        }

        public Builder fat(int f) {
            this.fat = f;
            return this;
        }

        public Builder carbo(int c) {
            this.carbo = c;
            return this;
        }

        public NutritionalFacts build() {
            return new NutritionalFacts(this);
        }
    }

    private NutritionalFacts(Builder b) {
        this.sodium = b.sodium;
        this.fat = b.fat;
        this.carbo = b.carbo;
    }
} // Updated code snippet to fix the error.

Causes

The Builder class is not static, which means it cannot be instantiated without an instance of the enclosing class `NutritionalFacts`.
The default constructor of `NutritionalFacts` is private, which can only be accessed through the `Builder` class.

Solutions

Make the `Builder` class a static nested class to allow it to be instantiated without an instance of `NutritionalFacts`. This allows easier access to the Builder for creating `NutritionalFacts` objects.
Ensure that the attributes in the Builder class are consistently named and initialized to avoid confusion.

Common Mistakes

Mistake: Not making the Builder class static.

Solution: If the Builder is static, it can be used without needing an instance of the enclosing class.

Mistake: Overlooking the use of proper constructors in the main class.

Solution: Ensure Builder is correctly referenced and that main method correctly creates an instance.

Helpers

Builder pattern
Effective Java
NutritionalFacts class
Java design patterns
Java compilation error