Specification Pattern

The Specification Pattern is a software design pattern that allows you to encapsulate business rules and criteria. This pattern is particularly useful when you need to create complex queries or filters without having to modify the underlying data model.

• Encapsulation of Business Logic: The pattern encapsulates complex business rules into reusable specifications.
• Separation of Concerns: It promotes separation between the specification logic and the core business logic.
• Composable Specifications: Specifications can be combined using logical operators (AND, OR, NOT).

3.1 Defining the Specification Interface

interface Specification {
    boolean isSatisfiedBy(T candidate);
    Specification and(Specification other);
    Specification or(Specification other);
    Specification not();
}

3.2 Implementing Concrete Specifications

class AgeSpecification implements Specification {
    private final int age;

    public AgeSpecification(int age) {
        this.age = age;
    }

    @Override
    public boolean isSatisfiedBy(Person candidate) {
        return candidate.getAge() >= age;
    }

    @Override
    public Specification and(Specification other) {
        return new AndSpecification<>(this, other);
    }

    @Override
    public Specification or(Specification other) {
        return new OrSpecification<>(this, other);
    }

    @Override
    public Specification not() {
        return new NotSpecification<>(this);
    }
}

3.3 Using the Specification

List people = ...; // Assume this is a list of people
Specification ageSpec = new AgeSpecification(18);
List adults = people.stream()
    .filter(ageSpec::isSatisfiedBy)
    .collect(Collectors.toList());

• Keep specifications simple and focused on a single responsibility.
• Use composition to create more complex specifications from simpler ones.
• Document the specifications to clarify their purpose and usage.