Open-Closed Principle (OCP)

• states that software entities should be open for extension, but closed for modification

Ways to Implement OCP

Interface Example

Interfaces are one way to follow OCP

Non-Compliant Way

public interface CalculatorOperation {}

public class Addition implements CalculatorOperation {
    private double left;
    private double right;
    private double result = 0.0;

    public Addition(double left, double right) {
        this.left = left;
        this.right = right;
    }

    // getters and setters
}

public class Subtraction implements CalculatorOperation {
    private double left;
    private double right;
    private double result = 0.0;

    public Subtraction(double left, double right) {
        this.left = left;
        this.right = right;
    }

    // getters and setters
}

public class Calculator {
    public void calculate(CalculatorOperation operation) {
        if (operation instanceof Addition) {
            Addition addition = (Addition) operation;
            addition.setResult(addition.getLeft() + addition.getRight());
        } else if (operation instanceof Subtraction) {
            Subtraction subtraction = (Subtraction) operation;
            subtraction.setResult(subtraction.getLeft() - subtraction.getRight());
        }
    }
}

When a new requirement of adding multiplication or divide functionality comes in, we’ve no way besides changing the calculate method of the Calculator class

Compliant Way

public interface CalculatorOperation {
    void perform();
}

public class Addition implements CalculatorOperation {
    private double left;
    private double right;
    private double result;

    // constructor, getters and setters

    @Override
    public void perform() {
        result = left + right;
    }
}

public class Division implements CalculatorOperation {
    private double left;
    private double right;
    private double result;

    // constructor, getters and setters

    @Override
    public void perform() {
        if (right != 0) {
            result = left / right;
        }
    }
}

public class Calculator {
    public void calculate(CalculatorOperation operation) {
        if (operation == null) {
            throw new InvalidParameterException("Cannot perform operation");
        }
        operation.perform();
    }
}

Our Calculator class doesn’t need to implement new logic as we introduce new operators

Dependency Injection

see Dependency Injection Principle (DIP) - Inversion of Control (IoC)

Resources

• https://www.baeldung.com/java-open-closed-principle