Калькулятор используя паттерн проектирования Цепочка обязанностей (Chain of Responsibility)

import java.util.Scanner;
 
public class Main {
    public static void main(String[] args) {
        Calculator calculator = new Calculator();
        Scanner sc = new Scanner(System.in);
        String inputLine = sc.nextLine();
        calculator.process(new Expression(inputLine));
    }
}

public class Calculator {
    Processor chain;
 
    public Calculator() {
        buildChain();
    }
 
    private void buildChain() {
        chain = new AdditionProcessor(new SubtractionProcessor(new MultiplicationProcessor(new DivisionProcessor(null))));
    }
 
    public void process(Expression request) {
        chain.process(request);
    }
}

public class AdditionProcessor extends Processor {
    public AdditionProcessor(Processor nextProcessor) {
        super(nextProcessor);
    }
 
    public void process(Expression request) {
        if (request.getOperation() == Operation.ADDITION) {
            double result = request.getA() + request.getB();
            System.out.println("Addition: " + request.getInput() + " = " + result);
        } else {
            super.process(request);
        }
    }
}

class SubtractionProcessor extends Processor {
    public SubtractionProcessor(Processor nextProcessor) {
        super(nextProcessor);
    }
 
    public void process(Expression request) {
        if (request.getOperation() == Operation.SUBTRACTION) {
            double result = request.getA() - request.getB();
            System.out.println("Subtraction: " + request.getInput() + " = " + result);
        } else {
            super.process(request);
        }
    }
}

class MultiplicationProcessor extends Processor {
 
    public MultiplicationProcessor(Processor nextProcessor) {
        super(nextProcessor);
    }
 
    public void process(Expression request) {
        if (request.getOperation() == Operation.MULTIPLICATION) {
            double result = request.getA() * request.getB();
            System.out.println("Multiplication: " + request.getInput() + " = " + result);
        } else {
            super.process(request);
        }
    }
}

class DivisionProcessor extends Processor {
 
    public DivisionProcessor(Processor nextProcessor) {
        super(nextProcessor);
    }
 
    public void process(Expression request) {
        if (request.getOperation() == Operation.DIVISION) {
            if (request.getB() != 0) {
                double result = request.getA() / request.getB();
                System.out.println("Division: " + request.getInput() + " = " + result);
            } else {
                throw new IllegalArgumentException("Division by zero");
            }
        } else {
            System.out.println("Check that you entered a valid operation");
        }
    }
}

public abstract class Processor {
    private Processor nextProcessor;
 
    public Processor(Processor nextProcessor) {
        this.nextProcessor = nextProcessor;
    }
 
    ;
 
    public void process(Expression request) {
        if (nextProcessor != null)
            nextProcessor.process(request);
    }
}

public class Expression {
    private double a;
    private double b;
    private Operation operation;
    private String input;
 
    public Expression(String input) {
        this.input = input;
        String[] partsOfExpression = input.split(" ");
        a = Double.parseDouble(partsOfExpression[0]);
        b = Double.parseDouble(partsOfExpression[2]);
 
        switch (partsOfExpression[1]) {
            case "+":
                operation = Operation.ADDITION;
                break;
            case "-":
                operation = Operation.SUBTRACTION;
                break;
            case "*":
                operation = Operation.MULTIPLICATION;
                break;
            case "/":
                operation = Operation.DIVISION;
                break;
        }
    }
 
    public double getB() {
        return b;
    }
 
    public Operation getOperation() {
        return operation;
    }
 
    public String getInput() {
        return input;
    }
 
    public double getA() {
        return a;
    }
}

public enum Operation {
    ADDITION,
    SUBTRACTION,
    MULTIPLICATION,
    DIVISION
}