Calculator Simples

Calculator.java

import java.util.Arrays;
import java.util.List;

public class Calculator {

    public static void main(String[] args) {
        Calculator calculator = new Calculator();
        //Double result = calculator.calcWithArray(Arrays.asList("5", "*", "4", "+", "2", "-", "9", "+", "7", "/", "2")); // 10
        Double result = calculator.calcWithArray(Arrays.asList("1", "/", "0"));
        System.out.println(result);
    }

    public Double calcWithArray(List<String> values) {

        Double total = null;

        if (values == null || values.isEmpty() || values.size() < 3) {
            throw new IllegalArgumentException("Nothing to do!");
        }

        Double actualValue1 = null;
        Operation actualOperation = null;
        Double actualValue2 = null;

        for (String value : values) {
            if (isCharOperation(value)) {
                actualOperation = Operation.fromCharactere(value);
            } else {
                if (actualValue1 == null) {
                    actualValue1 = Double.parseDouble(value);
                } else if (actualValue2 == null) {
                    actualValue2 = Double.parseDouble(value);
                }
            }

            // Se existem parametros para realizar um calculo, faremos
            if (actualValue1 != null && actualValue2 != null && actualOperation != null) {
                // Acrescentando ao resultado
                total = 0.0D;
                total += calc(actualValue1, actualValue2, actualOperation);
                actualValue1 = null;
                actualValue2 = null;
                actualOperation = null;
            } else if (total != null && actualOperation != null && actualValue1 != null) {
                total = calc(total, actualValue1, actualOperation);
                actualValue1 = null;
                actualValue2 = null;
                actualOperation = null;
            }
        }
        return total;
    }

    Double calc(Double value1, Double value2, Operation operation) {
        Double total = null;
        switch (operation) {
        case SOMA:
            total = value1 + value2;
            break;
        case SUBTRACAO:
            total = value1 - value2;
            break;
        case MULTIPLICACAO:
            total = value1 * value2;
            break;
        case DIVISAO:
            if (value2 == 0D) {
                throw new IllegalArgumentException("Division by Zero!");
            }
            total = value1 / value2;
            break;
        default:
            System.out.println("Operation undefined!");
        }
        return total;
    }

    Boolean isCharOperation(String value) {
        return Operation.fromCharactere(value) != null;
    }
}

enum Operation {
    SOMA("+"),
    SUBTRACAO("-"),
    MULTIPLICACAO("*"),
    DIVISAO("/");

    private String value;

    private Operation(String value) {
        this.value = value;
    }

    public static Operation fromCharactere(String op) {
        for (Operation operation : Operation.values() ) {
            if (operation.value.equals(op)) {
                return operation;
            }
        }
        return null;
    }
}