Greetings to everyone who reads this post! Due to my work, I often run into novice programmers and look at their code. Sometimes it is beautiful, sometimes not, but I understand that a person learns, and the further he masters the best practices and technologies, the better his code becomes. But I would still like to pay some attention to such a simple task that almost all novice programmers face - writing a calculator
Here's what novice programmers write:
import java.util.Scanner;
public class Calculator {
public static void main(String[] args) {
int sum = 0;
int number1 = 0, number2 = 0;
boolean correct = true;
char operation1;
Scanner sc = new Scanner(System.in);
System.out.println("Enter first number:");
while (!sc.hasNextInt()) {
System.out.println("Sorry, it is not a digit. Try again, please");
sc.next();
}
number1 = sc.nextInt();
System.out.println("Thanks! You entered number: " + number1);
System.out.println("Enter a number:");
while (!sc.hasNextInt()) {
System.out.println("It is not a digit. Try again");
sc.next();
}
number2 = sc.nextInt();
System.out.println("Thanks! It is a digit " + number2);
do {
correct = true;
System.out.println("What would you like to do? + - / *");
operation1 = sc.next().charAt(0);
switch (operation1) {
case '+':
sum = number1 + number2;
System.out.println("Sum of this two equals to " + sum);
break;
case '-':
sum = number1 - number2;
System.out.println("The difference between equals to: " + sum);
break;
case '/':
if (number2 != 0) {
sum = number1 / number2;
System.out.println("Divied equals to: " + sum);
} else {
System.out.println("Divided by zero is not allowed.");
correct = false;
}
break;
case '*':
sum = number1 * number2;
System.out.println("Multiplication of this two: " + sum);
break;
default:
System.out.println("Unknow operation");
correct = false;
break;
}
} while (correct != true);
}
}
What problems do you see in this code? I see the following problems
- Everything is written in the main method, and certainly represents more than 10 lines
- There are no tests for this code, that is, there is no confidence that it will work correctly
- Structural programming style - the switch statement encourages this
- Using the Scanner, but it can be forgiven because it has convenient methods for getting numbers right away.
How would you fix this code? I would fix the given code as follows
- Would create an Operation interface for all operations on numbers
public interface Operation {
int execute(int a, int b);
}
And accordingly I would implement this interface in the following classes
public class Addition implements Operation {
@Override
public int execute(int a, int b) {
return a + b;
}
}
public class Deduction implements Operation {
@Override
public int execute(int a, int b) {
return a - b;
}
}
public class Division implements Operation {
@Override
public int execute(int a, int b) {
return a / b;
}
}
public class Multiplication implements Operation {
@Override
public int execute(int a, int b) {
return a * b;
}
}
I would do the same with input and output methods, but before we start writing them, let's add a very useful library - Apache Commons
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-lang3</artifactId>
<version>3.11</version>
</dependency>
The interface for the operation with input-output of information will look like this
public interface IO {
int getNumber(String message, String errorMessage);
Operation getOperation(String message);
void printResult(String result);
}
And this is how its implementation will look like
public class ConsoleIO implements IO {
private final BufferedReader bufferedReader;
private final List<String> allowedOperation;
private Map<String, Operation> operationMap;
public ConsoleIO(final List<String> allowedOperation) {
this.bufferedReader = new BufferedReader(new InputStreamReader(System.in));
this.allowedOperation = allowedOperation;
fullFillServiceMap();
}
@Override
public int getNumber(String message, String errorMessage) {
if (StringUtils.isNotBlank(message)) {
System.out.println(message);
}
boolean validNumber = false;
int numberResult = 0;
while (!validNumber) {
try {
String line = bufferedReader.readLine();
if (StringUtils.isNumeric(line)) {
numberResult = Integer.parseInt(line);
validNumber = true;
}
} catch (IOException e) {
System.out.println(errorMessage);
throw new RuntimeException(e);
}
}
return numberResult;
}
@Override
public Operation getOperation(String operationKey) {
if (this.allowedOperation == null || this.allowedOperation.isEmpty()) {
throw new IllegalStateException("Не заданы разрешенные операции");
}
boolean validOperation = false;
while (!validOperation) {
if (this.allowedOperation.contains(operationKey)) {
validOperation = true;
}
}
return operationMap.get(operationKey);
}
@Override
public void printResult(String result) {
System.out.println(result);
}
private void fullFillServiceMap() {
this.operationMap = new HashMap<>();
this.operationMap.put("+", new Addition());
this.operationMap.put("-", new Deduction());
this.operationMap.put("*", new Multiplication());
this.operationMap.put("/", new Division());
}
}
The only thing left is to write the calculator itself, that is, the operation handler
public class Calculator {
private IO io;
private Map<Integer, String> idOperationKeys;
public Calculator(IO io) {
this.io = io;
fillMap();
}
public void launch() {
boolean working = true;
while (working) {
int number = io.getNumber("1.Addition\n2.Substraction\n3.Multiplication\n4.Divison\n5.Exit", "Not valid number");
if (number == 5) {
working = false;
} else {
int result = 0;
try {
result = io.getOperation(idOperationKeys.get(number))
.execute(
io.getNumber("Enter first number", "Not valid number"),
io.getNumber("Enter second number", "Not valid number")
);
io.printResult("The result of you operation equals to " + result);
} catch (RuntimeException runtimeException) {
io.printResult(runtimeException.getMessage());
}
}
}
}
private void fillMap() {
idOperationKeys = new HashMap<>();
idOperationKeys.put(1, "+");
idOperationKeys.put(2, "-");
idOperationKeys.put(3, "*");
idOperationKeys.put(4, "/");
}
}
Well, in order for our program to finally work, we will edit the main method
public class App {
public static void main(String[] args) {
new Calculator(
new ConsoleIO(
Arrays.asList("+", "-", "*", "/")
)
).launch();
}
}
My implementation does not claim to be the best solution, however I see several advantages in it:
- This code is easier to write tests
- The code is open to add new functionality
- No dependencies on any implementation - we use abstraction for input and output of information
That's all for me, thanks for your attention!
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