# How to make an arithmetic function in Java

## The challenge

Given two numbers and an arithmetic operator (the name of it, as a string), return the result of the two numbers having that operator used on them.

`a`

and `b`

will both be positive integers, and `a`

will always be the first number in the operation, and `b`

always the second.

The four operators are “add”, “subtract”, “divide”, “multiply”.

A few examples:

```
arithmetic(5, 2, "add") => returns 7
arithmetic(5, 2, "subtract") => returns 3
arithmetic(5, 2, "multiply") => returns 10
arithmetic(5, 2, "divide") => returns 2.5
```

```
ArithmeticFunction.arithmetic(5, 2, "add") => returns 7
ArithmeticFunction.arithmetic(5, 2, "subtract") => returns 3
ArithmeticFunction.arithmetic(5, 2, "multiply") => returns 10
ArithmeticFunction.arithmetic(5, 2, "divide") => returns 2
```

## The solution in Java code

Option 1:

```
class ArithmeticFunction {
public static int arithmetic(int a, int b, String operator) {
switch(operator) {
case "add":
return a+b;
case "subtract":
return a-b;
case "multiply":
return a*b;
case "divide":
return a/b;
}
return 0;
}
}
```

Option 2:

```
class ArithmeticFunction {
public static int arithmetic(int m, int n, String s) {
return s == "add" ? m + n : s == "multiply" ? m * n : s == "subtract" ? m - n : m / n;
}
}
```

Option 3:

```
class ArithmeticFunction {
private static enum Operation {
add {
@Override
int apply(int a, int b) {
return a + b;
}
}, subtract {
@Override
int apply(int a, int b) {
return a - b;
}
}, multiply {
@Override
int apply(int a, int b) {
return a * b;
}
}, divide {
@Override
int apply(int a, int b) {
return a / b;
}
};
abstract int apply(int a, int b);
}
public static int arithmetic(int a, int b, String operator) {
return Operation.valueOf(operator).apply(a, b);
}
}
```

## Test cases to validate our solution

```
import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;
public class SolutionTest {
@Test
public void testBasic() {
assertEquals("'add' should return the two numbers added together!", 3, ArithmeticFunction.arithmetic(1, 2, "add"));
assertEquals("'subtract' should return a minus b!", 6, ArithmeticFunction.arithmetic(8, 2, "subtract"));
assertEquals("'multiply' should return a multiplied by b!", 10, ArithmeticFunction.arithmetic(5, 2, "multiply"));
assertEquals("'divide' should return a divided by b!", 4, ArithmeticFunction.arithmetic(8, 2, "divide"));
}
@Test
public void testRandom() {
String[] commands = new String[]{"add","subtract","multiply","divide"};
for (int i = 0; i < 40; i++) {
int a = randInt(0, 10);
int b = randInt(1, 10);
String op = commands[randInt(0,3)];
assertEquals("It should work for random inputs too", solution(a,b,op), ArithmeticFunction.arithmetic(a,b,op));
}
}
private static int randInt(int min, int max) {
return (int)(min + Math.random() * ((max - min) + 1));
}
private static int solution(int a, int b, String operator) {
if (operator.equals("add"))
return a + b;
if (operator.equals("subtract"))
return a - b;
if (operator.equals("multiply"))
return a * b;
return a / b;
}
}
```