fix(calculator): fix calculator.

Calculator.java

@@ -18,18 +18,57 @@ class Calculator {
     }
 
     public void subtract(int value) {
-        this.result += value;
+        this.result -= value;
     }
 
     public void multiply(int value) {
-        for (int i=1; i<value; i++) {
-            add(value);
+        // Multiply with zero.
+        if (0 == value) {
+            this.result = 0;
+
+            return;
+        }
+
+        int currentResult = this.result;
+        int currentAbsoluteResult = getAbsoluteValue(currentResult);
+        int absoluteValue = getAbsoluteValue(value);
+
+        this.result = currentAbsoluteResult;
+
+        for (int i = 1; i < absoluteValue; i++) {
+            add(currentAbsoluteResult);
+        }
+
+        boolean isPositiveNumberMultiplyWithNegativeNumber = (value > 0 && currentResult < 0)
+                || (value < 0 && currentResult > 0);
+
+        if (isPositiveNumberMultiplyWithNegativeNumber) {
+            this.result = -this.result;
         }
     }
 
     public void exponent(int value) {
+        if (value < 0) {
+            throw new RuntimeException("Does not support.");
+        }
+
+        // Zero exponent
+        if (0 == value) {
+            this.result = 1;
+        }
+
+        int currentResult = this.result;
+
         for (int i = 1; i < value; i++) {
-            multiply(value);
+            multiply(currentResult);
+        }
     }
+
+    private int getAbsoluteValue(int value) {
+        if (value < 0) {
+            value = -value;
+        }
+
+        return value;
     }
 }

CalculatorTest.java

+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CalculatorTest {
+
+    @Test
+    void reset() {
+        Calculator calculator = new Calculator();
+        calculator.add(10);
+        calculator.reset();
+
+        assertEquals(0, calculator.getResult());
+    }
+
+    @Test
+    void getResult() {
+        Calculator calculator = new Calculator();
+
+        assertEquals(0, calculator.getResult());
+    }
+
+    @Test
+    void add() {
+        Calculator calculator = new Calculator();
+        calculator.add(10);
+        calculator.add(7);
+        calculator.add(-25);
+
+        assertEquals(-8, calculator.getResult());
+    }
+
+    @Test
+    void subtract() {
+        Calculator calculator = new Calculator();
+        calculator.add(10);
+        calculator.add(7);
+        calculator.subtract(18);
+        calculator.subtract(-5);
+
+        assertEquals(4, calculator.getResult());
+    }
+
+    @Test
+    void multiplyPositiveNumberWithNegativeNumber() {
+        Calculator calculator = new Calculator();
+        calculator.add(10);
+        calculator.multiply(-2);
+
+        assertEquals(-20, calculator.getResult());
+    }
+
+    @Test
+    void multiplyPositiveNumberWithPositiveNumber() {
+        Calculator calculator = new Calculator();
+        calculator.add(10);
+        calculator.multiply(2);
+
+        assertEquals(20, calculator.getResult());
+    }
+
+    @Test
+    void multiplyNegativeNumberWithNegativeNumber() {
+        Calculator calculator = new Calculator();
+        calculator.add(-10);
+        calculator.multiply(-2);
+
+        assertEquals(20, calculator.getResult());
+    }
+
+    @Test
+    void multiplyNegativeNumberWithZero() {
+        Calculator calculator = new Calculator();
+        calculator.add(-10);
+        calculator.multiply(0);
+
+        assertEquals(0, calculator.getResult());
+    }
+
+    @Test
+    void multiplyPositiveNumberWithZero() {
+        Calculator calculator = new Calculator();
+        calculator.add(10);
+        calculator.multiply(0);
+
+        assertEquals(0, calculator.getResult());
+    }
+
+    @Test
+    void multiplyNegativeNumberWithOne() {
+        Calculator calculator = new Calculator();
+        calculator.add(-10);
+        calculator.multiply(1);
+
+        assertEquals(-10, calculator.getResult());
+    }
+
+    @Test
+    void multiplyPositiveNumberWithOne() {
+        Calculator calculator = new Calculator();
+        calculator.add(10);
+        calculator.multiply(1);
+
+        assertEquals(10, calculator.getResult());
+    }
+
+    @Test
+    void multiplyNegativeNumberWithMinusOne() {
+        Calculator calculator = new Calculator();
+        calculator.add(-10);
+        calculator.multiply(-1);
+
+        assertEquals(10, calculator.getResult());
+    }
+
+    @Test
+    void multiplyPositiveNumberWithMinusOne() {
+        Calculator calculator = new Calculator();
+        calculator.add(10);
+        calculator.multiply(-1);
+
+        assertEquals(-10, calculator.getResult());
+    }
+
+    @Test
+    void exponentPositiveNumberWithPositiveNumber() {
+        Calculator calculator = new Calculator();
+        calculator.add(2);
+        calculator.exponent(3);
+
+        assertEquals(8, calculator.getResult());
+    }
+
+    @Test
+    void exponentPositiveNumberWithOne() {
+        Calculator calculator = new Calculator();
+        calculator.add(2);
+        calculator.exponent(1);
+
+        assertEquals(2, calculator.getResult());
+    }
+
+    @Test
+    void exponentPositiveNumberWithZero() {
+        Calculator calculator = new Calculator();
+        calculator.add(2);
+        calculator.exponent(0);
+
+        assertEquals(1, calculator.getResult());
+    }
+
+    @Test
+    void exponentNegativeNumberWithPositiveNumber() {
+        Calculator calculator = new Calculator();
+        calculator.add(-2);
+        calculator.exponent(3);
+
+        assertEquals(-8, calculator.getResult());
+    }
+
+    @Test
+    void exponentNegativeNumberWithOne() {
+        Calculator calculator = new Calculator();
+        calculator.add(-2);
+        calculator.exponent(1);
+
+        assertEquals(-2, calculator.getResult());
+    }
+
+    @Test
+    void exponentNegativeNumberWithZero() {
+        Calculator calculator = new Calculator();
+        calculator.add(-2);
+        calculator.exponent(0);
+
+        assertEquals(1, calculator.getResult());
+    }
+
+    @Test
+    void exponentNumberWithNegativeNumber() {
+        Calculator calculator = new Calculator();
+        calculator.add(2);
+
+        assertThrows(RuntimeException.class, () -> {calculator.exponent(-1);});
+    }
+}