added multiplying two ComplexNumbers with imaginary parts

src/main/java/com/ugsbo/complexnumcalc/ComplexNumber.java

@@ -100,8 +100,8 @@ public class ComplexNumber {
     public ComplexNumber multiply(ComplexNumber faktor) {
         Double productRealPart, productImaginaryPart;
 
-        productRealPart = this.realPart * faktor.realPart + this.imaginaryPart * faktor.imaginaryPart * -1.0;
-        productImaginaryPart = 0.0;
+        productRealPart = this.realPart * faktor.realPart - this.imaginaryPart * faktor.imaginaryPart;
+        productImaginaryPart = this.realPart * faktor.imaginaryPart + this.imaginaryPart * faktor.realPart;
 
         ComplexNumber product = new ComplexNumber(productRealPart, productImaginaryPart);
 

src/test/java/com/ugsbo/complexnumcalc/MultiplyComplexNumbersTest.java

@@ -16,4 +16,15 @@ public class MultiplyComplexNumbersTest {
 
         assertTrue("The product is not as expected", product.equals(expected));
     }
+
+    @Test
+    public void multiplyTwoComplexNumbersWithImaginaryPart() {
+        ComplexNumber firstFaktor = new ComplexNumber(Double.valueOf(5), Double.valueOf(3));
+        ComplexNumber secoundFaktor = new ComplexNumber(Double.valueOf(6), Double.valueOf(2));
+        ComplexNumber expected = new ComplexNumber(Double.valueOf(24.0), Double.valueOf(28.0));
+
+        ComplexNumber product = firstFaktor.multiply(secoundFaktor);
+
+        assertTrue("The product is not as expected", product.equals(expected));
+    }
 }