Added the multiply functionality for ComplexNumbers it can multiply ComplexNumbers without an Imaginary part

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

@@ -91,4 +91,21 @@ public class ComplexNumber {
         return differenz;
     }
 
+    /**
+     * Multiplies the faktor with this Instance.
+     * 
+     * @param faktor The ComplexNumber by wich this Instance will get multiplyed
+     * @return The product of this Instance and the faktor
+     */
+    public ComplexNumber multiply(ComplexNumber faktor) {
+        Double productRealPart, productImaginaryPart;
+
+        productRealPart = this.realPart * faktor.realPart + this.imaginaryPart * faktor.imaginaryPart * -1.0;
+        productImaginaryPart = 0.0;
+
+        ComplexNumber product = new ComplexNumber(productRealPart, productImaginaryPart);
+
+        return product;
+    }
+
 }

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

@@ -0,0 +1,19 @@
+package com.ugsbo.complexnumcalc;
+
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+public class MultiplyComplexNumbersTest {
+
+    @Test
+    public void multiplyTwoComplexNumbersWithoutImaginaryPart() {
+        ComplexNumber firstFaktor = new ComplexNumber(Double.valueOf(5), Double.valueOf(0));
+        ComplexNumber secoundFaktor = new ComplexNumber(Double.valueOf(6), Double.valueOf(0));
+        ComplexNumber expected = new ComplexNumber(Double.valueOf(30), Double.valueOf(0));
+
+        ComplexNumber product = firstFaktor.multiply(secoundFaktor);
+
+        assertTrue("The product is not as expected", product.equals(expected));
+    }
+}