#1313 Add Separated Interface design pattern

separated-interface/src/test/java/com/iluwatar/separatedinterface/invoice/InvoiceGeneratorTest.java

@@ -0,0 +1,25 @@
+package com.iluwatar.separatedinterface.invoice;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import static org.mockito.Mockito.*;
+
+public class InvoiceGeneratorTest {
+
+  private InvoiceGenerator target;
+
+  @Test
+  public void testGenerateTax() {
+    var productCost = 50.0;
+    var tax = 10.0;
+    TaxCalculator taxCalculatorMock = mock(TaxCalculator.class);
+    doReturn(tax).when(taxCalculatorMock).calculate(productCost);
+
+    target = new InvoiceGenerator(productCost, taxCalculatorMock);
+
+    Assertions.assertEquals(target.getAmountWithTax(), productCost + tax);
+    verify(taxCalculatorMock, times(1)).calculate(productCost);
+  }
+
+}