2: SortNumOfComparisons

mockito-tutorial/app/src/main/java/quality/software/NaturalOrder.java

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+package quality.software;
+
+import java.util.Comparator;
+
+/**
+ * Copied from JDK23 Arrays.java
+ */
+public final class NaturalOrder implements Comparator<Object> {
+    @SuppressWarnings("unchecked")
+    public int compare(Object first, Object second) {
+        return ((Comparable<Object>) first).compareTo(second);
+    }
+
+    public static final NaturalOrder INSTANCE = new NaturalOrder();
+}

mockito-tutorial/app/src/test/java/quality/software/X2_SortNumOfComparisons.java

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+package quality.software;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Comparator;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
+
+public class X2_SortNumOfComparisons {
+    Integer[] array = {3, 2, 1};
+
+    @Test
+    public void mockingNotIdealHere() {
+        // Create a mocked Comparator.
+        // - Forward calls of Comparator.compare() to an actual comparator.
+        // - Record how many times compare() is called.
+        Comparator<Object> comparator = mock(Comparator.class);
+        when(comparator.compare(any(Integer.class), any(Integer.class)))
+                .thenAnswer(invocation ->
+                        NaturalOrder.INSTANCE.compare(
+                                invocation.getArgument(0, Integer.class),
+                                invocation.getArgument(1, Integer.class)));
+
+
+        Arrays.sort(array, comparator);
+
+        verify(comparator, times(2)).compare(any(Integer.class), any(Integer.class));
+    }
+
+    @Test
+    public void useSpyInstead(){
+        Comparator<Object> comparator = spy(NaturalOrder.INSTANCE);
+
+        Arrays.sort(array, comparator);
+
+        verify(comparator, times(2)).compare(any(), any());
+    }
+}