BAEL-6874 test code for article (#15118)

* BAEL-6819 convert from int to Long in Java

* BAEL-6819 update package name

* BAEL-6874 test code for article

* BAEL-6874 rename test class

core-java-modules/core-java-numbers-6/src/test/java/com/baeldung/genericnumberscomparator/GenericNumbersComparatorUnitTest.java

@@ -0,0 +1,122 @@
+package com.baeldung.genericnumberscomparator;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.lang.reflect.Method;
+import java.lang.reflect.Proxy;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Optional;
+import java.util.function.BiFunction;
+import java.util.function.BiPredicate;
+import java.util.function.Function;
+
+import org.junit.jupiter.api.Test;
+
+class GenericNumbersComparatorUnitTest {
+
+    public int compareDouble(Number num1, Number num2) {
+        return Double.compare(num1.doubleValue(), num2.doubleValue());
+    }
+
+    @Test
+    void givenNumbers_whenUseCompareDouble_thenWillExecuteComparison() {
+        assertEquals(0, compareDouble(5, 5.0));
+    }
+
+    public int compareTo(Integer int1, Integer int2) {
+        return int1.compareTo(int2);
+    }
+
+    @Test
+    void givenNumbers_whenUseCompareTo_thenWillExecuteComparison() {
+        assertEquals(-1, compareTo(5, 7));
+    }
+
+    Map<Class<? extends Number>, BiFunction<Number, Number, Integer>> comparisonMap = new HashMap<>();
+
+    public int compareUsingMap(Number num1, Number num2) {
+        comparisonMap.put(Integer.class, (a, b) -> ((Integer) num1).compareTo((Integer) num2));
+
+        return comparisonMap.get(num1.getClass())
+            .apply(num1, num2);
+    }
+
+    @Test
+    void givenNumbers_whenUseCompareUsingMap_thenWillExecuteComparison() {
+        assertEquals(-1, compareUsingMap(5, 7));
+    }
+
+    public interface NumberComparator {
+        int compare(Number num1, Number num2);
+    }
+
+    @Test
+    void givenNumbers_whenUseProxy_thenWillExecuteComparison() {
+        NumberComparator proxy = (NumberComparator) Proxy.newProxyInstance(NumberComparator.class.getClassLoader(), new Class[] { NumberComparator.class },
+            (p, method, args) -> Double.compare(((Number) args[0]).doubleValue(), ((Number) args[1]).doubleValue()));
+        assertEquals(0, proxy.compare(5, 5.0));
+    }
+
+    public int compareUsingReflection(Number num1, Number num2) throws Exception {
+        Method method = num1.getClass()
+            .getMethod("compareTo", num1.getClass());
+        return (int) method.invoke(num1, num2);
+    }
+
+    @Test
+    void givenNumbers_whenUseCompareUsingReflection_thenWillExecuteComparison() throws Exception {
+        assertEquals(-1, compareUsingReflection(5, 7));
+    }
+
+    interface NumberComparatorFactory {
+        Comparator<Number> getComparator();
+    }
+
+    class IntegerComparatorFactory implements NumberComparatorFactory {
+        @Override
+        public Comparator<Number> getComparator() {
+            return (num1, num2) -> ((Integer) num1).compareTo((Integer) num2);
+        }
+    }
+
+    @Test
+    void givenNumbers_whenUseComparatorFactory_thenWillExecuteComparison() {
+        NumberComparatorFactory factory = new IntegerComparatorFactory();
+        Comparator<Number> comparator = factory.getComparator();
+        assertEquals(-1, comparator.compare(5, 7));
+    }
+
+    Function<Number, Double> toDouble = Number::doubleValue;
+    BiPredicate<Number, Number> isEqual = (num1, num2) -> toDouble.apply(num1)
+        .equals(toDouble.apply(num2));
+
+    @Test
+    void givenNumbers_whenUseIsEqual_thenWillExecuteComparison() {
+        assertEquals(true, isEqual.test(5, 5.0));
+    }
+
+    private Number someNumber = 5;
+    private Number anotherNumber = 5.0;
+
+    Optional<Number> optNum1 = Optional.ofNullable(someNumber);
+    Optional<Number> optNum2 = Optional.ofNullable(anotherNumber);
+    int comparisonResult = optNum1.flatMap(n1 -> optNum2.map(n2 -> Double.compare(n1.doubleValue(), n2.doubleValue())))
+        .orElse(0);
+
+    @Test
+    void givenNumbers_whenUseComparisonResult_thenWillExecuteComparison() {
+        assertEquals(0, comparisonResult);
+    }
+
+    private boolean someCondition = true;
+    Function<Number, ?> dynamicFunction = someCondition ? Number::doubleValue : Number::intValue;
+    Comparator<Number> dynamicComparator = (num1, num2) -> ((Comparable) dynamicFunction.apply(num1)).compareTo(dynamicFunction.apply(num2));
+
+    @Test
+    void givenNumbers_whenUseDynamicComparator_thenWillExecuteComparison() {
+        assertEquals(0, dynamicComparator.compare(5, 5.0));
+    }
+
+}