Added example and test for method ref.

core-java/functional-programming/functional-interfaces/src/test/java/io/reflectoring/function/custom/MethodReferenceTest.java

@@ -0,0 +1,53 @@
+package io.reflectoring.function.custom;
+
+import java.math.BigInteger;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+/** The Method reference test. */
+public class MethodReferenceTest {
+  @Test
+  void staticMethodReference() {
+    List<Integer> numbers = List.of(1, -2, 3, -4, 5);
+    List<Integer> positiveNumbers = numbers.stream().map(Math::abs).toList();
+    positiveNumbers.forEach(
+        number -> Assertions.assertTrue(number > 0, "Number should be positive."));
+  }
+
+  @Test
+  void instanceMethodReference() {
+    List<String> numbers = List.of("One", "Two", "Three");
+    List<Integer> numberChars = numbers.stream().map(String::length).toList();
+    numberChars.forEach(length -> Assertions.assertTrue(length > 0, "Number text is not empty."));
+  }
+
+  @Test
+  void instanceMethodArbitraryObjectParticularType() {
+    List<Number> numbers = List.of(1, 2L, 3.0f, 4.0d);
+    List<Integer> numberIntValues = numbers.stream().map(Number::intValue).toList();
+    Assertions.assertEquals(List.of(1, 2, 3, 4), numberIntValues, "Int values are not same.");
+  }
+
+  @Test
+  void constructorReference() {
+    List<String> numbers = List.of("1", "2", "3");
+    Map<String, BigInteger> numberMapping =
+        numbers.stream()
+            .map(BigInteger::new)
+            .collect(Collectors.toMap(BigInteger::toString, Function.identity()));
+    Map<String, BigInteger> expected =
+        new HashMap<>() {
+          {
+            put("1", BigInteger.valueOf(1));
+            put("2", BigInteger.valueOf(2));
+            put("3", BigInteger.valueOf(3));
+          }
+        };
+    Assertions.assertEquals(expected, numberMapping, "Mapped numbers do not match.");
+  }
+}