BAEL-7396, Instantiate an Inner Class With Reflection in Java

core-java-modules/core-java-reflection-3/src/main/java/com/baeldung/reflection/innerclass/Person.java

@@ -0,0 +1,20 @@
+package com.baeldung.reflection.innerclass;
+
+public class Person {
+    String name;
+    Address address;
+
+    public Person() {
+    }
+
+    public class Address {
+        String zip;
+
+        public Address(String zip) {
+            this.zip = zip;
+        }
+    }
+
+    public static class Builder {
+    }
+}

core-java-modules/core-java-reflection-3/src/test/java/com/baeldung/reflection/innerclass/CreateInnerClassWithReflectionUnitTest.java

@@ -0,0 +1,67 @@
+package com.baeldung.reflection.innerclass;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
+import java.util.Arrays;
+import java.util.stream.Collectors;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class CreateInnerClassWithReflectionUnitTest {
+
+    static Logger logger = LoggerFactory.getLogger(CreateInnerClassWithReflectionUnitTest.class);
+
+    @Test
+    void givenInnerClass_whenUseReflection_thenShowConstructors() {
+        final String personBuilderClassName = "com.baeldung.reflection.innerclass.Person$Builder";
+        final String personAddressClassName = "com.baeldung.reflection.innerclass.Person$Address";
+        assertDoesNotThrow(() -> logConstructors(Class.forName(personAddressClassName)));
+        assertDoesNotThrow(() -> logConstructors(Class.forName(personBuilderClassName)));
+    }
+
+    private static void logConstructors(Class<?> clazz) {
+        Arrays.stream(clazz.getDeclaredConstructors())
+            .map(c -> formatConstructorSignature(c))
+            .forEach(logger::info);
+    }
+
+    private static String formatConstructorSignature(Constructor<?> constructor) {
+        String params = Arrays.stream(constructor.getParameters())
+            .map(parameter -> parameter.getType().getSimpleName() + " " + parameter.getName())
+            .collect(Collectors.joining(", "));
+        return constructor.getName() + "(" + params + ")";
+    }
+
+    @Test
+    void givenStaticInnerClass_whenUseReflection_thenInstantiate()
+        throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException,
+            InstantiationException, IllegalAccessException {
+        final String personBuilderClassName = "com.baeldung.reflection.innerclass.Person$Builder";
+        Class<Person.Builder> personBuilderClass = (Class<Person.Builder>) Class.forName(personBuilderClassName);
+        Person.Builder personBuilderObj = personBuilderClass.getDeclaredConstructor().newInstance();
+        assertTrue(personBuilderObj instanceof Person.Builder);
+    }
+
+    @Test
+    void givenNonStaticInnerClass_whenUseReflection_thenInstantiate()
+        throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException,
+            InstantiationException, IllegalAccessException {
+        final String personClassName = "com.baeldung.reflection.innerclass.Person";
+        final String personAddressClassName = "com.baeldung.reflection.innerclass.Person$Address";
+
+        Class<Person> personClass = (Class<Person>) Class.forName(personClassName);
+        Person personObj = personClass.getConstructor().newInstance();
+
+        Class<Person.Address> personAddressClass = (Class<Person.Address>) Class.forName(personAddressClassName);
+
+        assertThrows(NoSuchMethodException.class, () -> personAddressClass.getDeclaredConstructor(String.class));
+
+        Constructor<Person.Address> constructorOfPersonAddress = personAddressClass.getDeclaredConstructor(Person.class, String.class);
+        Person.Address personAddressObj = constructorOfPersonAddress.newInstance(personObj, "751003");
+        assertTrue(personAddressObj instanceof Person.Address);
+    }
+
+}