diff --git a/JavaOOP/src/Java21.java b/JavaOOP/src/Java21.java
new file mode 100644
index 00000000..6bd158cb
--- /dev/null
+++ b/JavaOOP/src/Java21.java
@@ -0,0 +1,22 @@
+import myObject.Animal;
+import myObject.Cat;
+import myObject.Dog;
+
+public class Java21 {
+    public static void main(String[] args) {
+        // 1. 다형성 인수
+        Dog dog = new Dog();
+        Cat cat = new Cat();
+
+        display(dog);
+        display(cat);
+    }
+
+    public static void display(Animal animal) { // 다형성 인수 upcasting
+        animal.eat();
+        // Cat 타입 인지 확인 instanceof -> Cat 타입 일때만 실행
+        if (animal instanceof Cat) {
+            ((Cat) animal).say(); // downcasting
+        }
+    }
+}
diff --git a/JavaOOP/src/Java22.java b/JavaOOP/src/Java22.java
new file mode 100644
index 00000000..1003571f
--- /dev/null
+++ b/JavaOOP/src/Java22.java
@@ -0,0 +1,19 @@
+import myObject.Animal;
+import myObject.Cat;
+import myObject.Dog;
+
+public class Java22 {
+    public static void main(String[] args) {
+        // 2. 다형성 배열
+        Animal[] animals = new Animal[2];
+        animals[0] = new Dog();
+        animals[1] = new Cat();
+
+        for (Animal animal : animals) {
+            animal.eat();
+            if (animal instanceof Cat) {
+                ((Cat) animal).say();
+            }
+        }
+    }
+}