diff --git a/JavaOOP/src/Java27.java b/JavaOOP/src/Java27.java
index 163a3dfa..8294368d 100644
--- a/JavaOOP/src/Java27.java
+++ b/JavaOOP/src/Java27.java
@@ -1,4 +1,4 @@
-import myObject.A;
+import poly.A;
 
 public class Java27 {
     public static void main(String[] args) {
diff --git a/JavaOOP/src/Java28.java b/JavaOOP/src/Java28.java
new file mode 100644
index 00000000..daa08c3d
--- /dev/null
+++ b/JavaOOP/src/Java28.java
@@ -0,0 +1,20 @@
+import myObject.A;
+import myObject.B;
+
+public class Java28 {
+    public static void main(String[] args) {
+        A a = new A();
+        display(a);
+
+        B b = new B();
+        display(b);
+    }
+
+    private static void display(Object o) {
+        if (o instanceof A) {
+            ((A) o).go();
+        } else {
+            ((B) o).go();
+        }
+    }
+}
diff --git a/JavaOOP/src/Java29.java b/JavaOOP/src/Java29.java
new file mode 100644
index 00000000..7990f13b
--- /dev/null
+++ b/JavaOOP/src/Java29.java
@@ -0,0 +1,31 @@
+import myObject.A;
+import myObject.B;
+
+public class Java29 {
+    public static void main(String[] args) {
+        // A, B 클래스를 저장할 배열
+        Object[] o = new Object[2];
+        o[0] = new A();
+        o[1] = new B();
+
+        for (int i = 0; i < o.length; i++) {
+            if (o[i] instanceof A) {
+                ((A) o[i]).go();
+            } else {
+                ((B) o[i]).go();
+            }
+        }
+
+        printGo(o);
+    }
+
+    private static void printGo(Object[] o) {
+        for (int i = 0; i < o.length; i++) {
+            if (o[i] instanceof A) {
+                ((A) o[i]).go();
+            } else {
+                ((B) o[i]).go();
+            }
+        }
+    }
+}
diff --git a/JavaOOP/src/myObject/A.java b/JavaOOP/src/myObject/A.java
new file mode 100644
index 00000000..daa55c0b
--- /dev/null
+++ b/JavaOOP/src/myObject/A.java
@@ -0,0 +1,7 @@
+package myObject;
+
+public class A {
+    public void go() {
+        System.out.println("A의 go 메소드");
+    }
+}
diff --git a/JavaOOP/src/myObject/B.java b/JavaOOP/src/myObject/B.java
new file mode 100644
index 00000000..ce0710d3
--- /dev/null
+++ b/JavaOOP/src/myObject/B.java
@@ -0,0 +1,7 @@
+package myObject;
+
+public class B {
+    public void go() {
+        System.out.println("B의 go 메소드");
+    }
+}
diff --git a/JavaOOP/src/poly/A.java b/JavaOOP/src/poly/A.java
index eb326cfb..dc2c943c 100644
--- a/JavaOOP/src/poly/A.java
+++ b/JavaOOP/src/poly/A.java
@@ -1,4 +1,4 @@
-package myObject;
+package poly;
 
 public class A{