diff --git a/practice/extends1/Child.java b/practice/extends1/Child.java
new file mode 100644
index 000000000..dd71b2ac2
--- /dev/null
+++ b/practice/extends1/Child.java
@@ -0,0 +1,12 @@
+package extends1;
+
+public class Child extends Parent {
+    public Child() {
+        System.out.println("Child Constructor");
+    }
+
+    @Override
+    public void printProtectedName() {
+        System.out.println("Parent printProtectedName()");
+    }
+}
diff --git a/practice/extends1/ChildArg.java b/practice/extends1/ChildArg.java
new file mode 100644
index 000000000..b7448489a
--- /dev/null
+++ b/practice/extends1/ChildArg.java
@@ -0,0 +1,13 @@
+package extends1;
+
+public class ChildArg extends ParentArg {
+    public ChildArg() {
+        super("ChildArgName ~~~ ");
+        System.out.println("Child Constructor");
+    }
+
+    @Override
+    public void printName() {
+        System.out.println("ChildArg printName() Overriding");
+    }
+}
diff --git a/practice/extends1/Friend.java b/practice/extends1/Friend.java
new file mode 100644
index 000000000..10e284e7c
--- /dev/null
+++ b/practice/extends1/Friend.java
@@ -0,0 +1,7 @@
+package extends1;
+
+public class Friend {
+    public Friend() {
+        System.out.println("Friend Constructor");
+    }
+}
diff --git a/practice/extends1/GrandChild.java b/practice/extends1/GrandChild.java
new file mode 100644
index 000000000..e3e9384c4
--- /dev/null
+++ b/practice/extends1/GrandChild.java
@@ -0,0 +1,11 @@
+package extends1;
+
+public class GrandChild extends Child {
+    public GrandChild() {
+        System.out.println("GrandChild Constructor");
+    }
+
+    public void print() {
+        System.out.println("GrandChild!!!!!");
+    }
+}
diff --git a/practice/extends1/Parent.java b/practice/extends1/Parent.java
new file mode 100644
index 000000000..05afb8a51
--- /dev/null
+++ b/practice/extends1/Parent.java
@@ -0,0 +1,19 @@
+package extends1;
+
+public class Parent {
+    public Parent() {
+        System.out.println("Parent Constructor");
+    }
+
+    public void printName() {
+        System.out.println("Parent printName()");
+    }
+
+    private void printPrivateName() {
+        System.out.println("Parent printPrivateName()");
+    }
+
+    protected void printProtectedName() {
+        System.out.println("Parent printProtectedName()");
+    }
+}
diff --git a/practice/extends1/ParentArg.java b/practice/extends1/ParentArg.java
new file mode 100644
index 000000000..6439b7cd2
--- /dev/null
+++ b/practice/extends1/ParentArg.java
@@ -0,0 +1,12 @@
+package extends1;
+
+public class ParentArg {
+
+    public ParentArg(String name) {
+        System.out.println("ParentArg("+name+") Constructor");
+    }
+
+    public void printName() {
+        System.out.println("ParentArg printName()");
+    }
+}
diff --git a/practice/extends1/RunExtends.java b/practice/extends1/RunExtends.java
new file mode 100644
index 000000000..a56108167
--- /dev/null
+++ b/practice/extends1/RunExtends.java
@@ -0,0 +1,30 @@
+package extends1;
+
+public class RunExtends {
+  public static void main(String[] args) {
+    // 1. 매개변수가 없는 경우
+    Child child = new Child();
+//    child.printName();
+////    child.printPrivateName(); 불가능!
+    child.printProtectedName();
+
+    // 2. 매개변수가 있는 경우
+    ChildArg childArg = new ChildArg();
+    childArg.printName();
+
+//    // 3. 손자의 경우
+//    GrandChild grandChild = new GrandChild();
+//    grandChild.printProtectedName();
+
+    // 4. 상속 구조가 아닌 경우
+    Friend friend = new Friend();
+//    friend.printProtectedName(); 불가능!
+
+    // 5. 형변환
+    // (1) 가능
+    Parent parent = new Child();
+
+    // (2) 불가능
+    Child child = new Parent();
+  }
+}