From 9037fb958d890995835e86328b7f1a51be0825a7 Mon Sep 17 00:00:00 2001
From: daadaadaah <daadaadaah@gmail.com>
Date: Mon, 20 Feb 2023 19:30:22 +0900
Subject: [PATCH 1/9] =?UTF-8?q?=EA=B8=B0=EB=B3=B8=EC=83=9D=EC=84=B1?=
 =?UTF-8?q?=EC=9E=90=20=EC=9E=90=EB=8F=99=20=ED=98=B8=EC=B6=9C=20=EB=B0=8F?=
 =?UTF-8?q?=20=EB=B6=80=EB=AA=A8=20=ED=81=B4=EB=9E=98=EC=8A=A4=EC=97=90=20?=
 =?UTF-8?q?=EC=9E=88=EB=8A=94=20=EB=A9=94=EC=86=8C=EB=93=9C=20=EC=9E=90?=
 =?UTF-8?q?=EC=8B=9D=EC=9D=B4=20=EC=82=AC=EC=9A=A9=ED=95=A0=20=EC=88=98=20?=
 =?UTF-8?q?=EC=9E=88=EC=9D=8C?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

- Parant 클래스를 호출하지 않았는데도 부모 클래스의 기본 생성자가 호출 된다.
- 자식 클래스에 구현하지 않았는데도 부모 클래스에 있는 printName() 메소드를 자식 클래스에서 사용할 수 있다.

> 출력 결과
Parent Constructor
Child Constructor
Parent printName()
---
 practice/extends1/Child.java      |  7 +++++++
 practice/extends1/Parent.java     | 11 +++++++++++
 practice/extends1/RunExtends.java |  8 ++++++++
 3 files changed, 26 insertions(+)
 create mode 100644 practice/extends1/Child.java
 create mode 100644 practice/extends1/Parent.java
 create mode 100644 practice/extends1/RunExtends.java

diff --git a/practice/extends1/Child.java b/practice/extends1/Child.java
new file mode 100644
index 000000000..10e6d2ee3
--- /dev/null
+++ b/practice/extends1/Child.java
@@ -0,0 +1,7 @@
+package extends1;
+
+public class Child extends Parent {
+    public Child() {
+        System.out.println("Child Constructor");
+    }
+}
diff --git a/practice/extends1/Parent.java b/practice/extends1/Parent.java
new file mode 100644
index 000000000..f778f3d1b
--- /dev/null
+++ b/practice/extends1/Parent.java
@@ -0,0 +1,11 @@
+package extends1;
+
+public class Parent {
+    public Parent() {
+        System.out.println("Parent Constructor");
+    }
+
+    public void printName() {
+        System.out.println("Parent printName()");
+    }
+}
diff --git a/practice/extends1/RunExtends.java b/practice/extends1/RunExtends.java
new file mode 100644
index 000000000..aaac1f53b
--- /dev/null
+++ b/practice/extends1/RunExtends.java
@@ -0,0 +1,8 @@
+package extends1;
+
+public class RunExtends {
+  public static void main(String[] args) {
+    Child child = new Child();
+    child.printName();
+  }
+}

From 5d47349b51eb1a05538539e2666a25a1dd6cc160 Mon Sep 17 00:00:00 2001
From: daadaadaah <daadaadaah@gmail.com>
Date: Mon, 20 Feb 2023 19:40:51 +0900
Subject: [PATCH 2/9] =?UTF-8?q?public,=20protected=20=EB=A1=9C=20=EC=84=A0?=
 =?UTF-8?q?=EC=96=B8=EB=90=9C=20=EB=A9=94=EC=86=8C=EB=93=9C=EB=A7=8C=20?=
 =?UTF-8?q?=EC=9E=90=EC=8B=9D=20=ED=81=B4=EB=9E=98=EC=8A=A4=EA=B0=80=20?=
 =?UTF-8?q?=EC=82=AC=EC=9A=A9=ED=95=A0=20=EC=88=98=20=EC=9E=88=EA=B3=A0,?=
 =?UTF-8?q?=20private=20=EB=A1=9C=20=EC=84=A0=EC=96=B8=EB=90=9C=20?=
 =?UTF-8?q?=EA=B1=B4=20=EC=9E=90=EC=8B=9D=EB=8F=84=20=EB=AA=BB=20=EC=82=AC?=
 =?UTF-8?q?=EC=9A=A9=ED=95=A8.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

> 출력 결과
Parent Constructor
Child Constructor
Parent printName()
Parent printProtectedName()
---
 practice/extends1/Child.java      | 2 ++
 practice/extends1/Parent.java     | 8 ++++++++
 practice/extends1/RunExtends.java | 2 ++
 3 files changed, 12 insertions(+)

diff --git a/practice/extends1/Child.java b/practice/extends1/Child.java
index 10e6d2ee3..246074722 100644
--- a/practice/extends1/Child.java
+++ b/practice/extends1/Child.java
@@ -4,4 +4,6 @@ public class Child extends Parent {
     public Child() {
         System.out.println("Child Constructor");
     }
+
+
 }
diff --git a/practice/extends1/Parent.java b/practice/extends1/Parent.java
index f778f3d1b..05afb8a51 100644
--- a/practice/extends1/Parent.java
+++ b/practice/extends1/Parent.java
@@ -8,4 +8,12 @@ public Parent() {
     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/RunExtends.java b/practice/extends1/RunExtends.java
index aaac1f53b..133257376 100644
--- a/practice/extends1/RunExtends.java
+++ b/practice/extends1/RunExtends.java
@@ -4,5 +4,7 @@ public class RunExtends {
   public static void main(String[] args) {
     Child child = new Child();
     child.printName();
+//    child.printPrivateName(); 불가능!
+    child.printProtectedName();
   }
 }

From e429058a5426cd4e7c504637686e6e5f41a8cdb5 Mon Sep 17 00:00:00 2001
From: daadaadaah <daadaadaah@gmail.com>
Date: Mon, 20 Feb 2023 19:55:25 +0900
Subject: [PATCH 3/9] =?UTF-8?q?=EB=B6=80=EB=AA=A8=ED=81=B4=EB=9E=98?=
 =?UTF-8?q?=EC=8A=A4=EC=97=90=20=EB=A7=A4=EA=B0=9C=20=EB=B3=80=EC=88=98?=
 =?UTF-8?q?=EA=B0=80=20=EC=9E=88=EB=8A=94=20=EC=83=9D=EC=84=B1=EC=9E=90?=
 =?UTF-8?q?=EB=A7=8C=20=EC=9E=88=EC=9D=84=20=EA=B2=BD=EC=9A=B0,=20super?=
 =?UTF-8?q?=EB=A5=BC=20=EC=9D=B4=EC=9A=A9=ED=95=B4=EC=84=9C=20=EB=B6=80?=
 =?UTF-8?q?=EB=AA=A8=20=EC=83=9D=EC=84=B1=EC=9E=90=EB=A5=BC=20=EA=BC=AD=20?=
 =?UTF-8?q?=ED=98=B8=EC=B6=9C=ED=95=B4=EC=A4=98=EC=95=BC=ED=95=9C=EB=8B=A4?=
 =?UTF-8?q?.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 practice/extends1/ChildArg.java   |  9 +++++++++
 practice/extends1/ParentArg.java  | 12 ++++++++++++
 practice/extends1/RunExtends.java | 14 ++++++++++----
 3 files changed, 31 insertions(+), 4 deletions(-)
 create mode 100644 practice/extends1/ChildArg.java
 create mode 100644 practice/extends1/ParentArg.java

diff --git a/practice/extends1/ChildArg.java b/practice/extends1/ChildArg.java
new file mode 100644
index 000000000..4af26ac66
--- /dev/null
+++ b/practice/extends1/ChildArg.java
@@ -0,0 +1,9 @@
+package extends1;
+
+public class ChildArg extends ParentArg {
+    public ChildArg() {
+        super("ChildArgName ~~~ ");
+        System.out.println("Child Constructor");
+    }
+
+}
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
index 133257376..2d1941cb1 100644
--- a/practice/extends1/RunExtends.java
+++ b/practice/extends1/RunExtends.java
@@ -2,9 +2,15 @@
 
 public class RunExtends {
   public static void main(String[] args) {
-    Child child = new Child();
-    child.printName();
-//    child.printPrivateName(); 불가능!
-    child.printProtectedName();
+    // 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();
+
   }
 }

From aa42a43b79d4edf7126c933e7fb0f6248a5d3ba1 Mon Sep 17 00:00:00 2001
From: daadaadaah <daadaadaah@gmail.com>
Date: Mon, 20 Feb 2023 19:58:02 +0900
Subject: [PATCH 4/9] =?UTF-8?q?=EC=9E=90=EC=8B=9D=20=ED=81=B4=EB=9E=98?=
 =?UTF-8?q?=EC=8A=A4=EC=97=90=EC=84=9C=20=EB=B6=80=EB=AA=A8=20=ED=81=B4?=
 =?UTF-8?q?=EB=9E=98=EC=8A=A4=EC=9D=98=20=EB=A9=94=EC=84=9C=EB=93=9C?=
 =?UTF-8?q?=EB=A5=BC=20Overriding=20=ED=95=98=EB=A9=B4,=20=EC=9E=90?=
 =?UTF-8?q?=EC=8B=9D=20=ED=81=B4=EB=9E=98=EC=8A=A4=EC=97=90=20=EA=B5=AC?=
 =?UTF-8?q?=ED=98=84=EB=90=9C=20=EB=A9=94=EC=86=8C=EB=93=9C=EA=B0=80=20?=
 =?UTF-8?q?=ED=98=B8=EC=B6=9C=EB=90=9C=EB=8B=A4.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

> 출력 결과
[Overriding 전]
ParentArg(ChildArgName ~~~ ) Constructor
Child Constructor
ParentArg printName()

[Overriding 후]
ParentArg(ChildArgName ~~~ ) Constructor
Child Constructor
ChildArg printName() Overriding
---
 practice/extends1/ChildArg.java | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/practice/extends1/ChildArg.java b/practice/extends1/ChildArg.java
index 4af26ac66..b7448489a 100644
--- a/practice/extends1/ChildArg.java
+++ b/practice/extends1/ChildArg.java
@@ -6,4 +6,8 @@ public ChildArg() {
         System.out.println("Child Constructor");
     }
 
+    @Override
+    public void printName() {
+        System.out.println("ChildArg printName() Overriding");
+    }
 }

From aa7861f3ceb1b0859e7187eee0235cae079cb2af Mon Sep 17 00:00:00 2001
From: daadaadaah <daadaadaah@gmail.com>
Date: Mon, 20 Feb 2023 23:52:20 +0900
Subject: [PATCH 5/9] =?UTF-8?q?=EC=86=90=EC=9E=90=EC=9D=98=20=EA=B2=BD?=
 =?UTF-8?q?=EC=9A=B0=EC=97=90=EB=8F=84=20Parent=EC=9D=98=20protected=20?=
 =?UTF-8?q?=EB=A9=94=EC=84=9C=EB=93=9C=20=EC=A0=91=EA=B7=BC=20=EA=B0=80?=
 =?UTF-8?q?=EB=8A=A5=ED=95=98=EB=8B=A4.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 practice/extends1/GrandChild.java | 11 +++++++++++
 practice/extends1/RunExtends.java |  3 +++
 2 files changed, 14 insertions(+)
 create mode 100644 practice/extends1/GrandChild.java

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/RunExtends.java b/practice/extends1/RunExtends.java
index 2d1941cb1..9b8a3562f 100644
--- a/practice/extends1/RunExtends.java
+++ b/practice/extends1/RunExtends.java
@@ -12,5 +12,8 @@ public static void main(String[] args) {
     ChildArg childArg = new ChildArg();
     childArg.printName();
 
+    // 3. 손자의 경우
+    GrandChild grandChild = new GrandChild();
+    grandChild.printProtectedName();
   }
 }

From 23851c02a54d6a3c0cc91b71feae735755fe3802 Mon Sep 17 00:00:00 2001
From: daadaadaah <daadaadaah@gmail.com>
Date: Tue, 21 Feb 2023 00:06:58 +0900
Subject: [PATCH 6/9] =?UTF-8?q?protected=EB=A1=9C=20=EC=84=A0=EC=96=B8?=
 =?UTF-8?q?=EB=90=9C=20=EB=A9=94=EC=84=9C=EB=93=9C=EB=8A=94=20=EC=83=81?=
 =?UTF-8?q?=EC=86=8D=20=EA=B5=AC=EC=A1=B0=EA=B0=80=20=EC=95=84=EB=8B=8C=20?=
 =?UTF-8?q?=ED=81=B4=EB=9E=98=EC=8A=A4=EC=97=90=EC=84=9C=EB=8A=94=20?=
 =?UTF-8?q?=EC=A0=91=EA=B7=BC=20=EB=B6=88=EA=B0=80=EB=8A=A5=ED=95=98?=
 =?UTF-8?q?=EB=8B=A4.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 practice/extends1/Friend.java     | 7 +++++++
 practice/extends1/RunExtends.java | 4 ++++
 2 files changed, 11 insertions(+)
 create mode 100644 practice/extends1/Friend.java

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/RunExtends.java b/practice/extends1/RunExtends.java
index 9b8a3562f..068ede503 100644
--- a/practice/extends1/RunExtends.java
+++ b/practice/extends1/RunExtends.java
@@ -15,5 +15,9 @@ public static void main(String[] args) {
     // 3. 손자의 경우
     GrandChild grandChild = new GrandChild();
     grandChild.printProtectedName();
+
+    // 4. 상속 구조가 아닌 경우
+    Friend friend = new Friend();
+//    friend.printProtectedName(); 불가능!
   }
 }

From ff04ff218cffdeed30b140c9f4e1e5a5f0c3eaab Mon Sep 17 00:00:00 2001
From: daadaadaah <daadaadaah@gmail.com>
Date: Tue, 21 Feb 2023 00:25:59 +0900
Subject: [PATCH 7/9] =?UTF-8?q?=EB=B6=80=EB=AA=A8=20=ED=81=B4=EB=9E=98?=
 =?UTF-8?q?=EC=8A=A4=20=EB=B3=B4=EB=8B=A4=20=EC=A0=91=EA=B7=BC=20=EC=A0=9C?=
 =?UTF-8?q?=EC=96=B4=EC=9E=90=EA=B0=80=20=EC=B6=95=EC=86=8C=EB=90=98?=
 =?UTF-8?q?=EB=8A=94=20=EA=B2=83=EC=9D=80=20=EB=AC=B8=EC=A0=9C=EA=B0=80=20?=
 =?UTF-8?q?=EB=90=9C=EB=8B=A4.=20protected=20->=20pacakge-private=EB=A1=9C?=
 =?UTF-8?q?=20=EC=B6=95=EC=86=8C=20=EB=90=A8?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 practice/extends1/Child.java | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/practice/extends1/Child.java b/practice/extends1/Child.java
index 246074722..a0ee972fa 100644
--- a/practice/extends1/Child.java
+++ b/practice/extends1/Child.java
@@ -4,6 +4,9 @@ public class Child extends Parent {
     public Child() {
         System.out.println("Child Constructor");
     }
-
-
+    
+    @Override
+    void printProtectedName() { //  attempting to assign weaker access privileges ('package-private'); was 'protected' 라고 하면서 에러 남!
+        System.out.println("Parent printProtectedName()");
+    }
 }

From 071717ed6c11fecb2f1b6691e32460bb358eb5a6 Mon Sep 17 00:00:00 2001
From: daadaadaah <daadaadaah@gmail.com>
Date: Tue, 21 Feb 2023 00:27:07 +0900
Subject: [PATCH 8/9] =?UTF-8?q?=EB=B6=80=EB=AA=A8=20=ED=81=B4=EB=9E=98?=
 =?UTF-8?q?=EC=8A=A4=EB=B3=B4=EB=8B=A4=20=EC=A0=91=EA=B7=BC=20=EC=A0=9C?=
 =?UTF-8?q?=EC=96=B4=EC=9E=90=EA=B0=80=20=EB=8D=94=20=ED=99=95=EB=8C=80?=
 =?UTF-8?q?=EB=90=98=EB=8A=94=20=EA=B2=83=EC=9D=80=20=EB=AC=B8=EC=A0=9C=20?=
 =?UTF-8?q?=EB=90=98=EC=A7=80=20=EC=95=8A=EB=8A=94=EB=8B=A4.=20protected?=
 =?UTF-8?q?=20->=20public?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 practice/extends1/Child.java | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/practice/extends1/Child.java b/practice/extends1/Child.java
index a0ee972fa..dd71b2ac2 100644
--- a/practice/extends1/Child.java
+++ b/practice/extends1/Child.java
@@ -4,9 +4,9 @@ public class Child extends Parent {
     public Child() {
         System.out.println("Child Constructor");
     }
-    
+
     @Override
-    void printProtectedName() { //  attempting to assign weaker access privileges ('package-private'); was 'protected' 라고 하면서 에러 남!
+    public void printProtectedName() {
         System.out.println("Parent printProtectedName()");
     }
 }

From ef8320964d4e0a96201525159e5849d42c8c96f2 Mon Sep 17 00:00:00 2001
From: daadaadaah <daadaadaah@gmail.com>
Date: Tue, 21 Feb 2023 00:31:13 +0900
Subject: [PATCH 9/9] =?UTF-8?q?=EB=B6=80=EB=AA=A8=EB=8A=94=20=EC=9E=90?=
 =?UTF-8?q?=EC=8B=9D=EC=9D=84=20=EB=8B=B4=EC=9D=84=20=EC=88=98=20=EC=9E=88?=
 =?UTF-8?q?=EC=9C=BC=EB=82=98,=20=EC=9E=90=EC=8B=9D=EC=9D=80=20=EB=B6=80?=
 =?UTF-8?q?=EB=AA=A8=EB=A5=BC=20=EB=8B=B4=EC=A7=80=20=EB=AA=BB=ED=95=9C?=
 =?UTF-8?q?=EB=8B=A4.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

왜냐하면, 자식에게만 존재하는 변수나 메서드가 있는데, 이를 부모로부터 참조할 수 없으므로,
---
 practice/extends1/RunExtends.java | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/practice/extends1/RunExtends.java b/practice/extends1/RunExtends.java
index 068ede503..a56108167 100644
--- a/practice/extends1/RunExtends.java
+++ b/practice/extends1/RunExtends.java
@@ -3,21 +3,28 @@
 public class RunExtends {
   public static void main(String[] args) {
     // 1. 매개변수가 없는 경우
-//    Child child = new Child();
+    Child child = new Child();
 //    child.printName();
 ////    child.printPrivateName(); 불가능!
-//    child.printProtectedName();
+    child.printProtectedName();
 
     // 2. 매개변수가 있는 경우
     ChildArg childArg = new ChildArg();
     childArg.printName();
 
-    // 3. 손자의 경우
-    GrandChild grandChild = new GrandChild();
-    grandChild.printProtectedName();
+//    // 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();
   }
 }