Extends

practice/extends1/Child.java

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+package extends1;
+
+public class Child extends Parent {
+    public Child() {
+        System.out.println("Child Constructor");
+    }
+
+    @Override
+    public void printProtectedName() {
+        System.out.println("Parent printProtectedName()");
+    }
+}

practice/extends1/ChildArg.java

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+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");
+    }
+}

practice/extends1/Friend.java

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+package extends1;
+
+public class Friend {
+    public Friend() {
+        System.out.println("Friend Constructor");
+    }
+}

practice/extends1/GrandChild.java

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+package extends1;
+
+public class GrandChild extends Child {
+    public GrandChild() {
+        System.out.println("GrandChild Constructor");
+    }
+
+    public void print() {
+        System.out.println("GrandChild!!!!!");
+    }
+}

practice/extends1/Parent.java

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+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()");
+    }
+}

practice/extends1/ParentArg.java

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+package extends1;
+
+public class ParentArg {
+
+    public ParentArg(String name) {
+        System.out.println("ParentArg("+name+") Constructor");
+    }
+
+    public void printName() {
+        System.out.println("ParentArg printName()");
+    }
+}

practice/extends1/RunExtends.java

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+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();
+  }
+}