add extends demo 1

README.md

@@ -107,18 +107,21 @@
         - [Constructor2](./src/main/java/Basics/Constructor2.java)
         - [Constructor3](./src/main/java/Basics/Constructor3.java)
         - [ConstructorDemo1](./src/main/java/Basics/ConstructorDemo1.java)
+
     - Encapsulation
         - [Encapsulation1](./src/main/java/Basics/Encapsulation1.java)
         - In short :
             - hide the things need to hide : users don't need to know how does the library/Class... do the implementation
             - export the things need to export : users only need to know the how/where (e.g. : `API`) to use the library/Class.
         - pros : make the code extenable, scalable, easy to maintain
+
     - JavaBean
         - [CustomerBean](./src/main/java/Basics/CustomerBean.java)
         - A java class that has below properties
             - 1. the class is a `public` class
             - 2. with a `no argument` `public` constructor
             - 3. has corresponding getter, setter methods
+
     - This
         - 1. `this` can be used in `attr`, `class`, `method`, `constructor`
         - 2. `this` on  `attr`, `method`
@@ -137,6 +140,10 @@
         - [thisDemo4](./src/main/java/Basics/thisDemo4.java)
         - [thisDemo5](./src/main/java/Basics/thisDemo5)
 
+    - Extends
+        - [Extends_demo1](./src/main/java/Basics/Extends_demo1)
+        - the "children" class can `reuse`, `overwrite` the `attr/method` that their "parent" class already defined
+
 - [JavaHelloWorld](https://github.com/yennanliu/JavaHelloWorld/tree/main/src) : basic3
     - Static demo
         - [staticDemo 1](./src/main/java/Basics/staticDemo1.java)

src/main/java/Basics/Extends_demo1/ExtendsTest.java

@@ -0,0 +1,20 @@
+package Basics.Extends_demo1;
+
+// https://www.youtube.com/watch?v=ArTiU9HR5Kw&list=PLmOn9nNkQxJH0qBIrtV6otI0Ep4o2q67A&index=259
+
+public class ExtendsTest {
+    public static void main(String[] args){
+        Person p1 = new Person();
+        p1.age = 30;
+        System.out.println("p1.age = " + p1.age);
+        p1.eat();
+
+        System.out.println("==================");
+
+        Student s1 = new Student();
+        s1.age = 20;
+        System.out.println("s1.age = " +  s1.age);
+        s1.sleep();
+        s1.study();
+    }
+}

src/main/java/Basics/Extends_demo1/Person.java

@@ -0,0 +1,27 @@
+package Basics.Extends_demo1;
+
+// https://www.youtube.com/watch?v=ArTiU9HR5Kw&list=PLmOn9nNkQxJH0qBIrtV6otI0Ep4o2q67A&index=259
+
+public class Person {
+    // attr
+    String name;
+    int age;
+
+    // constructor
+    public Person(){
+    }
+
+    public Person(String name, int age){
+        this.name = name;
+        this.age = age;
+    }
+
+    // method
+    public void eat(){
+        System.out.println("eat @@@@");
+    }
+
+    public void sleep(){
+        System.out.println("sleep !!!!!");
+    }
+}

src/main/java/Basics/Extends_demo1/Student.java

@@ -0,0 +1,23 @@
+package Basics.Extends_demo1;
+
+// https://www.youtube.com/watch?v=ArTiU9HR5Kw&list=PLmOn9nNkQxJH0qBIrtV6otI0Ep4o2q67A&index=259
+
+public class Student extends Person {
+    // attr  (we can use the attr, method that Person already defined)
+    String major;
+
+    // constructor
+    public Student(){
+    }
+
+    public Student(String name, int age, String major){
+        this.name = name;
+        this.age = age;
+        this.major = major;
+    }
+
+    // method  (we can use the attr, method that Person already defined)
+    public void study(){
+        System.out.println("i am study ~~~~");
+    }
+}