第2课第3节_Java面向对象编程_继承性_P【学习笔记】

摘要:韦东山android视频学习笔记 

面向对象程序的三大特性之继承性：继承性的主要作用就是复用代码.继承性也有一定的限制，如图一

图一

1、我们在第2课第2节_Java面向对象编程_封装性_P 中定义了persion.我在这代码基础上定义一个学习类，并继承persion类。

 class Persion{
     private int age;

     public void setAge(int age){
         if (age < 0 || age > 200)
             age = 0;
         else
             this.age = age;
     }

     public int getAge(){
         return age;
     }
 }

 class Student extends Persion{

 }

 public class Ext{
     public static void main(String args[]){
         Student stu = new Student();
         stu.setAge(10);

         System.out.println(stu.getAge());

     }
 }

编译运行结果

2、在上面的代码基础上，我们的子类继承了父类，我们可以添加自己的属性还有方法.

 class Persion{
     private int age;

     public void setAge(int age){
         if (age < 0 || age > 200)
             age = 0;
         else
             this.age = age;
     }

     public int getAge(){
         return age;
     }
 }

 class Student extends Persion{
     private String school;

     public void setSchool(String school){
         this.school = school;
     }

     public String getSchool(){
         return school;
     }

     public Student(String school){
         this.school = school;
     }

 }

 public class Ext2{
     public static void main(String args[]){
         Student stu = new Student("ustc");
         stu.setAge(10);

         System.out.println(stu.getAge());
         System.out.println(stu.getSchool());

     }
 }

编译运行结果

3、子类继承父类的方法和属性都可以进行覆写，我们在子类覆写父类的printInfo方法。

 class Persion{
     private int age;

     public void setAge(int age){
         if (age < 0 || age > 200)
             age = 0;
         else
             this.age = age;
     }

     public int getAge(){
         return age;
     }

     public void printInfo(){
         System.out.println("age = " + age);
     }
 }

 class Student extends Persion{
     private String school;

     public void setSchool(String school){
         this.school = school;
     }

     public String getSchool(){
         return school;
     }

     public Student(String school){
         this.school = school;
     }

     public void printInfo(){
         System.out.println("school = " + school + " age = " + getAge());
     }
 }

 public class Ext3{
     public static void main(String args[]){
         Student stu = new Student("ustc");
         stu.setAge(10);

         System.out.println(stu.getAge());
         System.out.println(stu.getSchool());
         stu.printInfo();

     }
 }

编译运行结果

4、实例化子类对象时，先调用父类的构造方法，再调用子类的构造方法，super（）函数在子类中可以指定调用父类构造函数的类型。

 class Persion{
     private int age;

     public void setAge(int age){
         if (age < 0 || age > 200)
             age = 0;
         else
             this.age = age;
     }

     public int getAge(){
         return age;
     }

     public void printInfo(){
         System.out.println("age = " + age);
     }

     public Persion(){System.out.println("Persion()");};
     public Persion(int age){
         System.out.println("Persion(int age)");
         this.age = age;
     }
 }

 class Student extends Persion{
     private String school;

     public void setSchool(String school){
         this.school = school;
     }

     public String getSchool(){
         return school;
     }

     public Student(String school){
         super(5);
         System.out.println("Student(String school)");
         this.school = school;
     }

     public void printInfo(){
         System.out.println("school = " + school + " age = " + getAge());
     }
 }

 public class Ext4{
     public static void main(String args[]){
         Student stu = new Student("ustc");
         //stu.setAge(10);

         System.out.println(stu.getAge());
         System.out.println(stu.getSchool());
         stu.printInfo();

     }
 }

编译运行结果

 5、抽象类规定子类必须实现的方法，起“模板”作用，缺点不能实例化对象，子类必须覆写全部抽象方法

  abstract class Father {
     private int money;    

     public int getMoney() {return money; }
     public void setMoney(int money) {this.money = money; }

     public abstract void study();
 }

 class Son extends Father{
     public void study() {System.out.println("I am study"); }

 }

 public class Ext6 {
     public static void main (String args[]) {
         //Father f = new Father();
         Son son = new Son();
         son.study();

     }
 }

编译运行结果：

6、作用：跟抽象类相似，起“模板”作用；子类可以继承多个接口，突破“单继承”的限制

 abstract class Father{
     private int money;

     public int getMoney(){return money;}
     public void setMoney(int money){
         this.money = money;
     }
     public abstract void study();
 }

 interface A{
     public static final int i = 10;
     public abstract int getNum();
 }

 interface B{
     public static String name = "InterfaceB";
     public abstract String getName();
 }

 class Son extends Father implements A,B{
     public int getNum(){return i;}
     public String getName(){return name;}

     public void study(){
         System.out.println("I am study");
     }
 }

 public class Ext6{
     public static void main(String args[]){
         Son son = new Son();
         System.out.println(son.getName());
         System.out.println(son.getNum());
         son.study();

     }
 }

编译运行结果：

相关代码存放在github，可以下载https://github.com/zzb2760715357/100ask