learn java step by step day14

1.Extands

package com.gdledu.extend_;
//大学生-> 模拟大学生考试的简单情况
public class Gruduate {public String name;public int age;private double score;public void setScore(double score) {this.score = score;}public void testing(){System.out.println(name + "正在考英语四级");}public void showInfo(){System.out.println(name + " 年龄 " + age + " 成绩 " + score);}
}package com.gdledu.extend_;
//小学生->模拟小学生考试情况
public class Pupil {public String name;public int age;private double score;public void setScore(double score) {this.score = score;}public void testing(){System.out.println(name + "正在考英文");}public void showInfo(){System.out.println(name + " 年龄 " + age + " 成绩 " + score);}
}
//相同属性多，复用性差，引出继承

package com.gdledu.extend_.improve_;
//父类，是pupil和graduate的父类
public class Student {//共有属性public String name;public int age;private double score;//共有方法public void setScore(double score) {this.score = score;}public void showInfo(){System.out.println(name + " 年龄 " + age + " 成绩 " + score);}
}package com.gdledu.extend_.improve_;
//让Pupil继承Student
public class Pupil extends Student{public void testing(){System.out.println(name + "正在考英语四级");}
}package com.gdledu.extend_.improve_;
//让Graduate继承Student
public class Graduate extends Student{public void testing(){System.out.println(name + "正在考英语四级");}}package com.gdledu.extend_.improve_;import com.gdledu.extend_.Gruduate;
import com.gdledu.extend_.Pupil;public class Extends01 {public static void main(String[] args) {com.gdledu.extend_.Pupil pupil = new Pupil();pupil.name = "小明";pupil.age = 12;pupil.testing();pupil.setScore(88);pupil.showInfo();Gruduate gruduate = new Gruduate();gruduate.name = "大明";gruduate.age = 19;gruduate.testing();gruduate.setScore(500);gruduate.showInfo();}
}

The details of Extends:

1.子类继承了所有的属性和方法，非私有的属性和方法可以在子类直接访问, 但是私有属性和方法不能在子类直接访问，要通过父类提供公共的方法去访问

package com.gdledu.extend_;public class Base { //父类//4个属性public int n1 = 100;protected int n2 = 200;int n3 = 300;private int n4 = 400;public Base() {//无参构造器System.out.println("base()....");}//父类提供一个public的方法public int getN4() {return n4;}public void test100() {System.out.println("test100");}protected void test200() {System.out.println("test200");}void test300() {System.out.println("test300");}private void test400() {System.out.println("test400");}public void calltest400(){test400();}
}package com.gdledu.extend_;public class Sub extends Base{public Sub(){System.out.println("sub()....");}public void sayOK(){//子类方法//非私有的属性和方法可以在子类直接访问System.out.println(n1 + n2 + n3);//n4 错误test100();test200();test300();//test400; 错误//要通过父类提供的public方法访问n4System.out.println("n4 = " + getN4());//通过父类提供的public方法访问test400calltest400();}
}package com.gdledu.extend_;public class ExtendsDetail {public static void main(String[] args) {Sub sub = new Sub();sub.sayOK();}
}

2.子类必须调用父类的构造器，完成父类的初始化

3.当创建子类对象时，不管使用子类的哪个构造器，默认情况下总会去调用父类的无参构造器，如果父类没有提供无参构造器，则必须在子类的构造器中用 super 去指定使用父类的哪个构造器完成对父类的初始化工作，否则，编译不会通过。

public class Base { //父类
// public Base() {//无参构造器
//      System.out.println("父类base()构造器被调用....");
//  }public Base(String name, int age) {System.out.println("父类Base(String name, int age)构造器被调用....");}public class Sub extends Base{public Sub(){//无参构造器//super(); 默认调用父类的无参构造器super("ccc", 18);//当父类没有无参构造器时，使用super指向指定构造器调用System.out.println("子类sub()构造器被调用....");}public Sub(String name){super("cca", 19);//do noting...System.out.println("子类Sub(String name)构造器被调用...");}package com.gdledu.extend_;public class ExtendsDetail {public static void main(String[] args) {Sub sub = new Sub();System.out.println("===========");Sub sub1 = new Sub("chubby");}
}

4.如果希望指定去调用父类的某个构造器，则显式的调用一下 : super(参数列表)

5.super 在使用时，必须放在构造器第一行(super 只能在构造器中使用)

6.super() 和 this() 都只能放在构造器第一行，因此这两个方法不能共存在一个构造器

public class Sub extends Base {public Sub(String name, int age) {//1.调用父类的无参构造器//super();//父类的无参构造器//2.调用父类的Base(String name)构造器//super("ccc");//3.调用父类Base(String name, int age)构造器//super必须放在第一行super("ada",99);//this("ccc");//报错}
}

7.java 所有类都是 Object 类的子类, Object 是所有类的基类. ctrl + h 查看

8. 父类构造器的调用不限于直接父类！将一直往上追溯直到 Object 类(顶级父类)

9.子类最多只能继承一个父类(指直接继承) ，即java 中是单继承机制

思考：如何让 A 类继承 B 类和 C 类？【A 继承 B ， B 继承 C】

10.不能滥用继承，子类和父类之间必须满足 is-a 的逻辑关系

The essence of extends:

package com.gdledu.extend_;/*** 讲解继承的本质*/
public class ExtendsTheory {public static void main(String[] args) {Son son = new Son();//内存的布局
//?->  这时请大家注意，要按照查找关系来返回信息
//(1)  首先看子类是否有该属性
//(2)  如果子类有这个属性，并且可以访问，则返回信息
//(3)  如果子类没有这个属性，就看父类有没有这个属性(如果父类有该属性，并且可以访问，就返回信息..)
//(4)  如果父类没有就按照(3)的规则，继续找上级父类，直到 Object...
//(5)  如果找到但是为private 则报错，不会再往上级父类找System.out.println(son.name);//返回就是大头儿子
//System.out.println(son.age);//返回的就是 39
//System.out.println(son.getAge());//返回的就是 39System.out.println(son.hobby);//返回的就是旅游}
}
class GrandPa { //爷类String name = "大头爷爷";String hobby = "旅游";
}
class Father extends GrandPa {//父类String name = "大头爸爸";private int age = 39;public int getAge() {return age;}
}
class Son extends Father { //子类String name = "大头儿子";
}

package com.gdledu.extend_.exercise;public class Computer {//编写 Computer 类，包含 CPU 、内存、硬盘等属性，//getDetails 方法用于返回 Computer 的详细信息private String cpu;private int memory;private int disk;public Computer(String cpu, int memory, int disk) {this.cpu = cpu;this.memory = memory;this.disk = disk;}public String getDetails(){return "cpu=" + cpu + " memory=" + memory + " disk=" + disk;}public String getCpu() {return cpu;}public void setCpu(String cpu) {this.cpu = cpu;}public int getMemory() {return memory;}public void setMemory(int memory) {this.memory = memory;}public int getDisk() {return disk;}public void setDisk(int disk) {this.disk = disk;}
}package com.gdledu.extend_.exercise;
//编写 PC 子类，继承 Computer 类，添加特有属性【品牌 brand】
public class PC extends Computer{private String brand;//这里IDEA根据继承的规则，自动吧构造器的调用写好//这里也体现： 继承设计的基本思想，父类的构造器完成父类属性的初始化//子类的构造器完成子类属性初始化public PC(String cpu, int memory, int disk, String brand) {super(cpu, memory, disk);this.brand = brand;}public String getBrand() {return brand;}public void setBrand(String brand) {this.brand = brand;}public String printInfo(){return "电脑信息为：" + getDetails() + " brand=" + brand;}
}package com.gdledu.extend_.exercise;
//编写 NotePad 子类，继承 PC 类，添加特有属性【品牌 color】
public class NotePad extends PC{private String color;public NotePad(String cpu, int memory, int disk, String brand, String color) {super(cpu, memory, disk, brand);this.color = color;}public String getColor() {return color;}public void setColor(String color) {this.color = color;}public void showInfo(){System.out.println(printInfo() + " color=" + color);}
}package com.gdledu.extend_.exercise;public class ExtendsExercise {public static void main(String[] args) {NotePad notePad = new NotePad("inter", 999, 999, "离谱", "bule");notePad.showInfo();//输出电脑信息}
}

2.super

super 代表父类的引用，用于访问父类的属性、方法、构造器

3.Override

package com.gdledu.override;public class Animal {public void cry(){System.out.println("动物叫唤...");}
}package com.gdledu.override;public class Dog extends Animal{public void cry(){//1.因为Dog是Animal的子类//2.Dog的cry 方法和Animal的cry方法定义形式一样(名称、返回类型、参数)//3.这时我们就说Dog的cry方法重写了Animal的cry方法System.out.println("小狗汪汪叫...");}
}package com.gdledu.override;public class Override01 {public static void main(String[] args) {Dog dog = new Dog();dog.cry();//输出:小狗汪汪叫}
}

Detail

package com.gdledu.override;public class Person {private String name;private int age;public Person(String name, int age) {this.name = name;this.age = age;}public String say(){return "我叫" + name + "今年" + age + "岁";}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
}package com.gdledu.override;public class Student extends Person{private String id;private double score;public Student(String name, int age, String id, double score) {super(name, age);this.id = id;this.score = score;}public String say(){return super.say() + "id是" + id + "分数为" + score + "分";}public String getId() {return id;}public void setId(String id) {this.id = id;}public double getScore() {return score;}public void setScore(double score) {this.score = score;}
}package com.gdledu.override;public class OverrideExercise {public static void main(String[] args) {Person person = new Person("chubby",18);System.out.println(person.say());Student student = new Student("ccc",19,"2125110",99);System.out.println(student.say());}
}

4.Polymorphic

package com.gdledu.poly_.objectpoly_;public class Animal {public void cry(){System.out.println("Animal cry() 动物在叫唤...");}
}package com.gdledu.poly_.objectpoly_;public class Cat extends Animal{public void cry() {System.out.println("Cat cry() 小猫喵喵叫...");;}
}package com.gdledu.poly_.objectpoly_;public class Dog extends Animal{public void cry() {System.out.println("Dog cry() 小狗汪汪叫...");}
}package com.gdledu.poly_.objectpoly_;public class PolyObject {public static void main(String[] args) {//体验对象多态特点//animal 编译类型就是Animal , 运行类型 DogAnimal animal = new Dog();//因为运行时，执行到该行时，animal运行类型是Dog，所以cry就是Dog的cryanimal.cry();//Dog cry() 小狗汪汪叫...//animal 编译类型 Animal，运行类型是 Catanimal = new Cat();animal.cry();//Cat cry() 小猫喵喵叫...}
}

package com.gdledu.poly_.detail;public class Animal {String name = "动物";int age = 10;public void sleep(){System.out.println("睡");}public void run(){System.out.println("跑");}public void eat(){System.out.println("吃");}public void show(){System.out.println("hello, 你好");}
}package com.gdledu.poly_.detail;public class Cat extends Animal{public void eat(){//猫捉老鼠System.out.println("猫吃鱼");}public void catchMouse(){//Cat特有方法System.out.println("猫捉老鼠");}
}package com.gdledu.poly_.detail;public class PolyDetail {public static void main(String[] args) {//向上转型：父类的引用指向了子类的对象//语法：父类类型引用名 = new 子类类型()Animal animal = new Cat();Object obj = new Cat();//也可以//可以调用父类中的所有成员(需要遵守访问权限)//但是不能调用子类的特有的成员//因为在编译阶段，能调用哪些成员变量，是由编译类型来决定的//animal.catchMouse() 错误//最终的运行效果看子类的具体实现animal.eat();//猫吃鱼}
}

属性没有重写之说！属性的值看编译类型

package com.gdledu.poly_.detail;public class PolyDetail02 {public static void main(String[] args) {Base base = new Sub();System.out.println(base.count);//10Sub sub = new Sub();System.out.println(sub.count);//20}
}
class Base {//父类int count = 10;//属性
}
class Sub extends Base{//子类int count = 20;//属性
}

instanceOf 比较操作符，用于判断对象的运行类型是否为 XX 类型或 XX 类型的子类型

package com.gdledu.poly_.detail;public class PolyDetail03 {public static void main(String[] args) {BB bb = new BB();System.out.println(bb instanceof BB);//tureSystem.out.println(bb instanceof AA);//ture//aa 编译类型AA 运行类型BBAA aa = new BB();System.out.println(aa instanceof AA);//tureSystem.out.println(aa instanceof BB);//tureObject obj = new Object();System.out.println(obj instanceof AA);//falseString str = "hello";System.out.println(str instanceof Object);//ture}
}
class AA{}//父类
class BB extends AA{}//子类

DynamicBinding

PolyArray

数组的定义类型为父类类型，里面保存的实际元素类型为子类类型

public class Person {private String name;private int age;public Person(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String say(){return name + "\t" + age;}
}package com.gdledu.poly_.polyarr_;public class Teacher extends Person{private double salary;public Teacher(String name, int age, double salary) {super(name, age);this.salary = salary;}public double getSalary() {return salary;}public void setSalary(double salary) {this.salary = salary;}@Overridepublic String say() {return super.say() + "\t" + salary;}public void teach(){System.out.println("老师" + getName() + "正在授课");}
}package com.gdledu.poly_.polyarr_;public class Student extends Person{private double score;public Student(String name, int age, double score) {super(name, age);this.score = score;}public double getScore() {return score;}public void setScore(double score) {this.score = score;}@Overridepublic String say() {return super.say() + "\t" + score;}public void study(){System.out.println("学生" + getName() + "正在学习");}
}package com.gdledu.poly_.polyarr_;public class PolyArray {public static void main(String[] args) {Person[] persons = new Person[5];persons[0] = new Person("aaa",18);persons[1] = new Student("bbb",18,90);persons[2] = new Student("ccc",18,80);persons[3] = new Teacher("ddd",18,30000);persons[4] = new Teacher("eee",18,60000);for (int i = 0; i < persons.length; i++) {System.out.println(persons[i].say());if(persons[i] instanceof Student){((Student)persons[i]).study();//等价于//Student student = (Student)person[i]//student.study()}else if (persons[i] instanceof Teacher){((Teacher)persons[i]).teach();}}}
}

PolyParameter

package com.gdledu.poly_.polyparameter;public class Employee {private String name;private double salary;public Employee(String name, double salary) {this.name = name;this.salary = salary;}public String getName() {return name;}public void setName(String name) {this.name = name;}public double getSalary() {return salary;}public void setSalary(double salary) {this.salary = salary;}public double getAnnual(){return 12 * salary;}
}package com.gdledu.poly_.polyparameter;public class Worker extends Employee{public Worker(String name, double salary) {super(name, salary);}public void work(){System.out.println("普通员工" + getName() + " is working");}@Overridepublic double getAnnual() {return super.getAnnual();}
}package com.gdledu.poly_.polyparameter;public class Manager extends Employee{private double bonus;public Manager(String name, double salary, double bonus) {super(name, salary);this.bonus = bonus;}public double getBonus() {return bonus;}public void setBonus(double bonus) {this.bonus = bonus;}public void manage(){System.out.println("经理" + getName() + " is managing");}@Overridepublic double getAnnual() {return super.getAnnual() + bonus;}
}package com.gdledu.poly_.polyparameter;public class PolyParameter {public static void main(String[] args) {Worker aaa = new Worker("aaa", 5000);Manager bbb = new Manager("bbb", 20000, 50000);PolyParameter polyParameter = new PolyParameter();polyParameter.showEmpAnnual(aaa);polyParameter.showEmpAnnual(bbb);polyParameter.testWork(aaa);polyParameter.testWork(bbb);}public void showEmpAnnual(Employee e){System.out.println(e.getAnnual());}public void testWork(Employee e){if(e instanceof Worker){((Worker) e).work();}else if(e instanceof Manager){((Manager) e).manage();}}
}

The knowledge learning in the article comes from：【零基础快速学Java】韩顺平零基础30天学会Java_哔哩哔哩_bilibilihttps://www.bilibili.com/video/BV1fh411y7R8?p=32