面向对象进阶9—多态、内部类、常用API

多态的概述

什么是多态

多态的优势

多态下的自动类型转换

代码演示

父类

package com.peihj.polymorphic_advantage;

public abstract class Animals {
    public String name = "父类动物";
    public abstract void run();
}

子类狗

package com.peihj.polymorphic_advantage;

public class dog extends Animals{

    public String name = "狗名称";
    @Override
    public void run() {
        System.out.println("🐕跑到快");
    }

    /*
    * 独有功能
    * */
    public void watch(){
        System.out.println("🐕再看门");
    }


}

子类乌龟

package com.peihj.polymorphic_advantage;

public class tortoise extends Animals{
    public String name = "乌龟";
    @Override
    public void run() {
        System.out.println("🐢跑的特别慢");
    }

    /*
    * 独有功能
    * */
    public void layeggs(){
        System.out.println("🐢再下蛋");
    }
}

测试类

package com.peihj.polymorphic_advantage;

public class Test {
    public static void main(String[] args) {
        Animals t = new dog();  // 编译看左边，运行看右边。
//        t.run();
//        System.out.println(t.name);  // 编译看左边，运行也看左边。（多态侧重行为多态）
        go(t);
//        dog dog = (dog) t;// 强制类型转化，从父类到子类
//        dog.watch();// 可以调用独有功能
        trans(t);

        Animals t1 = new tortoise();
//        t1.run();
//        System.out.println(t1.name);
        go(t1);
//        tortoise tortoise = (tortoise) t1;
//        tortoise.layeggs();
        trans(t1);
    }

    public static void go(Animals a){
        System.out.println("比赛开始");
        a.run();
        System.out.println("比赛结束");
    }

    public static void trans(Animals a){
        if (a instanceof tortoise){
            tortoise t = (tortoise) a;  // 从父类类型到子类必须强制类型转换
            t.layeggs();
        }else if (a instanceof dog){
            dog d = (dog) a;
            d.watch();
        }
    }

}

运行结果

多态的案例

USB接口代码

package com.peihj.duotai;


/*
* USB接口规范
* */

public interface USB {
    // 接入，接出
    void connect();
    void unconnect();

}

鼠标代码

package com.peihj.duotai;

public class mouse implements USB{
    private String name;

    public mouse() {
    }

    public mouse(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    /*
    * 独有功能
    * */
    public void dbclick(){
        System.out.println(name+"按了一下左键");
    }

    @Override
    public void connect() {
        System.out.println(name+"成功连接了电脑");
    }

    @Override
    public void unconnect() {
        System.out.println(name+"取消连接了电脑");
    }
}

键盘代码

package com.peihj.duotai;

/*
* 实现类：键盘
* */
public class Keyboard implements USB {
    private String name;

    public Keyboard() {
    }

    public Keyboard(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    /*
    * 独有功能
    * */
    public void click(){
        System.out.println(name+"敲击了电脑");
    }

    @Override
    public void connect() {
        System.out.println(name+"成功连接了电脑。");
    }

    @Override
    public void unconnect() {
        System.out.println(name+"从电脑拔出。");
    }
}

电脑代码

package com.peihj.duotai;

public class computer {
    private String name;

    public computer() {
    }

    public computer(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void start(){
        System.out.println(name+"启动了----");
    }

    /*
    * 提供安装USB的接口
    * */
    public void install(USB usb){
        // 多态对象：可能是电脑也可以是鼠标
        usb.connect();
        // 独有功能先判断再调用
        if (usb instanceof Keyboard){
            ((Keyboard) usb).click();
        }else if(usb instanceof mouse){
            ((mouse) usb).dbclick();
        }
        usb.unconnect();
    }

}

测试类

package com.peihj.duotai;

public class Test {
    public static void main(String[] args) {
        Keyboard k = new Keyboard("戴尔键盘");

        mouse m = new mouse("罗技鼠标");

        computer c = new computer("外星人电脑");

        c.start();
        c.install(k);
        System.out.println("===============================");
        c.install(m);


    }
}

结果演示

内部类

内部类的概述

静态内部类

静态内部类代码表述

package com.peihj.innerclass_static;

public class Outer {
    public static class inner{
        private String name;
        private int age;
        public static String SchoolName;

        public inner() {
        }

        public inner(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 static String getSchoolName() {
            return SchoolName;
        }

        public static void setSchoolName(String schoolName) {
            SchoolName = schoolName;
        }
    }
}

package com.peihj.innerclass_static;

public class TEST {
    public static void main(String[] args) {
        Outer.inner in = new Outer.inner();
        in.setName("乔巴");
        in.setAge(25);
        System.out.println(in.getAge());
        System.out.println(in.getName());
    }
}

静态内部类小结

成员内部类

成员内部类代码说明

package com.peihj.innerclass_static;

/*
* 成员内部类
* */

public class Outer1 {
    public static int num = 111;
    private String hobby = "唱歌";

    public class inner1{
        private String name;
        private int age;
        public static int a; // JDK16之后开始支持静态成员了

        public void show(){
            System.out.println("名称："+name);
            System.out.println("年龄："+age);
            System.out.println("爱好："+hobby);
            System.out.println("数量："+num);
        }

        public inner1() {
        }

        public inner1(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 static int getA() {
            return a;
        }

        public static void setA(int a) {
            inner1.a = a;
        }
    }
}

package com.peihj.innerclass_static;

public class TEST {
    public static void main(String[] args) {
        Outer1.inner1 IN = new Outer1().new inner1();
        IN.show();
    }
}

成员内部类代码演示2

package com.peihj.innerclass_static;

/*
* 成员内部类
* */

public class Outer1 {
    public static int num = 111;
    private String hobby;

    public Outer1(String hobby) {
        this.hobby = hobby;
    }

    public class inner1{
        private String name;
        private int age;
        public static int a; // JDK16之后开始支持静态成员了

        public void show(){
            System.out.println("名称："+name);
            System.out.println("年龄："+age);
            System.out.println("爱好："+hobby);
            System.out.println("数量："+num);
        }

        public inner1() {
        }

        public inner1(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 static int getA() {
            return a;
        }

        public static void setA(int a) {
            inner1.a = a;
        }
    }
}

package com.peihj.innerclass_static;

public class TEST {
    public static void main(String[] args) {
        Outer1.inner1 IN = new Outer1("跳舞").new inner1("小王",25);
        IN.show();
        System.out.println("====================");
    }
}

成员内部类小结

面试代码

package com.peihj.innerclass_static;

public class TRY {
    public static void main(String[] args) {
        
    }
}

class people{
    private int heartbeat = 150;
    /*
    * 成员内部类
    * */
    public class heart{
        private int heartbeat = 110;
        
        public void show(){
            int heartbeat = 78;
            System.out.println(heartbeat);  // 78
            System.out.println(this.heartbeat); // 110
            System.out.println(people.this.heartbeat); // 150
        }
    }
    
}

局部内部类

匿名内部类（重点）

匿名内部类演示代码

package com.peihj.interclass_anonymous;

public class test {
    public static void main(String[] args) {
        Animals t = new tiger();
        t.run();

        // 匿名内部类，简化代码写法
        Animals t1 = new Animals() {
            @Override
            void run() {
                System.out.println("🐅跑的贼快");
            }
        };

        t1.run();


    }
}

class tiger extends Animals{
    @Override
    void run() {
        System.out.println("🐅跑的贼快；");
    }
}

abstract class Animals{
    abstract void run();
}

结果演示

匿名内部类小结

匿名内部类常见使用形式

package com.peihj.interclass_anonymous;

public class test2 {
    public static void main(String[] args) {
       swimming s = new Student();
       go(s);

       swimming s1 = new swimming() {
           @Override
           public void swim() {
               System.out.println("学生再快乐的自由泳");
           }
       };
       s1.swim();

    }
    /*
    * 多态老师与学生可以一起比赛
    * */
    public static void go(swimming s){
        System.out.println("开始");
        s.swim();
        System.out.println("结束");
    }
}


class teacher implements swimming{
    @Override
    public void swim() {
        System.out.println("老师游泳熟读非常快");
    }
}

class Student implements swimming{
    @Override
    public void swim() {
        System.out.println("学生游泳速度中等");
    }
}

// 定义一个接口
interface swimming{
    void swim();
}

匿名内部类真实使用场景

常用API

什么是api

object类的作用

toString

toString代码（快捷键：tos）

package com.peihj.API;

public class Student {
    private String name;
    private char sex;
    private int age;

    public Student() {
    }

    public Student(String name, char sex, int age) {
        this.name = name;
        this.sex = sex;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public char getSex() {
        return sex;
    }

    public void setSex(char sex) {
        this.sex = sex;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", sex=" + sex +
                ", age=" + age +
                '}';
    }
}

测试类

package com.peihj.API;

public class test {
    public static void main(String[] args) {
        Student student = new Student("乔巴",'男',25);
        String ts = student.toString();
        System.out.println(ts);

    }
}

结果输出

toString小结

equals

equals代码演示

package com.peihj.API;

public class Student {
    private String name;
    private char sex;
    private int age;

    public Student() {
    }

    public Student(String name, char sex, int age) {
        this.name = name;
        this.sex = sex;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public char getSex() {
        return sex;
    }

    public void setSex(char sex) {
        this.sex = sex;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", sex=" + sex +
                ", age=" + age +
                '}';
    }
}

测试

package com.peihj.API;

public class test {
    public static void main(String[] args) {
        Student student = new Student("乔巴",'男',25);
        Student student1 = new Student("乔巴",'男',25);
        System.out.println((student == student1));
        System.out.println(student.equals(student1));
    }
}

结果演示

equals方法改写

equals改写快捷键：equ+回车+回车

package com.peihj.API;

public class test {
    public static void main(String[] args) {
        Student student = new Student("乔巴",'男',25);
        Student student1 = new Student("乔巴",'男',25);
        System.out.println((student == student1));
        System.out.println(student.equals(student1));
    }
}

改写后比较内容

结果演示

阶段小结

objects概述

objects常见方法

代码演示

package com.peihj.API;

import java.util.Objects;

public class test2 {
    public static void main(String[] args) {
        String s1 = null;
        String s2 = new String("peihj");

        // System.out.println(s1.equals(s2));
        // Cannot invoke "String.equals(Object)" because "s1" is null

        System.out.println(Objects.equals(s1, s2));  // 更安全，结果也是对的

        System.out.println("=======================");

        System.out.println(Objects.isNull(s1));
        System.out.println(s1 == null);
        System.out.println(Objects.isNull(s2));
        System.out.println(s2 == null);
    }
}

结果演示

objects小结

StringBuilder概述

StringBuilder常见用法

代码演示

package com.peihj.API;

public class StringBuildDemo {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        sb.append("a");
        sb.append("b");
        sb.append("c");
        sb.append("我爱你中国");
        sb.append("false");

        System.out.println(sb);
        System.out.println("======");

        // 支持链式编程
        sb.append("a").append("b").append("c").append("111111");
        System.out.println(sb);

        // 反转
        System.out.println(sb.reverse().append("2222222"));

        // 长度
        System.out.println(sb.length());
    }
}

必须先转换成字符串

package com.peihj.API;

public class StringBuildDemo {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        sb.append("123").append("456");
        String rs = sb.toString();
        check(rs);
    }

    public static void check(String data){
        System.out.println(data);
    }
}

结果演示

字符串拼接原理图

StringBuilder提高效率原理图

小结

案例：打印整个数组内容

package com.peihj.API;

public class Test3 {
    public static void main(String[] args) {
        int[] arr = {10,20,30,40,50};
        int[] arr1 = null;
        int[] arr2 = {};
        String a = ToString(arr);
        String b  =ToString(arr1);
        String c = ToString(arr2);
        System.out.println(a);
        System.out.println(b);
        System.out.println(c);
    }
    /*
    * 定义方法接收数组返回数组内容格式
    * */
    public static String ToString(int[] arr){
        // 定义StringBuilder接收数组内容
        StringBuilder sb = new StringBuilder("[");
        if (arr != null){
            for (int i = 0; i < arr.length; i++) {
                sb.append(arr[i]).append(i == arr.length-1 ? "":",");
            }
            sb.append("]");
            String SB = sb.toString();
            return SB;
        }else {
            return null;
        }

    }
}

Math库

Math库程序演示

package com.peihj.API;

public class MathDemo {
    public static void main(String[] args) {
        // 取绝对值返回正数
        System.out.println(Math.abs(10));  // 10
        System.out.println(Math.abs(-10));  // 10

        // 向上取整
        System.out.println(Math.ceil(4.000000001)); // 5.0
        System.out.println(Math.ceil(4.9999999999));  // 5.0

        // 向下取整
        System.out.println(Math.floor(4.99999999));  // 4.0

        // 求指数次方
        System.out.println(Math.pow(2,3));  // 8

        // 四舍五入
        System.out.println(Math.round(4.49999));  // 4
        System.out.println(Math.round(4.500001)); // 5

        // random 左闭右开
        System.out.println(Math.random()); //0.0 --- 1.0

        // 扩展：取3--9之间的随机数
        int data = (int)(Math.random()*7) + 3;
        System.out.println(data);

    }
}

System 类概述

程序演示

package com.peihj.API;

public class SystemDemo {
    public static void main(String[] args) {
        System.out.println("程序开始");

        // System.exit(0); // jvm终止
        // 计算机认为时间有起源，返回1970.1-1 00：00：00 到现在的总毫秒值，时间毫秒值
        long time = System.currentTimeMillis();
        System.out.println(time);

        long starttime = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            System.out.println("第"+i+"次");
        }

        long endtime= System.currentTimeMillis();

        System.out.println(((endtime - starttime) / 1000.0));
        System.out.println("程序结束");


    }
}

BigDecimal

代码演示

package com.peihj.API;

import java.math.BigDecimal;
import java.math.RoundingMode;

public class BigDecimalDemo {
    public static void main(String[] args) {
        System.out.println("=======================================");

        double a = 0.1;
        double b = 0.2;
        double c = a + b;   // 0.30000000000000004
        System.out.println(c);

        System.out.println("========================================");
        // 包装浮点型数据成为大数据对象
        BigDecimal a1=  BigDecimal.valueOf(a);
        BigDecimal b1=  BigDecimal.valueOf(b);
        BigDecimal c1 = a1.add(b1);
//        BigDecimal c2 = a1.subtract(b1);
//        BigDecimal c3 = a1.multiply(b1);
//        BigDecimal c4 = a1.divide(b1);

        System.out.println(c1); // 0.3
        System.out.println("============================================");
        // 目的：rs
        double rs = c1.doubleValue();
        System.out.println(rs);

        System.out.println("============================================");

        // 注意：BigDecimal是一定要精度运算的

        BigDecimal a11 = BigDecimal.valueOf(10.0);
        BigDecimal b11 = BigDecimal.valueOf(3.0);
        // BigDecimal c11 = a11.divide(b11); //  Non-terminating decimal expansion; no exact representable decimal result.
        BigDecimal c11 = a11.divide(b11,2, RoundingMode.HALF_UP); // 保留2位小数
        System.out.println(c11);


    }
}

参考

https://www.bilibili.com/video/BV1Cv411372m?p=120&t=101.3