多态

多态的基本使用

实体类

package Person;

/**
 * @author sakurasep
 * * @date 2024/3/13
 **/
public class Person {
    String name;
    int age;
    public void eat(){
        System.out.println("人吃饭");
    }

    public void walk(){
        System.out.println("人走路");
    }

}

package Person;

/**
 * @author sakurasep
 * * @date 2024/3/13
 **/
public class Man extends Person{
    boolean isSomking;

    public void eat(){
        System.out.println("男人吃肉");
    }

    public void walk(){
        System.out.println("男人走路");
    }

    public void play(){
        System.out.println("男人打游戏");
    }
}

package Person;

/**
 * @author sakurasep
 * * @date 2024/3/13
 **/
public class Woman extends Person{
    boolean isBeauty;
    public void eat(){
        System.out.println("女人吃甜点");
    }

    public void walk(){
        System.out.println("女人走路");
    }

    public void watch(){
        System.out.println("女人看剧");
    }
}

package Person;

/**
 * @author sakurasep
 * * @date 2024/3/13
 **/
public class PersonTest {
    public static void main(String[] args) {
        Person person = new Person();
        person.eat();
        person.walk();

        Man man = new Man();
        man.eat();
        man.walk();

        Woman woman = new Woman();
        woman.eat();
        woman.walk();

        System.out.println("*********多态性********");

        Person person1 = new Man();
        person1.eat();
        person1.walk();

        Person person2 = new Woman();
        person2.eat();
        person2.walk();
    }
}


测试方法

package Person;

/**
 * @author sakurasep
 * * @date 2024/3/13
 **/
public class PersonTest {
    public static void main(String[] args) {
        Person person = new Person();
        person.eat();
        person.walk();

        Man man = new Man();
        man.eat();
        man.walk();

        Woman woman = new Woman();
        woman.eat();
        woman.walk();

        System.out.println("*********多态性********");

        Person person1 = new Man();
        person1.eat();
        person1.walk();

        Person person2 = new Woman();
        person2.eat();
        person2.walk();

    }

}

输出结果

image-20240313164918351

多态的实际使用

测试方法

源码

当不存在多态性的时候

package Person;

/**
 * @author sakurasep
 * * @date 2024/3/13
 **/
public class AccountTest {
    public static void main(String[] args) {
        AccountTest test = new AccountTest();
        test.console(new Account());
        System.out.println("-------------");
        test.console(new CheckAccount());
        System.out.println("-------------");
        test.console(new SaveAccount());
    }
    public void console(Account account){
        account.saveMoney(100);
        account.getMoney(40);
    }

    public void console(CheckAccount account){
        account.saveMoney(100);
        account.getMoney(40);
    }

    public void console(SaveAccount account){
        account.saveMoney(100);
        account.getMoney(40);
    }

}
class Account {
    String username;
    String password;
    int money; // 余额

    // 存钱
    public void saveMoney(int money) {
        this.money = money;
        System.out.printf("普通账户余额为: %d\n", this.money);
    }

    // 取钱
    public void getMoney(int money) {
        if(money <= this.money) {
            this.money -= money;
            System.out.printf("普通账户余额为: %d\n", this.money);
        }
        else {
            System.out.println("普通账户余额不足");
        }
    }
}

class CheckAccount extends Account {
    int quota = 500; // 额度
    public void saveMoney(int money) {
        this.money = money + quota;
        System.out.printf("信用卡账户余额为: %d\n", this.money);
    }

    // 取钱
    public void getMoney(int money)
    {
        if(this.money >= money) {
            this.money -= money;
            System.out.printf("信用卡余额为: %d\n", this.money);
        }
        else {
            System.out.println("信用卡余额不足");
        }
    }

}

class SaveAccount extends Account {
    float money;
    float rate = 0.2F; // 利率
    float interest; // 利息

    public void saveMoney(int money) {
        interest = money * rate;
        this.money = money + interest;
        System.out.printf("储蓄卡预期余额为: %f\n", this.money);
    }

    public void getMoney(int money) {
        if(this.money - interest > money) {
            this.money -= money;
            System.out.printf("储蓄卡余额为: %f\n", this.money);
        }
        else {
            System.out.println("储蓄卡余额不足");
        }
    }
}

正常情况下，如果没有 Java 的多态性，那么对于不同对象的方法调用，就需要如下这样写多个同名方法，赋予不同对象的形参

public void console(Account account){
    account.saveMoney(100);
    account.getMoney(40);
}

public void console(CheckAccount account){
    account.saveMoney(100);
    account.getMoney(40);
}

public void console(SaveAccount account){
    account.saveMoney(100);
    account.getMoney(40);
}

而因为 Java 的多态性，这里的方法便可以省略重复调用，在实际的项目中，多态很有用，符合开闭原则。对于增加新的子类，方法都不需要进行改变，只定义一个父类即可。

public void console(Account account){
    account.saveMoney(100);
    account.getMoney(40);
}

输出结果

image-20240319110110588

Java 向下转型

测试方法

虽然创建的子类的对象，也加载了子类的特有的方法和属性，但是因为我们声明的是父类的引用，所以没有办法直接调用子类特有的属性和方法。

注意，在向下转型的时候可能会出现类型转换异常，即你要转换的类型并不是你声明的类型

所以在进行向下转型之前，为了程序的健壮性，最好使用 instanceof 进行判断

public class AccountTest {
    public static void main(String[] args) {
        AccountTest test = new AccountTest();
        test.console(new CheckAccount());
      	test.console(new SaveAccount());
    }
    public void console(Account account){
        account.saveMoney(100);
        account.getMoney(40);
				
				// 避免出现类型转换异常
        if (account instanceof CheckAccount){
            CheckAccount check_account = (CheckAccount)account;
            check_account.getQuota();
        }

        if (account instanceof SaveAccount){
            SaveAccount save_account = (SaveAccount)account;
            save_account.getInterest();
            save_account.getRate();
        }

    }

}

输出结果

image-20240319143347946

多态案例练习

题目

image-20240319134006979

关系图

image-20240319134130028

实体类

package Geometric;

/**
 * @author sakurasep
 * * @date 2024/3/19
 **/
public class GeometricObject {
    protected String color;
    protected double weight;

    protected GeometricObject(String color, double weight) {
        this.color = color;
        this.weight = weight;
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    public double getWeight() {
        return weight;
    }

    public void setWeight(double weight) {
        this.weight = weight;
    }

    public double findArea(){
        return 0;
    }
}

package Geometric;

/**
 * @author sakurasep
 * * @date 2024/3/19
 **/
public class Circle extends GeometricObject {
    private double radius;

    public Circle(String color, double weight, double radius) {
        super(color, weight);
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }

    public void setRadius(double radius) {
        this.radius = radius;
    }

    @Override
    public double findArea(){
        return 3.14 * radius * radius;
    }
}

package Geometric;

/**
 * @author sakurasep
 * * @date 2024/3/19
 **/
public class MyRectangle extends GeometricObject{
    private double width;
    private double height;

    public MyRectangle(String color, double weight, double width, double height) {
        super(color, weight);
        this.width = width;
        this.height = height;
    }

    public double getWidth() {
        return width;
    }

    public void setWidth(double width) {
        this.width = width;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }

    @Override
    public double findArea() {
        return width * height;
    }
}

测试方法

package Geometric;

import Person.Man;

/**
 * @author sakurasep
 * * @date 2024/3/19
 **/
public class GeometricTest {

    public static void main(String[] args) {
        GeometricTest test = new GeometricTest();
        Circle c1 = new Circle("red", 1.0, 4);
        Circle c2 = new Circle("bule", 1.0, 4);
        MyRectangle m1 = new MyRectangle("red", 1.0, 4, 4);
        test.displayGeometricObject(c1);
        test.displayGeometricObject(c2);
        test.displayGeometricObject(m1);

        boolean isEquals = test.equalsArea(c1, c2);
        if (isEquals){
            System.out.println("面积相等");
        }
        else {
            System.out.println("面积不等");
        }

    }
    public boolean equalsArea(GeometricObject object1, GeometricObject object2){
        return object1.findArea() == object2.findArea();
    }

    public void displayGeometricObject(GeometricObject object){
        System.out.println("几何图形的面积为: " + object.findArea());
    }
}

输出结果

image-20240319140928531

重写对象方法

重写 Object 类的 equals() 方法

对于 String，File ，包装类等已经重写了 Object 的 equals 方法。我们自己用的时候，通常也可以重写 equals 方法实现对象的应用。

当然 IDEA 自带重写 equals 方法

实体类

class User {
    private int age;
    private String name;

    public User() {
    }

    public User(int age, String name) {
        this.age = age;
        this.name = name;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }

        if (obj instanceof User) {
            User user = (User) obj;
            return this.age == user.age && this.name.equals(user.name);
        }

        return false;
    }
  
  	@Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        User user = (User) o;

        if (age != user.age) return false;
        return Objects.equals(name, user.name);
    }
}

测试方法

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

        User u1 = new User(10, "lihua");
        User u2 = new User(10, "xiaoli");
        User u3 = new User(10,"lihua");
        System.out.println(u1.equals(u2));
        System.out.println(u1.equals(u3));
    }
}

输出结果

image-20240319155034611

重写 toString() 方法

和 equals 一样，String，File 等也已经预先重写了 toString 方法，对于自定义类，我们同样可以进行重写

@Override
public String toString() {
      return "Use r" + "name: " + name + " age: " + age;
}

image-20240319183944770

当然，IDEA 也可以自动生成对象的 toString 方法

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

关键字 Static

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