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Java面向对象基础(初级)#

对面向对象和面向过程的理解

谈谈你对面向过程和面向对象的理解_智十七°的博客-CSDN博客_面向对象和面向过程的理解

面向对象的三大特征#

封装 (Encapsulation)
继承 (Inheritance)
多态 (Polymorphism)

初识对象和类#

image-20221207221107547

image-20221207221135590

image-20221207221142578

image-20221207222150537

初次使用对象和类#

如下，在下面这个代码中，我创建了一个猫类，然后利用猫类创建了两个猫对象，一个赋值给cat01，一个赋值给cat02

package cn.meowrain.Object_;

public class relean_01 {
    public static void main(String[] args) {
        //创建两猫对象
        Cat cat01 = new Cat();
        cat01.name = "小花";
        cat01.age = 1;
        cat01.color = "花色";
        Cat cat02 = new Cat();
        cat02.name = "小白";
        cat02.age = 2;
        cat02.color = "白色";
        //访问对象属性
        System.out.println(cat01.name);
        System.out.println(cat02.name);
        cat01.eat();
        cat02.eat();
    }
}

class Cat {
    public String name;
    public int age;
    public String color;
    public void eat(){
        System.out.println(name + "吃饭");
    }
    public void drink(){
        System.out.println(name + "喝水");
    }
}

new Cat() //创建一只猫对象

Cat cat01 = new Cat();//把创建的猫对象赋值给cat01

cat01是对象名(对象引用)

如何创建一个对象#

1) 先声明再创建
Cat cat ; //声明对象 cat
cat = new Cat(); //创建
2) 直接创建
Cat cat = new Cat();

补充：Java中实例化对象是什么意思

在Java语言中使用**new关键字创建/构造对象的过程叫做类的实例化**，该过程的本质，会在内存空间的**堆区申请一块存储区域**，用于**记录该对象独有的成员变量信息**。

通俗点就是，实例化：使用关键字new来创建对象。

例如：
Person person = new Person();

这样的语句称为创建对象。

对象是根据类创建的。在Java中使用关键字new创建对象。

new Person(); 是 声明一个Person类型的对象，在堆区区域。

new Person(); ()是指创建Person类型的对象后，自动调用Person类中的构造方法，来进行成员变量的初始化。

Person person ，Person为类名，person 为引用变量名。
————————————————
版权声明：本文为CSDN博主「智十七°」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/weixin_45812336/article/details/113954823

对象在内存中的存在形式#

image-20221207222229633

属性，成员变量#

package cn.meowrain.Object_;

public class relearn_02 {
}
class Hunman {
    //下面都属于Human类的属性
    String name; 
    double salary;
    int age;
    String country;
}

属性是类的一个组成部分，一般是基本数据类型,也可是引用类型(对象，数组)。上面这个代码中的name,salary等都是这个Human类的属性

属性的定义语法同变量，

示例：访问修饰符 属性类型 属性名

访问修饰符详情见Java 访问修饰符 - 喵雨の小屋|木更老婆！！！ (meowrain.cn)

属性的定义类型可以为任意类型，包含基本类型或引用类型

基本数据类型和引用数据类型详见Java数据类型：基本数据类型和引用数据类型 (biancheng.net)

Java数据类型结构图

Java数据类型（八种基本数据类型 + 四种引用数据类型）_火火笔记的博客-CSDN博客_引用数据类型有哪几种

属性如果不赋值，有默认值，规则和数组一致。

具体: int 0，short 0, byte 0, long 0, float 0.0,double 0.0，char \u0000， boolean false，String null

如何访问属性#

基本语法

对象名.属性名;

示范：

package cn.meowrain.Object_;

public class relearn_02 {
    Human human01 = new Human();
    human01.name = "mike";
    human01.age = 20;
    System.out.println(human.name + "is" + human01.age + "years old")
}
class Hunman {
    //下面都属于Human类的属性
    String name; 
    double salary;
    int age;
    String country;
}

类和对象的内存分配机制#

我们定义一个人类(Person)(包括 名字,年龄)

package cn.meowrain.Object_;

public class relearn_03 {
    public static void main(String[] args) {
        Person p1 = new Person();
        p1.age = 10;
        p1.name = "小明";
        Person p2 = p1;
        System.out.println(p2.age);

    }
}
class Person {
    public int age;
    public String name;

}

问题：p2.age 究竟是多少，画出内存图

image-20221207224810174

Java 内存的结构分析#

1) 栈： 一般存放基本数据类型(局部变量)
2) 堆： 存放对象(Cat cat , 数组等)
3) 方法区：常量池(常量，比如字符串)， 类加载信息

Java 创建对象的流程简单分析#

image-20221207230136236

image-20221207230219768

方法重载#

java中允许同一个类中，多个同名方法的存在，但是要求形参列表不一致

通过重载，我们减轻了起名的麻烦

案例：

package cn.meowrain.Object_;

public class relearn_06 {
    public static void main(String[] args) {
        rel rel = new rel();
        int m = rel.getSum(1,2,3);
        System.out.println(m);
        int n = rel.getSum(1,2);

    }
}
class rel {
    public int getSum(int n1,int n2){
        return n1+n2;
    }
    public int getSum(int n1,int n2,int n3){
        return n1+n2+n3;
    }
}

可变参数#

java允许把同一个类中多个同名同功能但是参数个数不同的方法，封装成一个方法

基本语法：

//int... 表示接受的是可变参数，类型是int，即可以接收多个int
//使用可变参数时候，可以当数组来使用，nums可以当做数组

package cn.meowrain.Object_;

public class relearn_07 {
    public static void main(String[] args) {
        rel_07 rel = new rel_07();
        int m = rel.getSum(1,2,3,4);//10
        int n = rel.getSum(5,6,7);//18
        int b = rel.getSum(0,1);//1
        System.out.println(m);
        System.out.println(n);
        System.out.println(b);
    }

}
class rel_07 {
    //int... 表示接受的是可变参数，类型是int，即可以接收多个int
    //使用可变参数时候，可以当数组来使用，nums可以当做数组
    public int getSum(int... nums){
        int res = 0;
        for(int i = 0;i<nums.length;i++){
            res+=nums[i];
        }
        return res;
    }
}

image-20221208221302540

作用域#

image-20221208221637490

package cn.meowrain.Object_;

public class relearn_08 {
    public static void main(String[] args) {

    }
}

class Dog {
    //全局变量：也就是属性，作用域为整个类体Cat类，cry eat等方法使用属性
    //属性在定义时，可以直接赋值
    int age = 10;
    double weight;//默认是0.0

    public void hi() {
        //局部变量必须赋值后才能使用，因为没有默认值
        //局部变量一般是指在成员方法中定义的变量
        int num = 1;
        String address = "Beijing's Dog";
        System.out.println("num=" + num);
        System.out.println("address" + address);
        System.out.println("weight=" + weight);
    }

    public void cry() {
        int n = 10;
        String name = "jack";
        System.out.println(name + "cry for" + n + "times");


    }

    public void eat() {
        String name = "meow";
        System.out.println(name + "eat");
    }
}

注意事项和使用细节#

image-20221208222256061

属性可以加修饰符，但是局部变量不能加修饰符

构造器#

构造方法又叫构造器(constructor)，是类的一种特殊的方法，

它的主要作用是完成对新对象的初始化。它有几个特点：

1) 方法名和类名相同
1) 没有返回值
1) 在创建对象时，系统会自动的调用该类的构造器完成对象的初始化。

image-20221208223933784

基本语法：

[修饰符] 方法名(形参列表) {
    方法体
}

image-20221208224012942

案例：

package cn.meowrain.Object_;

public class relearn_09 {
    public static void main(String[] args) {
        rel_09 rel = new rel_09("meowrain", 18);
        rel_09 rel_1 = new rel_09("meow", 20, "mouse");
        rel.getInfo();
        rel_1.getItem();
    }

}

class rel_09 {
    String name;
    int age;
    String item;

    public rel_09(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // 构造器重载
    public rel_09(String name, int age, String item) {
        this.name = name;
        this.age = age;
        this.item = item;
    }

    public void getInfo() {
        System.out.println(this.name + "  " + this.age);
    }

    public void getItem() {
        System.out.println(this.item);
    }
}

image-20221208224712870

练习题：#

image-20221208225109288

package cn.meowrain.Object_;

public class relearn_10 {
    public static void main(String[] args) {
        Person_n person01 = new Person_n("meow", 12);
        Person_n person02 = new Person_n();
        System.out.println(person01.name  + " is " + person01.age + "years old");
        System.out.println(person02.name + " is " + person02.age + "years old");
    }

}

class Person_n {
    String name;
    int age;

    public Person_n() {
        this.age = 18;
    }

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

}

image-20221208230513474

对象创建的流程分析#

image-20221208232146688

image-20221208232252577

this关键字#

image-20221208232330819

什么是this?

简单说：哪个对象调用，this就代表哪个对象

image-20221208232947980

image-20221208233027570

package cn.meowrain.Object_;

public class relearn12 {

}
class This01 {
    public static void main(String[] args) {
        Dog_new dog_new = new Dog_new("大黄", 1);
        dog_new.info();
    }
}
class Dog_new {
    String name;
    int age;
    public Dog_new(String name,int age) {
        this.name = name;
        this.age = age;
    }
    public void info(){
        System.out.println("this.hashCode=" + this.hashCode());
        System.out.println(name + "\t" + age + "\t");
    }
}

练习题：#

image-20221208233149601

package cn.meowrain.Object_;

public class relearn13 {
    public static void main(String[] args) {
        person_01 p1 = new person_01("mary", 18);
        person_01 p2 = new person_01("mary",18);
        System.out.println(p1.compareTo(p2)); //true
    }

}
class person_01 {
    String name;
    int age;
    public person_01(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public boolean compareTo(person_01 p){
        if(this.name.equals(p.name)&&this.age == p.age) {
            return true;
        }
        return false;
    }
}

作业#

作业1#

image-20221208233847713

package cn.meowrain.Object_.homework;

public class h1 {
    public static void main(String[] args) {
        double[] arr = { 1.0, -2.0, 4.5 };
        A01 a01 = new A01();
        double max = a01.max(arr);
        System.out.println(max);

    }
}

class A01 {
    public double max(double[] a) {
        double m = a[0];
        for (int i = 0; i < a.length; i++) {
            if (a[i] > m)
                m = a[i];
        }
        return m;
    }
}

作业2#

image-20221208233859783

package cn.meowrain.Object_.homework;

public class h2 {
    public static void main(String[] args) {
        String[] strArr = {"helloworld","meowrain"};
        A02 a02 = new A02();
        int a = a02.find(strArr);
        if(a == 0){
            System.out.println(true);
        }else {
            System.out.println(false);
        }
    }

}
class A02 {
    public int find(String []arr){
        for (int i = 0; i < arr.length; i++){
            if(arr[i].equals("helloworld")){
                return i;
            }
        }
        return -1;
    }
}

作业3#

image-20221208233908170

package cn.meowrain.Object_.homework;

public class h3 {
    public static void main(String[] args) {
        Book book = new Book("猫的故事", 200);
        Book book2 = new Book("狗的故事", 120);
        Book book3 = new Book("驴的故事", 20);
        book.updatePrice(book.price);
        book2.updatePrice(book2.price);
        book3.updatePrice(book3.price);
        System.out.println(book.name + " : " + book.price);
        System.out.println(book2.name + " : " + book2.price);
        System.out.println(book3.name + " : " + book3.price);
        /*
         *  猫的故事 : 150.0
            狗的故事 : 100.0
            驴的故事 : 20.0
         * 
         */
    }
}
class Book {
    String name;
    double price;
    public Book(String name,double price) {
        this.name = name;
        this.price = price;
    }
    public void updatePrice(double price){
        if(price > 150){
            this.price = 150;
        }else if(price > 100) {
            this.price = 100;
        }
    }
}

作业4#

image-20221208233917803

package cn.meowrain.Object_.homework;

public class h4 {
    public static void main(String[] args) {
        A03 a03 = new A03();
        int[] arr = {1,2,3,4,5};
        int[] a1 = a03.copyArr(arr);
        for(int i=0;i<a1.length; i++){
            System.out.println(a1[i]);
        }
    }
}
class A03 {
    public int[] copyArr(int[] arr) {
        int[] a = new int[arr.length];
        for (int i = 0; i < arr.length; i++){
            a[i] = arr[i];
        }
        return a;
    }

}

作业5#

image-20221208233925348

package cn.meowrain.Object_.homework;

public class h5 {
public static void main(String[] args) {
    Circle circle = new Circle(2);
    double perimeter = circle.perimeter();
    double area = circle.area();
    System.out.printf("%.2f\n",perimeter);
    System.out.printf("%.2f\n",area);
    /*
     *  12.57
        12.57
     */
}
}

class Circle {
    double r;

    public Circle(double r) {
        this.r = r;
    }

    public double perimeter() {
        return Math.PI*r*2;
    }
    public double area() {
        return Math.PI * r * r;
    }
}

作业6#

image-20221208233941030

package cn.meowrain.Object_.homework;

public class h6 {
    public static void main(String[] args) {
        Cale cale01 = new Cale(5, 2);
        Cale cale02 = new Cale(1, 0);
        cale01.divide();
        cale02.divide();
    }
}

class Cale {
    int a, b;

    public Cale(int a, int b) {
        this.a = a;
        this.b = b;
    }

    public int sum() {
        return a + b;

    }

    public int multiply() {
        return a * b;
    }

    public void divide() {
        if (b == 0) {
            System.out.println("输入的除数不能为0！");
        } else {
            System.out.println(a * 1.0 / b);
        }

    }
}

作业7#

image-20221208233951625

package cn.meowrain.Object_.homework;

public class h7 {
public static void main(String[] args) {
    Dog dog = new Dog("mike","white",2);
    dog.show();
}
}
class Dog {
    String name;
    String color;
    int age;
    public Dog(String name,String color,int age){
        this.name = name;
        this.color = color;
        this.age = age;
    }
    public void show(){
        System.out.println("the dog " + name + " is " + age + " " + "years old and its color is " + color);
    }
}

作业8#

image-20221208234033422

作业9#

image-20221208234041063

package cn.meowrain.Object_.homework;

public class h9 {

    public static void main(String[] args) {
        Music music01 = new Music("青花瓷", "4:00");
        Music music02 = new Music("鸡你太美", "3:00");
        music01.getInfo();
        music01.play();

    }
}
class Music
{
    String name;
    String times;
    public Music(String name,String times){
        this.name = name;
        this.times = times;
    }
    public void play(){
        System.out.println(name + "is palying for " + times);
    }
    public void getInfo(){
        System.out.println("the music name is " + name + "times is : " + times);
    }
}

作业10#

image-20221208234050200

package cn.meowrain.Object_.homework;

public class h8 {
    int i = 100;
    public void m(){
        int j = i++;
        System.out.println("i=" + i);
        System.out.println("j=" + j);

    }
}
class Test {
    public static void main(String[] args) {
        h8 demo = new h8();
        h8 demo2 = new h8();
        demo.m();
        System.out.println(demo.i);
        System.out.println(demo2.i);
    }
}

i=101
j=100
101  
100

作业11#

image-20221208234102159

不做了

作业12#

image-20221208234109474

package cn.meowrain.Object_.homework;

public class h12 {
    Employee_ emp1 = new Employee_("经理", "10000");


}
class Employee_ {
    String name;
    String sex;
    int age;
    String position;
    String salary;
    public Employee_(String name, String sex, int age, String position, String salary) {
        this.name = name;
        this.sex = sex;
        this.age = age;
        this.position = position;
        this.salary = salary;
    }
    public Employee_(String name, String sex, int age) {
        this.name = name;
        this.sex = sex;
        this.age = age;
    }
    public Employee_(String position, String salary) {
        this.position = position;
        this.salary = salary;
    }



}

作业13#

image-20221208234123863

package cn.meowrain.Object_.homework;

public class h13 {
    public static void main(String[] args) {
        Circle01 circle = new Circle01();
        PassObject pass = new PassObject();
        pass.printAreas(circle, 5);
    }
}
 class Circle01 {
    public double radius;
    public double findArea(){
        return radius*radius*Math.PI;
    }

 }
 class PassObject {
    public void printAreas(Circle01 c,int times) {
        for(int i = times;i>0;i--){
            c.radius = i;
            System.out.println("r:" + i + "  " + "S=" + c.findArea());
        }
    }
 }