定义类和创建对象-优快云博客

本文链接：https://blog.youkuaiyun.com/Darling912/article/details/144707391

定义类和创建对象

Class

类是定义了相同类型对象的构造
在Java类中可以去使用变量去定义数据区域和方法去定义行为
此外，类提供了一种特殊的方法也就是构造方法，成为构造函数，
调用这些方法可以从类中构造对象

package demo03.oop01;

/**
 * @program: interview
 * @author: spring
 * @create: 2024-12-23
 */
class Circle {

    /**
     * data field
     */
    // The radius of the circle
    double radius = 1.0;

    /**
     * construct
     */
    // Construct a circle object
    Circle() {
        // no args
    }
    
    // Construct a circle object
    Circle(double newRadius) {
        // args
        radius = newRadius;
    }

    /**
     * method
     */
    double getArea() {
        return radius * radius * 3.14;
    }
}

定义类和创建对象的过程

package demo03.oop01;

/**
 * @program: interview
 * @author: spring
 * @create: 2024-12-23
 */
public class SimpleCircle {

    // data field
    double radius;

    // no args construct
    SimpleCircle() {

    }

    // args construct
    SimpleCircle(double radius) {
        this.radius = radius;
    }

    // return the area of this circle
    double getRadius() {
        return radius * radius * Math.PI;
    }

    // return the perimeter of the circle
    double getPerimeter() {
        return 2 * radius * Math.PI;
    }

    // Set a new radius for this circle 
    void setRadius(double radius) {
        this.radius = radius;
    }
}

package demo03.oop01;

/**
 * @program: interview
 * @author: spring
 * @create: 2024-12-23
 */
public class TestSimpleCircle {
    public static void main(String[] args) {

        SimpleCircle circle1 = new SimpleCircle();
        System.out.println(circle1.radius + "  " + circle1.getRadius());

        SimpleCircle circle2 = new SimpleCircle(25);
        System.out.println(circle2.radius + "  " + circle2.getRadius());


    }
}

another example

package demo03.oop01;

/**
 * @program: interview
 * @author: spring
 * @create: 2024-12-23
 */
public class TV {

    private int channel = 1;
    private int volumeLevel = 1;
    private boolean on = false;

    public TV() {

    }

    public TV(int channel, int volumeLevel, boolean on) {
        this.channel = channel;
        this.volumeLevel = volumeLevel;
        this.on = on;
    }

    public int getChannel() {
        return channel;
    }

    public void setChannel(int channel) {
        // using if else to check 
        this.channel = channel;
    }

    public int getVolumeLevel() {
        return volumeLevel;
    }

    public void setVolumeLevel(int volumeLevel) {
        // using if else to check 
        this.volumeLevel = volumeLevel;
    }

    public boolean isOn() {
        return on;
    }

    public void setOn(boolean on) {
        this.on = on;
    }

    public void turnOn() {
        on = true;
    }

    public void turnOff() {
        on = false;
    }
    
    public void channelUp(){
        if (on&&channel<120){
            channel++;
        }
    }
    
    public void  channelDown(){
        if (on&&channel>1){
            channel--;
        }
    }

}

package demo03.oop01;

/**
 * @program: interview
 * @author: spring
 * @create: 2024-12-23
 */
public class TestTv {

    public static void main(String[] args) {

        TV tv1 = new TV();
        tv1.turnOn();
        tv1.setChannel(30);
        tv1.setVolumeLevel(3);

        TV tv2 = new TV();
        tv2.turnOn();
        tv2.channelUp();
        tv2.channelDown();
        tv2.setVolumeLevel(50);

        System.out.println(tv1.channel + " " + tv1.volumeLevel);
        System.out.println(tv2.channel + " " + tv2.volumeLevel);
    }
}

Construct

// no args construc
Circle(){
    
}

// args construct
Circle(double radius){
    this.radius = radius;
}

构造函数是用来构造对象的一种特殊的方法

没有形参的构造函数叫做无参构造函数

A constructor with no parameters is referred to as a no-arg constructor

the three feature

构造函数的名称必须和类名是相同的
构造函数没有返回值-甚至没有void
创建对象时使用new操作符调用构造函数，构造函数起到了初始化对象的作用
Constructor must be the same as the Class name
Constructor doesn't have the value of the return - even the void
create the object uses the opeator of the new to invoke the constructor , it play a role in the initalize the object

new ClassName();

// Example
new Circle();

new Circle(5.0);

默认构造函数 default constructor

A class may be defined without constructors . In this case, a no-arg constructor with an empty body is implicitly(implicitly) declared in the class. This construcor is called the default constuctor , is provided automatically only if no constructos are explicitly defined in the class

类在没有构造方法的时候，在这种情况下，JVM 会主动在在类中隐式的给我们添加上一个无参构造方法，这个无参构造方法不是我们手动的进行添加的，所以也叫做默认构造方法，仅存在类中没有显式的定义构造函数的时候，这个默认构造方法才会自己去进行添加

声明对象引用变量

Declaring OBject Reference Variables

要去引用对象的话，首先我们要把对象赋值给引用变量

To reference an object , assign the object to a reference variable

ClassName objRefVar
Circle myCirCle

Declare / create Object in a singel step

ClassName objRefVar = new ClassName();

new Circle // create the object
myCircle // assign object reference

reference the object' s data

objRefVar.data
myCircle.radius

invoke the object's method

objRefVar.method()
myCircle.getArea()

declare the refVar
Create the obj
assign the objRef to the refVar
change radius in youCircle

在面向对象之前所学习的所有的类，eg Math 这样的都是由static所修饰的静态类，是可以直接通过这个. dot运算符直接去进行调用的

但是我们现在所学的是没有加上static关键字，也就并非静态的

必须是要去通过对象去进行相关的调用的

Reference Data Field

数据字段可以是引用类型的

public class Student{
    String name;
    int age;
    boolean isScienceMajor;
    char gender;
}

如果引用类型的数据字段不引用任何的对象的话，则该数据字段将保存一个特殊的文字值

public class Person {
    // 引用类型字段
    String address;

    public static void main(String[] args) {
        Person person = new Person();
        
        // 在没有显式赋值的情况下，address字段默认值为null
        System.out.println(person.address);  // 输出 null
    }
}

数据字段的默认值

package demo03.oop01;

import demo02.test01.Student;

/**
 * @program: interview
 * @author: spring
 * @create: 2024-12-23
 */
public class Test {

    public static void main(String[] args) {


        Student student = new Student();
        System.out.println(student.getName());
        System.out.println(student.getAge());
        System.out.println(student.getGender());
    }
}