Satoshi

Java 面向对象编程的概念

2019/04/22 Java

目录

What Is an Object?

对象是由相关状态和行为组成的

  • 将代码块放到对象的好处
    1. 模块化
    2. 信息隐藏
    3. 代码重复利用
    4. 可插入和可调式

What Is a Class?

类是创建对象的蓝图或原型
在现实世界中，你会发现有许多相同特征的个体。比如:
所有的自行车都是按照一个蓝图进行设计，以下是自行车一种可能实现的类

class Bicycle {

    int cadence = 0;

    int speed = 0;

    int gear = 1;

    void changeCadence(int newValue) {
        cadence = newValue;
    }

    void changeGear(int newValue) {
        gear = newValue;
    }

    void speedUp(int increment) {
        speed = speed + increment;
    }

    void applyBrakes(int decrement) {
        speed = speed - decrement;
    }

    void printStates() {
        System.out.println("cadence:" +
                cadence + " speed:" +
                speed + " gear:" + gear);
    }
}

字段cadence、speed和gear表示自行车的状态
方法changeCadence、changeGear等表示他和外界的交互

下面是一个BicycleDemo，创建对象并调用他的方法

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

        // Create two different
        // Bicycle objects
        Bicycle bike1 = new Bicycle();
        Bicycle bike2 = new Bicycle();

        // Invoke methods on
        // those objects
        bike1.changeCadence(50);
        bike1.speedUp(10);
        bike1.changeGear(2);
        bike1.printStates();

        bike2.changeCadence(50);
        bike2.speedUp(10);
        bike2.changeGear(2);
        bike2.changeCadence(40);
        bike2.speedUp(10);
        bike2.changeGear(3);
        bike2.printStates();
    }
}

运行结果截图
BicycleDemo

What Is Inheritance?

继继承为组织和构造软件提供了一种强大的机制
在面向对象编程语言中，一个父类可以拥有许多子类，但只允许继承一个父类
创建子类的关键字为extends,如下列代码所示

class MountainBike extends Bicycle {

    // new fields and methods defining
    // a mountain bike would go here

}

What Is an Interface?

接口是连接类与外部的桥梁
接口是抽象方法的集合，下面声明了一个自行车的接口

interface Bicycle {

    //  wheel revolutions per minute
    void changeCadence(int newValue);

    void changeGear(int newValue);

    void speedUp(int increment);

    void applyBrakes(int decrement);
}

实现接口需要使用关键字implements,一个类可以实现多个接口
下面为实现上面接口的一个类

class ACMEBicycle implements Bicycle {

    int cadence = 0;
    int speed = 0;
    int gear = 1;

   // The compiler will now require that methods
   // changeCadence, changeGear, speedUp, and applyBrakes
   // all be implemented. Compilation will fail if those
   // methods are missing from this class.

    void changeCadence(int newValue) {
         cadence = newValue;
    }

    void changeGear(int newValue) {
         gear = newValue;
    }

    void speedUp(int increment) {
         speed = speed + increment;
    }

    void applyBrakes(int decrement) {
         speed = speed - decrement;
    }

    void printStates() {
         System.out.println("cadence:" +
             cadence + " speed:" +
             speed + " gear:" + gear);
    }
}

What Is a Package?

包是以逻辑的方式组织类和接口
Java Platform API Specification ,Java SE平台提供的所有包、接口、类、字段和方法的完整清

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