Java - 类和对象示例

Java的类和对象示例

在学习了一定的Java语言基础之后，接下来会继续学习Java的类和对象，这篇文章会简单介绍一下Java类中的概念：声明成员变量、方法、构造器。

类

通过一个自行车类以及其子类山地车来作为示例，在自行车类Bicycle中声明该类的成员变量，方法以及构造器。子类MountainBike继承Bicycle类的所有成员变量和方法，并且在构造器中使用super方法调用父类的构造器，作为子类构造器的一部分：

Bicycle类

public class Bicycle {
        
    // the Bicycle class has
    // three fields
    public int cadence;
    public int gear;
    public int speed;
        
    // the Bicycle class has
    // one constructor
    public Bicycle(int startCadence, int startSpeed, int startGear) {
        gear = startGear;
        cadence = startCadence;
        speed = startSpeed;
    }
        
    // the Bicycle class has
    // four methods
    public void setCadence(int newValue) {
        cadence = newValue;
    }
        
    public void setGear(int newValue) {
        gear = newValue;
    }
        
    public void applyBrake(int decrement) {
        speed -= decrement;
    }
        
    public void speedUp(int increment) {
        speed += increment;
    }
        
}

MountainBike类

public class MountainBike extends Bicycle {
        
    // the MountainBike subclass has
    // one field
    public int seatHeight;

    // the MountainBike subclass has
    // one constructor
    public MountainBike(int startHeight, int startCadence,
                        int startSpeed, int startGear) {
        super(startCadence, startSpeed, startGear);
        seatHeight = startHeight;
    }   
        
    // the MountainBike subclass has
    // one method
    public void setHeight(int newValue) {
        seatHeight = newValue;
    }   

}

对象

一个典型的Java应用创建了很多对象用于交互，通过这些对象使用内部实现的方法交互可以让我们实现GUI、动画、在网络上收发信息，一旦一个类完成了它的任务，那么这个类的资源将会被其他的对象回收利用。

下面这段伪代码，创建了三个类：

public class CreateObjectDemo {

    public static void main(String[] args) {
		
        // Declare and create a point object and two rectangle objects.
        Point originOne = new Point(23, 94);
        Rectangle rectOne = new Rectangle(originOne, 100, 200);
        Rectangle rectTwo = new Rectangle(50, 100);
		
        // display rectOne's width, height, and area
        System.out.println("Width of rectOne: " + rectOne.width);
        System.out.println("Height of rectOne: " + rectOne.height);
        System.out.println("Area of rectOne: " + rectOne.getArea());
		
        // set rectTwo's position
        rectTwo.origin = originOne;
		
        // display rectTwo's position
        System.out.println("X Position of rectTwo: " + rectTwo.origin.x);
        System.out.println("Y Position of rectTwo: " + rectTwo.origin.y);
		
        // move rectTwo and display its new position
        rectTwo.move(40, 72);
        System.out.println("X Position of rectTwo: " + rectTwo.origin.x);
        System.out.println("Y Position of rectTwo: " + rectTwo.origin.y);
    }
}