理解实现 Java 扩展类

Question

我目前正在学习 Java，我的任务是了解 OOP。 我了解类的理论，但我对实现有疑问。

以下代码为例：

class Vehicle {
  private String engine;
  private int wheels;
  private int seats;
  private int fuelTank;
  private String lights;

  public Vehicle() {
    this.engine = "petrol";
    this.wheels = 4;
    this.seats = 4;
    this.fuelTank = 35;
    this.lights = "LED";

  }

  public Vehicle(String engine, int wheels, int seats, int fuelTank, String lights) {

    this.engine = engine;
    this.wheels = wheels;
    this.seats = seats;
    this.fuelTank = fuelTank;
    this.lights = lights;
  }

  public String getEngine() {
    return engine;
  }

  public int getWheels() {
    return wheels;
  }

  public int getSeats() {
    return seats;
  }

  public int getFueTank() {
    return fuelTank;
  }

  public String getLights() {
    return lights;
  }

}

class Car extends Vehicle {
  private String steering;
  private String musicSystem;
  private String airConditioner;
  private String fridge;
  private String entertainmentSystem;

  public Car() {
    super();
    this.steering = "Power Steering";
  }

  public Car(String steering, String engine, int wheels, int seats, int fueTank, String lights) {
    super(engine, wheels, seats, fueTank, lights);
    this.steering = steering;
  }

  public String getSteering() {
    return steering;
  }
}

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

    Car car = new Car("Power steering", "deisel", 4, 4, 40, "LED");

    System.out.println("Steering: " + car.getSteering());
    System.out.println("Engine type: " + car.getEngine());
    System.out.println("Number of seats: " + car.getSeats());
    System.out.println("Fuel tank capacity: " + car.getFueTank());
    System.out.println("Head lamp type: " + car.getLights());
    System.out.println("Number of wheels: " + car.getWheels());
  }

}

我在这里了解到，您可以使用默认构造函数或参数化构造函数来创建 Vehicle 对象。我也可以对从Vehicle 扩展的Car 对象执行相同的操作。我知道通过在默认构造函数中使用super()，它将使用Vehicle 中的默认构造函数以及Car 对象转向变量默认值。与使用super(args) 的参数化构造函数相同。

我在理解如何在每个扩展级别的车辆中对车轮进行硬编码时遇到问题。

为了解释更多，我不想在创建Car 对象时在构造函数中包含轮子。我希望它默认为 4。此外，如果我要创建一个从 Vehicle 扩展的 Bike 对象，我希望轮子变量默认为 2。

Answer 1

如果您真的想使用您的超类属性，请将其访问修饰符类型更改为受保护，否则您将无法直接在您的子类中使用它们。

然后您可以使用 IIB（实例初始化块）为每个不同的子类定义您的轮子变量值。

示例解决方案：

Vehicle.java

public class Vehicle{

    //attributes with protected access modifier
    protected String engine;
    protected int wheels;
    protected int seats;
    protected int fuelTank;
    protected String lights;

    public Vehicle(){//No-args constructor

    }

    
     public Vehicle(String engine, int seats, int fuelTank, String lights) {

         this.engine = engine;
         this.seats = seats;
         this.fuelTank = fuelTank;
         this.lights = lights;

    }

     public Vehicle(String engine, int wheels, int seats, int fuelTank, String lights) {

         this.engine = engine;
         this.wheels = wheels;
         this.seats = seats;
         this.fuelTank = fuelTank;
         this.lights = lights;
     }

       
}

汽车.java

public class Car extends Vehicle{

     private String steering;
     private String musicSystem;
     private String airConditioner;
     private String fridge;
     private String entertainmentSystem;

    {//IIB block to initialize wheels with default value

        wheels = 4;//Defining the wheels variable 


    }

     public Car(String steering,String engine,int seats,int fuelTank,String lights) 
     {//Overloaded constructor
         super(engine,seats,fuelTank,lights);
         this.steering = steering;

     }

}

Answer 2

您可以按照以下方式进行操作。

private final String steering; // final = constant, will not change.

public Car(String steering, String engine, int seats, int fueTank, String lights) {
    super(engine, 4 /*wheels*/, seats, fueTank, lights);
    this.steering = steering;
}

public Car(String steering, String engine, int fueTank, String lights) {
    this(steering, engine, 4 /*seats*/, fueTank, lights);
}

正如您在此处看到的，多个构造函数和许多参数可能会变得很麻烦。

有时最好为特殊字段设置设置器。