多个实例不能使用相同的变量，变量不能改变

Question

我不太清楚这有什么问题，我相信它与范围有关。昨天我遇到了一个问题，即一个字段将自己初始化为零，这是因为我不止一次地回顾了该方法，因此无论调用哪种方法的时间，都会使该类字段保持其值，从而解决该问题。

现在我有相反的问题，我需要现场复位，因为另一个对象需要使用它（这是可能的/糟糕的做法？）

下面是代码：

public class TestDigitalCamera { 

    static String brand; 
    static double megaPixels; 
    static double price; 

    //create 2 camera instances with the values of the variables tied to the arguments. 
    static DigitalCamera camera = new DigitalCamera(brand, megaPixels); 
    static DigitalCamera camera2 = new DigitalCamera(brand, megaPixels); 


    public static void main(String[] args) { 

     //no idea what this technique is called, need to look back but I know what it does 
     //I could use a for loop and reuse the same object over and over(would that even work anyway?) but the task says 
     //that i require 4 instances, ofc I am just working with 2 atm for simplicity 
     camera = getInformation(camera); 
     displayInfo(); 
     camera2 = getInformation(camera2); 
     displayInfo(); 


    } 

    //it basically runs this for the camera instance...right? lol 
    public static DigitalCamera getInformation(DigitalCamera dc){ 
     Scanner userInput = new Scanner(System.in); 

     //self explanatory 
     System.out.println("Enter brand: "); 
     brand = userInput.next(); 
     System.out.println("Enter Mega Pixels: "); 
     megaPixels = userInput.nextDouble(); 

     //I have another class setup with getters/setters for this, which get used in the next method 
     dc.setBrand(brand); 
     dc.setMegaPixels(megaPixels); 
     return dc;  

    } 

    public static void displayInfo(){ 

     //users the getters to pull the values 
     //the price is calculated using an if statement 
     System.out.println("Brand: " + camera.getBrand() + "\n" 
       + "Megapixels : " + camera.getMegaPixels() + "\n" 
         + "Price : $" + camera.getPrice() + "\n"); 
    } 

} 

是这是由于范围？该变量对任何和所有对象都是可用的，但它只能用于1？什么是最好的方法来解决这个问题？

Answer 1

你有这样的代码：

camera = getInformation(camera); 
displayInfo(); 
camera2 = getInformation(camera2); 
displayInfo(); 

这里你的方法displayInfo()是不是真的采取任何参数，并从camera对象打印信息这两个时间的叫法。即使您在getInformation的第二个呼叫中获得camera2对象的参考，您并不是真的在打印它。

你可以声明displayInfo这样的：

public static void displayInfo(DigitalCamera camera) { 
    //users the getters to pull the values 
    //the price is calculated using an if statement 
    System.out.println("Brand: " + camera.getBrand() + "\n" 
     + "Megapixels : " + camera.getMegaPixels() + "\n" 
     + "Price : $" + camera.getPrice() + "\n"); 
} 

Answer 2

你应该重构你的代码是这样的：

public static void displayInfo(DigitalCamera camera){ 

    //users the getters to pull the values 
    //the price is calculated using an if statement 
    System.out.println("Brand: " + camera.getBrand() + "\n" 
      + "Megapixels : " + camera.getMegaPixels() + "\n" 
        + "Price : $" + camera.getPrice() + "\n"); 
} 

然后在main方法：

public static void main(String[] args) { 

    camera = getInformation(camera); 
    displayInfo(camera); 
    camera2 = getInformation(camera2); 
    displayInfo(camera2); 


} 

如果你声明您在中的两个DigitalCamera实例这样的方法

DigitalCamera camera = getInformation(camera); 

那么就不需要单独的静态变量了。

我会stronlgy建议你阅读类和实例成员在Java教程部分：http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html

Answer 3

我会写你的班级这样

公共类TestDigitalCamera {

public static test_display(){ 
    DigitalCamera camera = getCamera(); 
    System.out.println("Brand: " + camera.getBrand() + "\n" 
      + "Megapixels : " + camera.getMegaPixels() + "\n" 
      + "Price : $" + camera.getPrice() + "\n"); 

} 

public static getCamera(){ 
    Scanner userInput = new Scanner(System.in); 

    //self explanatory 
    System.out.println("Enter brand: "); 
    brand = userInput.next(); 
    System.out.println("Enter Mega Pixels: "); 
    megaPixels = userInput.nextDouble(); 

    //I have another class setup with getters/setters for this, which get used in the next method 
    DigitalCamera dc = new DigitalCamera(); 
    dc.setBrand(brand); 
    dc.setMegaPixels(megaPixels); 
    return dc; 
} 

public static void main(String[] args) { 
    test_displainfo(); 
} 

}