java打印接口的ArrayList。

huangapple
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英文:

java print arraylist of interface

问题

  1. // Creates an ArrayList to hold all observers
  2. observers = new ArrayList<Observer>();
  4. // ...
  6. public void notifyObserver() {
  7.     for(Observer observer : observers){
  8.         observer.updateViaObserver(ibmPrice, aaplPrice, googPrice);
  9.     }
  10. }
  1. // Doesn't work
  2. toString();
  1. // Called to update all observers
  2. public void updateViaObserver(double ibmPrice, double aaplPrice, double googPrice) {
  3.     this.ibmPrice = ibmPrice;
  4.     this.aaplPrice = aaplPrice;
  5.     this.googPrice = googPrice;
  7.     // this works
  8.     printThePrices();
  10.     // doesn't work
  11.     toString();
  12. }
  1. // Accessing the ArrayList elements inside notifyObserver method
  2. System.out.println(observers.get(0));
英文:

I'm learning observer design pattern from one of youtube videos and want to understand a bit more the behavior of interface arraylist, I think I quite understand how it works but when it comes the interface arraylist, its confusing to me how to access the value inside loop.

the subject and observer interfaces as follow;

  1. public interface Subject {
  2. 	
  3. 	public void register(Observer o);
  4. 	public void unregister(Observer o);
  5. 	public void notifyObserver();
  6. 	
  7. }
  1. public interface Observer {
  2. 	
  3. 	public void updateViaObserver(double ibmPrice, double aaplPrice, double googPrice);
  4. 	
  5. }
  1. package observer_pattern;
  3. import java.util.ArrayList;
  4. import java.util.Arrays;
  6. public class StockGrabber implements Subject{
  7. 	
  8. 	private ArrayList&lt;Observer&gt; observers;
  9. 	private double ibmPrice;
  10. 	private double aaplPrice;
  11. 	private double googPrice; 
  12. 	
  13. 	public StockGrabber(){
  14. 		
  15. 		// Creates an ArrayList to hold all observers
  17. 		observers = new ArrayList&lt;Observer&gt;();
  18. 		
  19. 	}
  20. 	
  21. 	public void register(Observer newObserver) {
  22. 		
  23. 		observers.add(newObserver);
  24. 	
  25. 		
  26. 	}
  28. 	public void notifyObserver() {
  29. 		
  30. 		for(Observer observer : observers){
  32. 			observer.updateViaObserver(ibmPrice, aaplPrice, googPrice);
  33. 		
  34. 		}
  35. 		
  37. 	}
  38. 	
  39. 	public void setPrice(double newIBMPrice, double newAAPLPrice, double newGOOGPrice){
  40. 		
  41. 		this.ibmPrice = newIBMPrice;
  42. 		this.aaplPrice = newAAPLPrice;
  43. 		this.googPrice = newGOOGPrice;
  44. 		
  45. 		notifyObserver();
  46. 		
  47. 	}
  48. 	
  49. 	
  50. }
  1. package observer_pattern;
  3. public class StockObserver implements Observer {
  4. 	
  5. 	private double ibmPrice;
  6. 	private double aaplPrice;
  7. 	private double googPrice;
  9. 	private Subject stockGrabber;
  10. 	
  11. 	public StockObserver(Subject stockGrabber){
  12. 		
  13. 		this.stockGrabber = stockGrabber;
  14. 		
  15. 		this.observerID = ++observerIDTracker;
  16. 		
  17. 		stockGrabber.register(this);
  18. 		
  19. 	}
  20. 	
  21. 	// Called to update all observers
  22. 	
  23. 	public void updateViaObserver(double ibmPrice, double aaplPrice, double googPrice) {
  24. 		
  25. 		this.ibmPrice = ibmPrice;
  26. 		this.aaplPrice = aaplPrice;
  27. 		this.googPrice = googPrice;
  28. 		
  29. 		// this works
  30. 		printThePrices();
  33. 		// doesn&#39;t work
  34.         toString();
  35. 	}
  37. 	public void printThePrices(){
  38. 		
  39. 		System.out.println(observerID + &quot;\nIBM: &quot; + ibmPrice + &quot;\nAAPL: &quot; + 
  40. 				aaplPrice + &quot;\nGOOG: &quot; + googPrice + &quot;\n&quot;);
  41. 		
  42. 	}
  43. 	
  44. 	public String toString() {
  45. 	    return &quot;StockObserver: ibmPrice=&quot; + ibmPrice + &quot; aaplPrice=&quot; + aaplPrice;
  46. 	}
  47. 	
  48. }

MAIN

  1. package observer_pattern;
  3. public class GrabStocks{
  4. 	
  5. 	public static void main(String[] args){
  6. 		StockGrabber stockGrabber = new StockGrabber();
  7. 	
  8. 		StockObserver observer1 = new StockObserver(stockGrabber);
  9. 		
  10. 		stockGrabber.setIBMPrice(197.00);
  11. 		stockGrabber.setAAPLPrice(677.60);
  12. 		stockGrabber.setGOOGPrice(676.40);
  14.         observer1.toString();
  17. 		
  18. 	}
  19. 	
  20. }

How to access value for my ArrayList&lt;Observer&gt; observers inside "notifyObserver" method? if I do this System.out.println(observers.get(0)); I get observer_pattern.StockObserver@446cdf90

答案1

得分: 0

  1. 如何在 "notifyObserver" 方法内部访问我的 ArrayList<Observer> observers 的值？
  3. 你已经在做了。
  5. public void notifyObserver() {
  6.     for (Observer observer : observers) {
  7.         observer.updateViaObserver(ibmPrice, aaplPrice, googPrice);
  8.     }
  9. }
  11. 如果我执行 System.out.println(observers.get(0));，我会得到 observer_pattern.StockObserver@446cdf90。
  13. 这是因为你的 `StockObserver` 类没有一个返回对象的可读表示的 `toString` 方法。可以像这样做：
  15. public String toString() {
  16.     return "StockObserver: ibmPrice=" + ibmPrice + " aaplPrice=" + aaplPrice;
  17. }
英文:

> How to access value for my ArrayList<Observer> observers inside "notifyObserver" method?

You're already doing it.

  1. public void notifyObserver() {
  2.     for(Observer observer : observers){
  3.         observer.updateViaObserver(ibmPrice, aaplPrice, googPrice);
  4.     }
  5. }

> if I do this System.out.println(observers.get(0)); I get observer_pattern.StockObserver@446cdf90

That's because your StockObserver class does not have a toString method that returns a human readable representation of the object. Something like this

  1. public String toString() {
  2.     return &quot;StockObserver: ibmPrice=&quot; + ibmPrice + &quot; aaplPrice=&quot; + aaplPrice;
  3. }
  5. </details>

huangapple
  • 本文由 发表于 2020年4月6日 23:00:04
  • 转载请务必保留本文链接：https://go.coder-hub.com/61062706.html
  • arraylist
  • interface
  • java
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