观察者模式类（JAVA）

Question

I'm stuck on this assignment. I'm given an abstract Observer class with only 1 constructor in it, a constructor with parameters/arguments. (refer below)

public static void main(String[] args) {    
    PairOfNumbers numbers1 = new PairOfNumbers();            
    PairOfNumbers numbers2 = new PairOfNumbers();        
    SumObserver sum = new SumObserver(numbers1);        
    ProductObserver prod = new ProductObserver(numbers2);    
    MultiSubjectObserver m = new MultiSubjectObserver();
    m.addSubject(numbers1);
    m.addSubject(numbers2);        
    numbers1.setNumbers(20, 10);
    numbers2.setNumbers(-10, 15);       
}    


class Subject {
    private List<Observer> observers=new ArrayList<Observer>();
    public void attachObserver(Observer observer) {
    this.observers.add(observer);
    }
    public void detachObserver(Observer observer) {
        this.observers.remove(observer);
    }
    public void notifyObservers() {
    for (Observer observer: this.observers)
        observer.update(this);
    }
}   

class PairOfNumbers extends Subject {
    private double number1, number2;
    public double getNumber1() { return this.number1; }
    public double getNumber2() { return this.number2; }
    public void setNumbers(double d1, double d2) {
    this.number1=d1; this.number2=d2;
    this.notifyObservers();  // don't forget to do this!
    }
} 

abstract class Observer {
    public Observer(Subject subject) {
    subject.attachObserver(this);
}

abstract public void update(Subject subject);
}

class SumObserver extends Observer {
    public SumObserver(PairOfNumbers pair) {
    super(pair);
    }
    public void update(Subject subject) {
    PairOfNumbers numbers=(PairOfNumbers)subject;
    System.out.println("New sum is: "+(numbers.getNumber1()+numbers.getNumber2()));
    }
}

class ProductObserver extends Observer {
    public ProductObserver(PairOfNumbers pair) {
        super(pair);        
    }
    public void update(Subject subject) {
   PairOfNumbers numbers=(PairOfNumbers)subject;
   System.out.println("New product is: "+(numbers.getNumber1()*numbers.getNumber2()));
    }
}

Okay, now I'm suppose to create another class which inherits from the above class.

class MultiSubjectObserver extends Observer{
    public MultiSubjectObserver(PairOfNumbers pair){
        super(pair);
    }   

    public void addSubject(PairOfNumbers pair){
        pair.attachObserver(this);
    }

    public void update(Subject subject){
        PairOfNumbers numbers=(PairOfNumbers)subject;
        System.out.println("MultiSubjectObserver activated with numbers: " +    (numbers.getNumber1())+", "+(numbers.getNumber2()));
    }        
}

Is there a way to create a constructor inside the MSO Class which requires no parameter/argument? For example

public MultiSubjectObserver(){
    //enter code here 
}

Please guide me on this one. Had been thinking for days. Thanks in advance! :D

The instruction is to: Modify the source code to handle any number of Subject objects per Observer.

Expected output:

New sum is: 30.0 
MultiSubjectObserver activated with numbers: 20.0, 10.0 
New product is: -150.0 
MultiSubjectObserver activated with numbers: -10.0, 15.0

Answer 1

Yes you can do this, create a no-arg child class, but you still must call the arg-needing super constructor within the child constructor.

This:

class Child extends Super {
   public Child() {
      super(args_are_needed);
   }
}

The tricky part would be -- what to pass into the super constructor in this default case? In your case this could be:

public MultiSubjectObserver(){
    super(null);
}

Caveat: and this will lead to a NullPointerException when the super's constructor is called, due to the line, subject.attachObserver(this); , so no, you can't do this.

A better solution: make sure that MultiSubjectObserver does not extend from Observer!

Perhaps something like:

class MultiSubjectObserver {
   private List<Observer> observerList = new ArrayList<Observer>();

   public void addSubject(PairOfNumbers numbers1) {
      observerList.add(new InnerObserver(numbers1));
   }

   private class InnerObserver extends Observer {
      public InnerObserver(Subject subject) {
         super(subject);
      }

      @Override
      public void update(Subject subject) {
         System.out.println("From multi-observer: " + subject);
      }
   }    
}

But for this to work, you'd have to give PairOfNumbers a decent toString method, perhaps,

@Override
public String toString() {
   return String.format("[%.4f, %.4f]", number1, number2);
}

---

Edit
Based on the output:

class MultiSubjectObserver {
   private static final String FORMAT_STRING = "MultiSubjectObserver activated with numbers: %.1f, %.1f%n";
   private List<Observer> observerList = new ArrayList<Observer>();

   public void addSubject(PairOfNumbers numbers1) {
      observerList.add(new InnerObserver(numbers1));
   }

   private class InnerObserver extends Observer {
      public InnerObserver(Subject subject) {
         super(subject);
      }

      @Override
      public void update(Subject subject) {
         System.out.printf(FORMAT_STRING, ((PairOfNumbers)subject).getNumber1(),  ((PairOfNumbers)subject).getNumber1());
      }
   }
}

Although that casting is a bit skanky. I like the toString() version much better.