似乎无法让.doubleValue（）工作（我有最新版本的Java）

Question

So I am having problem with a method I created, called averageCost(). The error I get is:

    Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
The method doubleValue() is undefined for the type Double

at queue.averageCost(queue.java:38)
at queue.main(queue.java:47)

And previously I tried just adding a double value to a Double object which obviously didn't work. So I found the .doubleValue method but I can't get it to work. I added all my code (everything else works just fine) in case that clears anything up. I have been stuck on this for a couple days now while working on other stuff please help ):

    import java.util.LinkedList;
    public class queue<Double> {

private LinkedList<Double> linklist;
private String symbol;
private String name;
private int shares;
private Double price;

public queue(String symbol2, String name2, int shares2, Double price2) { //so far, I have a linkedlist that works
    linklist = new LinkedList<Double>();
    shares = shares2;
    price = price2;
    symbol = symbol2;
    name = name2;
    bigAdd(shares, price);
}

public void add(Double e) {
    linklist.add(e);
}

public Double take() {
    return linklist.poll();
}       

public void bigAdd (int shares2, Double price2) { 
    while(shares2>0) {
        linklist.add(price2);
        shares2--;
    }
}

public double averageCost(int shares2) {
    double average = 0;
    int sizer = 0;
    while(sizer < shares2) {
        average = average + linklist.poll().doubleValue(); //this is where the problem lies
        sizer++;
    }
    average = average/shares2;
    return average;
}
    }

Answer 1

Your mistake comes from the class declaration.

Formal parameter of generic may be T:

public class queue<T> {

Now you see that T has no method named doubleValue because you haven't constrained it.

The following do what you want:

import java.util.LinkedList;

public class queue<T extends Number> {

   private final LinkedList<T> linklist;
   private final int           shares;
   private final T             price;

   public queue( int shares2, T price2 ) {
      linklist = new LinkedList<>();
      shares   = shares2;
      price    = price2;
      bigAdd( shares, price );
   }

   public void add( T e ) {
      linklist.add(e);
   }

   public T take() {
      return linklist.poll();
   }

   public void bigAdd( int shares2, T price2 ) {
      while(shares2>0) {
         linklist.add(price2);
         shares2--;
      }
   }

   public double averageCost( int shares2 ) {
      double average = 0;
      int sizer = 0;
      while( sizer < shares2 ) {
         final T v = linklist.poll();
         average = average + v.doubleValue();
         sizer++;
      }
      average = average/shares2;
      return average;
   }
}

Answer 2

You can just use linklist.poll() . The Double will get unboxed to a double .