Arrays and Generics

I'm having trouble with a generic method that can accept ArrayLists of type integer and type double.

The program fails to compile because it says myList is of type "T" and i is of type "double".

Here's a snippet of code:

Code :

---

public double calculateTotals(ArrayList<T> list)
{
       private ArrayList<T> myList = new ArrayList<T>(10);
       myList = list;
 
       for (double i : myList) {
        ....do stuff
       }
}
 
public static void main(String args[])
{
ArrayList<Double> doubleArrayList = new ArrayList<Double>(10);
...populate array...
 
double totals = calculateTotals(doubleArrayList);
 
}

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I'm trying to figure out what I'm doing wrong. I thought myList would become an arraylist of type double since that's what I'm passing in.

Any clues would be greatly appreciated!

Thanks!

First, you can't iterate over a generic list expecting it to be a double (this is the compile time checking at work that makes generics so powerful). You need to iterate over the list using the generic

Code :

---

for ( T t : myList ){
//do something
}

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Second, you are accessing the method in a static way - define it as static or define a class that implements the method. Lastly, I presume based upon the function name you want to calculate the total regardless of whether the list contains int's, double's, etc...in which case you might want to have T extends Number, so you can use the doubleValue method to calculate the total

Oh ok, that makes sense.

I changed my for loop to use type "T"...

once in the loop, I need to apply math operations to the data, but I can't use addition, subtraction, etc on type "T". How does one do math operations on generic types?

Thanks!

Quote:

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Originally Posted by copeg

First, you can't iterate over a generic list expecting it to be a double (this is the compile time checking at work that makes generics so powerful). You need to iterate over the list using the generic

Code :

---

for ( T t : myList ){
//do something
}

---

Second, you are accessing the method in a static way - define it as static or define a class that implements the method. Lastly, I presume based upon the function name you want to calculate the total regardless of whether the list contains int's, double's, etc...in which case you might want to have T extends Number, so you can use the doubleValue method to calculate the total

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Ok, well I figured my other issue out myself...basically when using:

Code :

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for (T t : myList)

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I had to do this when performing math operations:

Code :

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total = (Double)t - constantRate;

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Read my post above again - if you defined T to extends Number, there will be no need to cast (which defeats the purpose of generics anyways)
see Number (Java Platform SE 6)

Code :

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public class MyClass<T extends Number>{
   public void myFunction(List<T> list){
 
     for ( T t : list ){
         //use a method defined by number here
     }
   }
}

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