Pointer to a function with Android - function

Question

I have a problem to solve with Android, but it's really confusing.
Using the function below:
function accumulate(combiner, nullValue, list){
if(list.length == 0){
return nullValue;
}
var first = list.removeFirst();
return combiner(first, accumulate(combiner, nullValue, list));
}
Develop the function sumOfSquares which returns the sum of squares of a list (Example: 1² + 2² + 3²...)
sumOfSquares([1,2,3,4,5])
returns the number 55.
In this case, the function accumulate must be used. The variable "combiner" is a "pointer to a function". The implementation of the function "combiner" is part of the solution.
I have no problem with the basics, doing the sum of squares, etc, but the part "pointer to a function" really confused me.
If anyone can tell me which is the way to get to the answer, I will be thankful :)
I have done until the code below:
public class MainActivity extends Activity{
protected void onCreate(...){
....
List<Integer> list = new ArrayList<Integer>();
//Fill the list with values
long value = accumulate(sumOfSquares(list), 0, list);
//Show the value
}
private int sumOfSquares(List<Integer> list){
int sum = 0;
for(int i = 0; i < list.size(); i++){
sum += Math.pow(list.get(i), 2);
}
return sum;
}
private long accumulate(int combiner, long nullValue, List<Integer> list){
if(list.size() == 0){
return nullValue;
}
int first = list.get(0);
list.remove(0);
return combiner(first, accumulate(combiner, nullValue, list));
}
private long combiner(int first, int rest){
return first + rest;
}
}

Answer 1

In some languages, the notion of a pointer to a function makes sense, and you could write the code pretty much as you've given it in the example. Not in Java, though, which is what underlies Android. (Android is a bit of a weird choice for this, by the way...)
What you want to do in Java (without giving you the whole solution) is to define a
private int combiner(int first, int rest);
method that takes the first element of the list and the solution to the smaller problem defined by the rest of the list, and produces the answer from these two bits. In other words, if first is the first element, and rest is the sum of the squares of everything except the first element, what is the sum of the squares of the whole list (in terms of first and rest)?
Now your accumulate method does almost exactly what you've written out. It just removes the first element, recursively calls itself on the rest of the list, and returns the value of combining the first element with the result of the recursive call.
The nullValue is there to give you the sum of the squares of an empty list.
If you want to look up more of the details of the theory, you're basically doing functional programming but in an imperative language :)