AggregateFunctions

introduction

Apply functions over a collection.

This class allow apply a function over all elements of a collection and get the result. Functions like, sum, avg or anything else, only depends the implementation of a AggregateFunction.

Too use this is easy.

List<Integer> list = new ArrayList<Integer>();
list.add(10);
list.add(20);
list.add(30);
list.add(40);
list.add(50);

Integer sum = CollectionsUtil.aggregate(new FuncSum(), list).intValue();//.intValue cause FuncSum returns Number
Double avg = CollectionsUtil.aggregate(new FuncAvg(), list).doubleValue();
System.out.println("sum = " + sum);
System.out.println("avg = " + avg);

The implementations FuncSum(Sum) and FuncAvg(Average) are already implementeds in the project, and there's a third one, FuncConcat with concat Strings.

There's possible to apply the function over a collection different of the one with the AggregateFunc expects, to it, use the method.

CollectionsUtil.aggregate(List<Object> l, AggregateFunc func, String field)

And the result will be the function applyed over the list returned by CollectionsUtil.split(l, field).

About AggregateFunction, to create new implementations of functions, implements this class.

And this assignature is:

package com.towel.collections.aggr;

/**
 * Functions to work over an object, the object is of the type T
 * 
 * @author marcos.vasconcelos
 */
public interface AggregateFunc<T> {
	/**
	 * Init this Func with the initial values.
	 * This method is called when a new Calculation over a Collection is going to ve initiated.
	 * When implementing, reset all values to it initial value.
	 */
	public void init();
	/**
	 * Called over each value in a List.
	 * This method should calculate the new value with a previous value.
	 * @param obj
	 */
	public void update(T obj);
	/**
	 * Called when the iteration is over and the final value is done.
	 * @return The value of the function apllied over all objects passed in Func.update
	 */
	public T getResult();
}

• The method 'init' is invoked before the iterations.
• The method 'update' is invoked with each object of the collection.
• The method 'getResult' is invoked after iterations, and should return the processed result.

As example, this is the implementation of average.

package com.towel.collections.aggr;

public class FuncAvg implements AggregateFunc<Number>{
	private Number x;
	private int total;
	@Override
	public void update(Number obj) {
		x = new Double(x.doubleValue() + obj.doubleValue());
		total++;
	}

	@Override
	public Number getResult() {
		return x.doubleValue() / total;
	}

	@Override
	public void init() {
		x = new Double(0);
		total = 0;
	}
	
}