First与Last

FirstAggregate 实现了第一个分组元素聚合，LastAggregate 实现了最后一个分组元素聚合。

类定义

FirstAggregate

    public static class FirstAggregate implements Aggregate
    {
        public void initialize(Object val) 
        {
            first = val;
        }
 
        public void accumulate(Object val) 
        {
        }
 
        public Object result() 
        {
            return first;
        }
 
        public void merge(Aggregate other) 
        {
        }
 
        Object first;
    }

LastAggregate

    public static class LastAggregate implements Aggregate
    {
        public void initialize(Object val) 
        {
            last = val;
        }
 
        public void accumulate(Object val) 
        {
            last = val;
        }
 
        public Object result() 
        {
            return last;
        }
 
        public void merge(Aggregate other) 
        {
            last = ((LastAggregate)other).last;
        }
 
        Object last;
    }