Question

示例：过滤基于fromPrice和toPrice的价格的产品列表。它们既可以提供，也可以只提供一个。

查找价格大于fromPrice
查找价格低于“价格”的所有产品
查找价格介于价格和价格之间的所有产品

产品：

namespace App.Adapters
{
    class ExpandableListviewAdapter : BaseExpandableListAdapter
    {
        private Activity mContext;
        private List<ExpandableItem> mDataItems;

       public ExpandableListviewAdapter(Activity context, List<ExpandableItem> items)
            : base()
        {
            mContext = context;
            mDataItems = items;

            // Sort the groups and all of its child elements alphabetically.
            SortDataItems();
        }

        void SortDataItems()
        {
            // Sort each group alphabetically.
            mDataItems.Sort(delegate (ExpandableItem e1, ExpandableItem e2) 
            {
                return e1.GroupTitle.CompareTo(e2.GroupTitle);
            });

            // Iterate over each group of items and sort the 
            // children alphabetically.
            foreach (var d in mDataItems)
            {
                d.ChildItems.Sort(delegate (FMNavigationListItem i1, FMNavigationListItem i2)
                {
                    return i1.DisplayAttributes[0].CompareTo(i2.DisplayAttributes[0]);
                });
            }
        }

        public override View GetChildView(int groupPosition, int childPosition, bool isLastChild, View convertView, ViewGroup parent)
        {
            if (convertView == null)
                convertView = mContext.LayoutInflater.Inflate(Resource.Layout.adapter_expandable_listview_listitem, null);

            FMNavigationListItem item = mDataItems[groupPosition].ChildItems[childPosition];
            convertView.FindViewById<TextView>(Resource.Id.adapter_expandable_listview_listitem_textview1).Text = item.DisplayAttributes[0];
            convertView.FindViewById<TextView>(Resource.Id.adapter_expandable_listview_listitem_textview2).Text = item.DisplayAttributes[1];
            convertView.FindViewById<TextView>(Resource.Id.adapter_expandable_listview_listitem_textview3).Text = item.DisplayAttributes[2];

            ImageButton i = convertView.FindViewById<ImageButton>(Resource.Id.adapter_expandable_listview_listitem_overflow_button);
            i.Click += OverflowButtonClicked;
            return convertView;
        }

        private void OverflowButtonClicked(object sender, EventArgs e)
        {
            Toast.MakeText(
                mContext,
                "Overflow button clicked!",
                ToastLength.Short).Show();
        }

        public override View GetGroupView(int groupPosition, bool isExpanded, View convertView, ViewGroup parent)
        {
            if (convertView == null)
                convertView = mContext.LayoutInflater.Inflate(Resource.Layout.adapter_expandable_listview_header, null);

            // We expand all child objects from the group per default.
            // Normal default ist that all children are collapsed.
            ExpandableListView listView = (ExpandableListView)parent;
            listView.ExpandGroup(groupPosition);

            // Set the header title to be the group title.
            ExpandableItem item = mDataItems[groupPosition];
            convertView.FindViewById<TextView>(Resource.Id.adapter_expandable_listview_header_textview).Text = item.GroupTitle;
            return convertView;
        }

        public override int GroupCount
        {
            get
            {
                return mDataItems.Count;
            }
        }

        public override bool HasStableIds
        {
            get
            {
                return true;
            }
        }

        public override Java.Lang.Object GetChild(int groupPosition, int childPosition)
        {
            throw new NotImplementedException();
        }

        public override long GetChildId(int groupPosition, int childPosition)
        {
            return childPosition;
        }

        public override int GetChildrenCount(int groupPosition)
        {
            return mDataItems[groupPosition].ChildItems.Count;
        }

        public override bool IsChildSelectable(int groupPosition, int childPosition)
        {
            return true;
        }

        public override Java.Lang.Object GetGroup(int groupPosition)
        {
            throw new NotImplementedException();
        }

        public override long GetGroupId(int groupPosition)
        {
            return groupPosition;
        }

        public FMNavigationListItem ChildAt(int groupPosition, int childPosition)
        {
            return mDataItems[groupPosition].ChildItems[childPosition];
        }
    }

    /// <summary>
    /// 
    /// </summary>
    internal class ExpandableItem
    {
        public string GroupTitle;
        public List<FMNavigationListItem> ChildItems;
    }
}

PricePredicate：

public class Product {

    private String id;

    private Optional<BigDecimal> price;

    public Product(String id, BigDecimal price) {
        this.id = id;
        this.price = Optional.ofNullable(price);
    }
}

过滤器：

public class PricePredicate {

    public static Predicate<? super Product> isBetween(BigDecimal fromPrice, BigDecimal toPrice) {
        if (fromPrice != null && toPrice != null) {
            return product -> product.getPrice().isPresent() && product.getPrice().get().compareTo(fromPrice) >= 0 &&
                    product.getPrice().get().compareTo(toPrice) <= 0;
        }
        if (fromPrice != null) {
            return product -> product.getPrice().isPresent() && product.getPrice().get().compareTo(fromPrice) >= 0;
        }
        if (toPrice != null) {
            return product -> product.getPrice().isPresent() && product.getPrice().get().compareTo(toPrice) <= 0;
        }
        return null;
    }
}

有没有办法改进我的谓词而不是if if null检查？可以用选项做什么？

Answer 1

不，可选不用于替换空检查。

但是如果两个参数都为null，那么可以通过避免重复来避免代码的改进，并避免返回null（这显然不是Predicate的有效值）：

public static Predicate<Product> isBetween(BigDecimal fromPrice, BigDecimal toPrice) {
    Predicate<Product> result = product -> true;

    if (fromPrice != null) {
        result = result.and(product -> product.getPrice().isPresent() && product.getPrice().get().compareTo(fromPrice) >= 0);
    }

    if (toPrice != null) {
        result = result.and(product -> product.getPrice().isPresent() && product.getPrice().get().compareTo(toPrice) <= 0);
    }

    return result;
}

Answer 2

您可以使用Apache Commons Lang，它提供null安全比较：

ObjectUtils.compare(from, to)

null is assumed to be less than a non-value

Java 8：基于可选条件进行流式处理和过滤

2 个答案: