Predicate用法举例

Predicate 是jakarta commons工具包中一个非常有用的接口, 它主要在对容器类遍历的时候做一些"手脚", 比如根据一定的规则, 逻辑, 对容器对象进行过滤.
在实际开发中我们就碰到一个类似的场景:
一个商品可以根据发往所在地来设置邮费项, 比如一个手机电池从上海发往北京和发往新疆的邮费是不一样的, 于是我们可以设置一些邮费模板项, 但是这些邮费模板项必须是唯一, 比如我设置了北京, 沈阳, 天津三个目的地的邮费都是5块, 接着我又设置了沈阳, 黑龙江的邮费是6块, 则最终得到的邮费项分别是:北京, 天津邮费为5块, 沈阳, 黑龙江为6块, 即能自动针对目的地进行去重处理.去重之后必须遵循后添加的邮费项替换已有的邮费项.

public class RemoveDuplicateDestinationPredicate implements Predicate {
    class UniquePostageItem {
        String dest;
        int mode;

        UniquePostageItem(PostageItem item) {
            this.dest = item.getDestination();
            this.mode = item.getMode();
        }

        @Override
        public boolean equals(Object obj) {
            return EqualsBuilder.reflectionEquals(this, obj);
        }

        @Override
        public int hashCode() {
            return HashCodeBuilder.reflectionHashCode(this);
        }
    }
    List<PostageItem> completeList = new ArrayList<PostageItem>();
    Predicate up = new UniquePredicate();

    public boolean evaluate(Object object) {
        PostageItem current = (PostageItem) object;
        // 初级过滤去重
        if (!up.evaluate(new UniquePostageItem(current))) {
            return false;
        }

         // 对item中的每一个destincation进行过滤去重
        String[] currentDests = filterDuplicate(current);

        // 过滤之后目的地为空的直接排除
        String dest = StringUtils.join(currentDests);
        if (StringUtils.isEmpty(dest)) {
            return false;
        }

        current.setDestination(dest);
        completeList.add(current);
        return true;
    }

    private String[] filterDuplicate(PostageItem current) {
        String[] currentDests = current.getDesctinations();
        int currentMode = current.getMode();
        boolean found = false;
        for (int i = 0; i < currentDests.length; i++) {
            String currentDest = currentDests[i];

            for (PostageItem each : completeList) {
                if (each.getMode() == currentMode) {
                    String[] eachDests = each.getDesctinations();
                    for (String otherDest : eachDests) {
                        if (StringUtils.equals(currentDest, otherDest)) {
                            currentDests[i] = "";
                            found = true;
                            break;
                        }
                    }
                    if (found) {
                        break;
                    }
                }
            }
        }
        return currentDests;
    }
}

 用法如下:

    private static void checkPostageItems(Postage postage, Result<Postage> result) {
        List<PostageItem> uniqueItems = new ArrayList<PostageItem>();
        for (Iterator<PostageItem> it = removeDuplicateDestinationIterator(items); it.hasNext();) {
            final PostageItem item = it.next();
            uniqueItems.add(item);
	...
        }
        // 将顺序还原
        Collections.sort(uniqueItems, new Comparator<PostageItem>() {
                public int compare(PostageItem o1, PostageItem o2) {
                return o1.getIndex() - o2.getIndex();
                }
                        });
        postage.setPostageItems(uniqueItems);
    }

    protected static Iterator<PostageItem> removeDuplicateDestinationIterator(final List<PostageItem> items) {
        Collections.sort(items, new Comparator<PostageItem>() {
                public int compare(PostageItem o1, PostageItem o2) {
                return o2.getIndex() - o1.getIndex();
                }
                        });

        return new FilterIterator(items.iterator(), new RemoveDuplicateDestinationPredicate());
    }