java - 如何有效(性能)从Java中的列表中删除许多项？

Question

我有一个相当大的List命名项(> = 1,000,000个项)，并且用表示的某些条件选择要删除的项，并且对于列表中的许多(也许一半)项都是正确的。

我的目标是有效地删除由选择的项目并保留所有其他项目，可以修改源列表，可以创建新列表-应该根据性能选择最佳方法。

这是我的测试代码:

    System.out.println("preparing items");
    List<Integer> items = new ArrayList<Integer>(); // Integer is for demo
    for (int i = 0; i < 1000000; i++) {
        items.add(i * 3); // just for demo
    }

    System.out.println("deleting items");
    long startMillis = System.currentTimeMillis();
    items = removeMany(items);
    long endMillis = System.currentTimeMillis();

    System.out.println("after remove: items.size=" + items.size() + 
            " and it took " + (endMillis - startMillis) + " milli(s)");

和天真的实现:

public static <T> List<T> removeMany(List<T> items) {
    int i = 0;
    Iterator<T> iter = items.iterator();
    while (iter.hasNext()) {
        T item = iter.next();
        // <cond> goes here
        if (/*<cond>: */i % 2 == 0) {
            iter.remove();
        }
        i++;
    }
    return items;
}

如您所见，我使用项索引模2 == 0作为删除条件()-仅用于演示目的。

可以提供什么更好的removeMany版本，为什么这个更好的版本实际上更好呢？

Answer 1

好的，现在该是提出的方法的测试结果了。这里是我测试过的方法(每种方法的名称在我的资源中也是类名):

NaiveRemoveManyPerformer-带有迭代器和remove的ArrayList-我的问题中给出的第一个天真的实现。

BetterNaiveRemoveManyPerformer-带有向后迭代并从头到尾删除的ArrayList。

LinkedRemoveManyPerformer-天真的迭代器，可以删除但可以使用LinkedList。缺点:仅适用于LinkedList。

CreateNewRemoveManyPerformer-ArrayList作为副本制作(仅添加保留的元素)，迭代器用于遍历输入的ArrayList。

SmartCreateNewRemoveManyPerformer-更好的CreateNewRemoveManyPerformer-结果ArrayList的初始大小(容量)设置为最终列表大小。缺点:启动时必须知道列表的最终大小。

FasterSmartCreateNewRemoveManyPerformer-更好(？)SmartCreateNewRemoveManyPerformer-使用项目索引(items.get(idx))代替迭代器。

MagicRemoveManyPerformer-就ArrayList而言就地工作(无列表副本)，并从列表末尾开始压缩孔(已删除项目)。缺点:此方法更改列表中项目的顺序。

ForwardInPlaceRemoveManyPerformer-适用于ArrayList-移动保留项以填充孔，最后返回subList(没有最终移除或清除)。

GuavaArrayListRemoveManyPerformer-用于Iterables.removeIf的Google Guava ArrayList-与ForwardInPlaceRemoveManyPerformer几乎相同，但最终删除列表末尾的项目。

完整的源代码在此答案的末尾给出。

使用不同的列表大小(从10,000到10,000,000个项目)和不同的删除因子(指定必须从列表中删除多少个项目)执行测试。

正如我在评论中发布的其他答案一样，我认为将项目从ArrayList复制到第二个ArrayList的过程比迭代LinkedList并删除项目要快。 Sun的Java文档指出，与ArrayList实现相比，LinkedList的常数因子较低，但是令人惊讶的是，我的问题并非如此。

实际上，在大多数情况下，具有简单迭代和删除功能的LinkedList具有最佳性能(此方法在LinkedRemoveManyPerformer中实现)。通常，只有MagicRemoveManyPerformer的性能可与LinkedRemoveManyPerformer相提并论，其他方法则要慢得多。 Google Guava GuavaArrayListRemoveManyPerformer比手工制作的类似代码慢(因为我的代码不会删除列表末尾的不必要项目)。

从1,000,000个源项目中删除500,000个项目的示例结果:

NaiveRemoveManyPerformer:未执行测试-我不是那个病人，但是它的表现比BetterNaiveRemoveManyPerformer差。

BetterNaiveRemoveManyPerformer:226080毫升

LinkedRemoveManyPerformer:69毫升

CreateNewRemoveManyPerformer:246毫升

SmartCreateNewRemoveManyPerformer:112毫升

FasterSmartCreateNewRemoveManyPerformer:202毫升

MagicRemoveManyPerformer:74毫升

ForwardInPlaceRemoveManyPerformer:69毫升

GuavaArrayListRemoveManyPerformer:118毫升

从1,000,000个源项目中删除1个项目的示例结果(已删除第一个项目):

BetterNaiveRemoveManyPerformer:34毫升

LinkedRemoveManyPerformer:41毫升

CreateNewRemoveManyPerformer:253毫升

SmartCreateNewRemoveManyPerformer:108毫升

FasterSmartCreateNewRemoveManyPerformer:71毫升

MagicRemoveManyPerformer:43毫升

ForwardInPlaceRemoveManyPerformer:73毫升

GuavaArrayListRemoveManyPerformer:78毫升

从1,000,000个源项目中删除333,334个项目的示例结果:

BetterNaiveRemoveManyPerformer:253206毫升

LinkedRemoveManyPerformer:69毫升

CreateNewRemoveManyPerformer:245毫升

SmartCreateNewRemoveManyPerformer:111毫米

FasterSmartCreateNewRemoveManyPerformer:203毫升

MagicRemoveManyPerformer:69毫升

ForwardInPlaceRemoveManyPerformer:72毫升

GuavaArrayListRemoveManyPerformer:102毫升

从1,000,000个源项目中删除1,000,000个(所有)项目的示例结果(所有项目都被删除，但经过一个接一个的处理，如果您先验地知道要删除所有项目，则应该简单地清除列表):

BetterNaiveRemoveManyPerformer:58毫升

LinkedRemoveManyPerformer:88毫升

CreateNewRemoveManyPerformer:95毫升

SmartCreateNewRemoveManyPerformer:91毫升

FasterSmartCreateNewRemoveManyPerformer:48毫升

MagicRemoveManyPerformer:61毫升

ForwardInPlaceRemoveManyPerformer:49毫升

GuavaArrayListRemoveManyPerformer:133毫米

我的最终结论是:使用混合方法-如果处理LinkedList-最好进行简单的迭代和删除，如果处理ArrayList-取决于项目顺序是否重要-然后使用ForwardInPlaceRemoveManyPerformer-如果可以更改项目顺序-最佳选择是MagicRemoveManyPerformer。如果先验地知道移除因子(您知道将要移除多少个项目还是保留了多少个项目)，那么在特定情况下可以采用更多条件来选择效果更好的方法。但是已知的去除因子不是通常的情况... Google Guava Iterables.removeIf是这样一种混合解决方案，但是假设稍有不同(必须更改原始列表，不能创建新列表，并且始终按项目顺序排列)-这些是最常见的假设，因此removeIf在大多数现实生活中都是最佳选择。

还要注意，所有好的方法(天真的不好!)都足够好-在实际应用中，其中任何一个都做得很好，但是必须避免使用天真的方法。

最后-我的测试源代码。

package WildWezyrListRemovalTesting;

import com.google.common.base.Predicate;
import com.google.common.collect.Iterables;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

public class RemoveManyFromList {

    public static abstract class BaseRemoveManyPerformer {

        protected String performerName() {
            return getClass().getSimpleName();
        }

        protected void info(String msg) {
            System.out.println(performerName() + ": " + msg);
        }

        protected void populateList(List<Integer> items, int itemCnt) {
            for (int i = 0; i < itemCnt; i++) {
                items.add(i);
            }
        }

        protected boolean mustRemoveItem(Integer itemVal, int itemIdx, int removeFactor) {
            if (removeFactor == 0) {
                return false;
            }
            return itemIdx % removeFactor == 0;
        }

        protected abstract List<Integer> removeItems(List<Integer> items, int removeFactor);

        protected abstract List<Integer> createInitialList();

        public void testMe(int itemCnt, int removeFactor) {
            List<Integer> items = createInitialList();
            populateList(items, itemCnt);
            long startMillis = System.currentTimeMillis();
            items = removeItems(items, removeFactor);
            long endMillis = System.currentTimeMillis();
            int chksum = 0;
            for (Integer item : items) {
                chksum += item;
            }
            info("removing took " + (endMillis - startMillis)
                    + " milli(s), itemCnt=" + itemCnt
                    + ", removed items: " + (itemCnt - items.size())
                    + ", remaining items: " + items.size()
                    + ", checksum: " + chksum);
        }
    }
    private List<BaseRemoveManyPerformer> rmps =
            new ArrayList<BaseRemoveManyPerformer>();

    public void addPerformer(BaseRemoveManyPerformer rmp) {
        rmps.add(rmp);
    }
    private Runtime runtime = Runtime.getRuntime();

    private void runGc() {
        for (int i = 0; i < 5; i++) {
            runtime.gc();
        }
    }

    public void testAll(int itemCnt, int removeFactor) {
        runGc();
        for (BaseRemoveManyPerformer rmp : rmps) {
            rmp.testMe(itemCnt, removeFactor);
        }
        runGc();
        System.out.println("\n--------------------------\n");
    }

    public static class NaiveRemoveManyPerformer
            extends BaseRemoveManyPerformer {

        @Override
        public List<Integer> removeItems(List<Integer> items, int removeFactor) {
            if (items.size() > 300000 && items instanceof ArrayList) {
                info("this removeItems is too slow, returning without processing");
                return items;
            }
            int i = 0;
            Iterator<Integer> iter = items.iterator();
            while (iter.hasNext()) {
                Integer item = iter.next();
                if (mustRemoveItem(item, i, removeFactor)) {
                    iter.remove();
                }
                i++;
            }
            return items;
        }

        @Override
        public List<Integer> createInitialList() {
            return new ArrayList<Integer>();
        }
    }

    public static class BetterNaiveRemoveManyPerformer
            extends NaiveRemoveManyPerformer {

        @Override
        public List<Integer> removeItems(List<Integer> items, int removeFactor) {
//            if (items.size() > 300000 && items instanceof ArrayList) {
//                info("this removeItems is too slow, returning without processing");
//                return items;
//            }

            for (int i = items.size(); --i >= 0;) {
                Integer item = items.get(i);
                if (mustRemoveItem(item, i, removeFactor)) {
                    items.remove(i);
                }
            }
            return items;
        }
    }

    public static class LinkedRemoveManyPerformer
            extends NaiveRemoveManyPerformer {

        @Override
        public List<Integer> createInitialList() {
            return new LinkedList<Integer>();
        }
    }

    public static class CreateNewRemoveManyPerformer
            extends NaiveRemoveManyPerformer {

        @Override
        public List<Integer> removeItems(List<Integer> items, int removeFactor) {
            List<Integer> res = createResultList(items, removeFactor);
            int i = 0;

            for (Integer item : items) {
                if (mustRemoveItem(item, i, removeFactor)) {
                    // no-op
                } else {
                    res.add(item);
                }
                i++;
            }

            return res;
        }

        protected List<Integer> createResultList(List<Integer> items, int removeFactor) {
            return new ArrayList<Integer>();
        }
    }

    public static class SmartCreateNewRemoveManyPerformer
            extends CreateNewRemoveManyPerformer {

        @Override
        protected List<Integer> createResultList(List<Integer> items, int removeFactor) {
            int newCapacity = removeFactor == 0 ? items.size()
                    : (int) (items.size() * (removeFactor - 1L) / removeFactor + 1);
            //System.out.println("newCapacity=" + newCapacity);
            return new ArrayList<Integer>(newCapacity);
        }
    }

    public static class FasterSmartCreateNewRemoveManyPerformer
            extends SmartCreateNewRemoveManyPerformer {

        @Override
        public List<Integer> removeItems(List<Integer> items, int removeFactor) {
            List<Integer> res = createResultList(items, removeFactor);

            for (int i = 0; i < items.size(); i++) {
                Integer item = items.get(i);
                if (mustRemoveItem(item, i, removeFactor)) {
                    // no-op
                } else {
                    res.add(item);
                }
            }

            return res;
        }
    }

    public static class ForwardInPlaceRemoveManyPerformer
            extends NaiveRemoveManyPerformer {

        @Override
        public List<Integer> removeItems(List<Integer> items, int removeFactor) {
            int j = 0; // destination idx
            for (int i = 0; i < items.size(); i++) {
                Integer item = items.get(i);
                if (mustRemoveItem(item, i, removeFactor)) {
                    // no-op
                } else {
                    if (j < i) {
                        items.set(j, item);
                    }
                    j++;
                }
            }

            return items.subList(0, j);
        }
    }

    public static class MagicRemoveManyPerformer
            extends NaiveRemoveManyPerformer {

        @Override
        public List<Integer> removeItems(List<Integer> items, int removeFactor) {
            for (int i = 0; i < items.size(); i++) {
                if (mustRemoveItem(items.get(i), i, removeFactor)) {
                    Integer retainedItem = removeSomeFromEnd(items, removeFactor, i);
                    if (retainedItem == null) {
                        items.remove(i);
                        break;
                    }
                    items.set(i, retainedItem);
                }
            }

            return items;
        }

        private Integer removeSomeFromEnd(List<Integer> items, int removeFactor, int lowerBound) {
            for (int i = items.size(); --i > lowerBound;) {
                Integer item = items.get(i);
                items.remove(i);
                if (!mustRemoveItem(item, i, removeFactor)) {
                    return item;
                }
            }
            return null;
        }
    }

    public static class GuavaArrayListRemoveManyPerformer
            extends BaseRemoveManyPerformer {

        @Override
        protected List<Integer> removeItems(List<Integer> items, final int removeFactor) {
            Iterables.removeIf(items, new Predicate<Integer>() {

                public boolean apply(Integer input) {
                    return mustRemoveItem(input, input, removeFactor);
                }
            });

            return items;
        }

        @Override
        protected List<Integer> createInitialList() {
            return new ArrayList<Integer>();
        }
    }

    public void testForOneItemCnt(int itemCnt) {
        testAll(itemCnt, 0);
        testAll(itemCnt, itemCnt);
        testAll(itemCnt, itemCnt - 1);
        testAll(itemCnt, 3);
        testAll(itemCnt, 2);
        testAll(itemCnt, 1);
    }

    public static void main(String[] args) {
        RemoveManyFromList t = new RemoveManyFromList();
        t.addPerformer(new NaiveRemoveManyPerformer());
        t.addPerformer(new BetterNaiveRemoveManyPerformer());
        t.addPerformer(new LinkedRemoveManyPerformer());
        t.addPerformer(new CreateNewRemoveManyPerformer());
        t.addPerformer(new SmartCreateNewRemoveManyPerformer());
        t.addPerformer(new FasterSmartCreateNewRemoveManyPerformer());
        t.addPerformer(new MagicRemoveManyPerformer());
        t.addPerformer(new ForwardInPlaceRemoveManyPerformer());
        t.addPerformer(new GuavaArrayListRemoveManyPerformer());

        t.testForOneItemCnt(1000);
        t.testForOneItemCnt(10000);
        t.testForOneItemCnt(100000);
        t.testForOneItemCnt(200000);
        t.testForOneItemCnt(300000);
        t.testForOneItemCnt(500000);
        t.testForOneItemCnt(1000000);
        t.testForOneItemCnt(10000000);
    }
}