java - 转换逻辑以使用 Java 8 Streams

Question

我正在学习如何在 Java 8 中使用 Streams，并且想了解如何将以下示例转换为基于流的方式。我做了几次尝试，但无法获得基于流的编译方式。我想我被似乎丢失的嵌套循环和变量引用绊倒了。

import java.util.Collection;
import java.util.Objects;

import gov.geo.argcci.product.parts.layers.PlatformMarkerLayer;

public class Test {

    public void java7Method(final Collection<Item> items) {
        for (final LayerHolder layerHolder : getLayerHolders()) {
            if (layerHolder.getLayer() instanceof MyLayer) {
                final MyLayer myLayer = (MyLayer) layerHolder.getLayer();
                final Item current = myLayer.getItem();
                if (current != null) {
                    for (final Item item : items) {
                        if (Objects.equals(item.getSomeKey(), current.getSomeKey())
                                && Objects.equals(item.getSomeOtherKey(), current.getSomeOtherKey())) {
                            final SomeObject someObject = getObjectBasedOnItem(current);
                            someObject.doSomething(layerHolder);
                        }
                    }
                }
            }
        }
    }

    public void java8Method(final Collection<Item> items) {
        getLayerHolders()
        .stream()
        .filter(layerHolder -> layerHolder.getLayer() instanceof MyLayer)
                .map(layerHolder -> (MyLayer) layerHolder.getLayer())
                .map(layerHolder -> layerHolder.getItem())
                .filter(Objects::nonNull)
                .forEach(current->{
                    items.stream()
                    .filter(Objects.equals(item.getSomeKey(), current.getSomeKey()) && Objects.equals(item.getSomeOtherKey(), current.getSomeOtherKey()));              
//                      final SomeObject someObject = getObjectBasedOnItem(current);
//                      someObject.doSomething(layerHolder);
                });
    }

    // The Code below this line is auto-generated to make sure example can compile without errors
    private SomeObject getObjectBasedOnItem(Item current) {
        return null;
    }
    private Collection<LayerHolder> getLayerHolders() {
        return null;
    }
    private class Item {
        public Object getSomeKey() {
            return null;
        }
        public Object getSomeOtherKey() {
            return null;
        }
    }
    private class LayerHolder {
        public MyLayer getLayer() {
            return null;
        }
    }
    private class MyLayer {
        public Item getItem() {
            return null;
        }
    }
    private class SomeObject {
        public void doSomething(LayerHolder layerHolder) {}
    }
}

Answer 1

您的 Java 7 代码的直接翻译是

public void java8Method(final Collection<Item> items) {
    getLayerHolders().forEach(layerHolder -> {
        Optional.of(layerHolder)
                .map(LayerHolder::getLayer)
                .filter(MyLayer.class::isInstance)
                .map(l -> ((MyLayer)l).getItem())
                .ifPresent(current-> items.stream()
                    .filter(item ->
                        Objects.equals(item.getSomeKey(), current.getSomeKey())
                     && Objects.equals(item.getSomeOtherKey(), current.getSomeOtherKey()))
                    .forEach(ignored ->
                             getObjectBasedOnItem(current).doSomething(layerHolder))
            );
    });
}

除了这不会调用 getLayer() 两次。

但我有一种强烈的感觉，你真正想做的是

public void java8Method(final Collection<Item> items) {
    getLayerHolders().forEach(layerHolder ->
        Optional.of(layerHolder)
                .map(LayerHolder::getLayer)
                .filter(MyLayer.class::isInstance)
                .map(l -> ((MyLayer)l).getItem())
                .filter(current-> items.stream()
                    .anyMatch(item ->
                        Objects.equals(item.getSomeKey(), current.getSomeKey())
                     && Objects.equals(item.getSomeOtherKey(), current.getSomeOtherKey())))
                .ifPresent(current ->
                           getObjectBasedOnItem(current).doSomething(layerHolder))
    );
}

通常，您应该更多地考虑实际的程序逻辑，而不是使用代码将 Java 8 之前的代码转换为 Stream API……