如何使用Java Stream API过滤嵌套列表并聚合数据

{
  "name": "product A",
  "images": [
    {
      "imgId": "img1",
      "url": "url1",
      "types": [
        { "format": "jpg" },
        { "format": "png" }
      ]
    },
    {
      "imgId": "img2",
      "url": "url2",
      "types": [
        { "format": "mp4" },
        { "format": "png" }
      ]
    },
    {
      "imgId": "img3",
      "url": "url3",
      "types": [
        { "format": "jpg" },
        { "format": "mp4" }
      ]
    }
  ]
}

import java.util.List;
import java.util.Objects;
import java.util.Arrays;
import java.util.Collection;
import java.util.function.BinaryOperator;
import java.util.function.Predicate;
import java.util.stream.Collectors;

// 表示图片格式的类，例如 { "format": "jpg" }
class ImageType {
    private String format;

    public ImageType(String format) {
        this.format = format;
    }

    public String getFormat() {
        return format;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        ImageType imageType = (ImageType) o;
        return Objects.equals(format, imageType.format);
    }

    @Override
    public int hashCode() {
        return Objects.hash(format);
    }
}

// 表示一个图片对象
class Image {
    private String imgId;
    private String url;
    private List<ImageType> types;

    public Image(String imgId, String url, List<ImageType> types) {
        this.imgId = imgId;
        this.url = url;
        this.types = types;
    }

    public String getUrl() {
        return url;
    }

    public List<ImageType> getTypes() {
        return types;
    }
}

// 表示一个产品对象，包含图片列表
class Product {
    private String name;
    private List<Image> images;

    public Product(String name, List<Image> images) {
        this.name = name;
        this.images = images;
    }

    public List<Image> getImages() {
        return images;
    }
}

public class StreamNestedListFilter {

    // 定义一个Predicate，用于判断一个Image对象是否包含JPG格式
    private static final Predicate<Image> hasJpgType = image ->
            image.getTypes().stream() // 获取当前图片的类型列表的Stream
                 .anyMatch(type -> "jpg".equalsIgnoreCase(type.getFormat())); // 检查是否有任何类型是"jpg"

    // 定义一个BinaryOperator，用于将两个字符串通过逗号连接起来
    private static final BinaryOperator<String> commaReducer = (s1, s2) -> s1 + "," + s2;

    /**
     * 从图片集合中提取所有JPG格式图片的URL，并以逗号分隔返回。
     *
     * @param images 待处理的图片集合。
     * @return 逗号分隔的JPG图片URL字符串，如果未找到则返回"No Match!"。
     */
    public static String getJpgImageUrls(final Collection<Image> images) {
        // 处理输入集合为空或null的情况
        if (images == null || images.isEmpty()) {
            return "No Match!";
        }

        return images.stream()
                .filter(hasJpgType)           // 步骤1: 过滤出包含JPG格式的图片
                .map(Image::getUrl)           // 步骤2: 将过滤后的Image对象映射为其URL字符串
                .reduce(commaReducer)         // 步骤3: 使用自定义的BinaryOperator将所有URL聚合成一个逗号分隔的字符串
                .orElse("No Match!");         // 步骤4: 如果Stream为空（没有匹配的图片），则返回默认值"No Match!"
    }

    public static void main(String[] args) {
        // 构造示例数据
        ImageType jpg = new ImageType("jpg");
        ImageType png = new ImageType("png");
        ImageType mp4 = new ImageType("mp4");

        Image img1 = new Image("img1", "url1", Arrays.asList(jpg, png));
        Image img2 = new Image("img2", "url2", Arrays.asList(mp4, png));
        Image img3 = new Image("img3", "url3", Arrays.asList(jpg, mp4));

        List<Image> productAImages = Arrays.asList(img1, img2, img3);
        Product productA = new Product("product A", productAImages);

        // 调用方法获取JPG图片的URL
        String jpgUrls = getJpgImageUrls(productA.getImages());
        System.out.println("JPG Image URLs: " + jpgUrls); // 预期输出: url1,url3

        // 测试没有JPG图片的场景
        Image img4 = new Image("img4", "url4", Arrays.asList(mp4, png));
        List<Image> noJpgImages = Arrays.asList(img2, img4);
        String noMatchUrls = getJpgImageUrls(noJpgImages);
        System.out.println("No JPG Image URLs: " + noMatchUrls); // 预期输出: No Match!

        // 测试空列表
        String emptyListUrls = getJpgImageUrls(Arrays.asList());
        System.out.println("Empty list URLs: " + emptyListUrls); // 预期输出: No Match!

        // 测试null列表
        String nullListUrls = getJpgImageUrls(null);
        System.out.println("Null list URLs: " + nullListUrls); // 预期输出: No Match!
    }
}