Java中高效关联对象列表的策略：从嵌套流到Multimap优化

public class A implements Comparable<A> {
    private String id;
    // getter, setter, compareTo...
    public String getId() { return id; }
    public void setId(String id) { this.id = id; }
    @Override public int compareTo(A o) { return o.getId().compareTo(this.getId()); }
    @Override public String toString() { return "A{" + "id='" + id + '\'' + '}'; }
}

public class B implements Comparable<B> {
    private String id;
    private List<A> aList = new ArrayList<>();
    // getter, setter, compareTo...
    public String getId() { return id; }
    public void setId(String id) { this.id = id; }
    public List<A> getAList() { return aList; }
    public void addA(A a) { aList.add(a); }
    @Override public int compareTo(B o) { return o.getId().compareTo(this.getId()); }
    @Override public String toString() { return "B{" + "id='" + id + '\'' + ", aList=" + aList + '}'; }
}

public class Main {
    public static void main(String[] args) {
        SortedSet<A> aSet = new TreeSet<>();
        SortedSet<B> bSet = new TreeSet<>();

        // 填充aSet和bSet，此处省略具体填充逻辑
        // ... 假设aSet和bSet已包含大量数据

        // 初始的关联尝试：使用嵌套并行流
        long startTime = System.currentTimeMillis();
        bSet.parallelStream().forEach(b -> {
            aSet.parallelStream().filter(a -> b.getId().equals(a.getId()))
                                 .forEach(b::addA);
        });
        long endTime = System.currentTimeMillis();
        System.out.println("嵌套并行流耗时: " + (endTime - startTime) + " ms");
    }
}

import java.util.*;

public class MainOptimized {
    public static void main(String[] args) {
        // 使用TreeMap作为多值映射，将A对象的ID映射到A对象的列表
        // 如果不需要键的排序，HashMap通常提供更快的平均查找速度
        Map<String, List<A>> aMapById = new TreeMap<>(); 
        List<B> bList = new ArrayList<>();

        // 1. 填充数据并构建A对象的ID索引
        long buildStartTime = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
            UUID uuid = UUID.randomUUID();
            String uuidAsString = uuid.toString();

            // 创建A对象并添加到aMapById
            A a1 = new A();
            a1.setId(uuidAsString);
            aMapById.computeIfAbsent(a1.getId(), k -> new ArrayList<>()).add(a1);

            A a2 = new A();
            a2.setId(uuidAsString);
            aMapById.computeIfAbsent(a2.getId(), k -> new ArrayList<>()).add(a2);

            // 创建B对象并添加到bList
            B b = new B();
            b.setId(uuidAsString);
            bList.add(b);
        }
        long buildEndTime = System.currentTimeMillis();
        System.out.println("数据填充与A对象索引构建耗时: " + (buildEndTime - buildStartTime) + " ms");

        // 2. 遍历B对象列表，利用aMapById进行高效查找和关联
        long associateStartTime = System.currentTimeMillis();
        for (B b : bList) {
            List<A> matchingAs = aMapById.get(b.getId());
            if (matchingAs != null) {
                // 将所有匹配的A对象添加到B对象的aList中
                for (A a : matchingAs) {
                    b.addA(a);
                }
            }
        }
        long associateEndTime = System.currentTimeMillis();
        System.out.println("B对象关联A对象耗时: " + (associateEndTime - associateStartTime) + " ms");

        // 验证结果（可选）
        // bList.forEach(System.out::println);
    }
}

import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.Multimap;
import java.util.*;

public class MainGuavaOptimized {
    public static void main(String[] args) {
        Multimap<String, A> aMultimapById = ArrayListMultimap.create(); // 使用Guava的Multimap
        List<B> bList = new ArrayList<>();

        long buildStartTime = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
            UUID uuid = UUID.randomUUID();
            String uuidAsString = uuid.toString();

            A a1 = new A();
            a1.setId(uuidAsString);
            aMultimapById.put(a1.getId(), a1); // 直接put即可，Multimap会自动处理列表

            A a2 = new A();
            a2.setId(uuidAsString);
            aMultimapById.put(a2.getId(), a2);

            B b = new B();
            b.setId(uuidAsString);
            bList.add(b);
        }
        long buildEndTime = System.currentTimeMillis();
        System.out.println("数据填充与A对象索引构建耗时 (Guava): " + (buildEndTime - buildStartTime) + " ms");

        long associateStartTime = System.currentTimeMillis();
        for (B b : bList) {
            // Guava的get方法返回一个Collection，可以直接迭代
            Collection<A> matchingAs = aMultimapById.get(b.getId());
            if (!matchingAs.isEmpty()) {
                for (A a : matchingAs) {
                    b.addA(a);
                }
            }
        }
        long associateEndTime = System.currentTimeMillis();
        System.out.println("B对象关联A对象耗时 (Guava): " + (associateEndTime - associateStartTime) + " ms");
    }
}