Java中List转Map的几种常见方式与对比

Map<String, Entity> map = new HashMap<>();
for (Entity entity : list) {
    if (entity.getKey() != null) {
        map.put(entity.getKey(), entity);
    }
}

Map<String, Entity> map = list.stream()
    .filter(entity -> entity.getKey() != null)
    .collect(Collectors.toMap(Entity::getKey, Function.identity()));

Map<String, Entity> map = new HashMap<>();
CollectionUtils.toMap(list, Entity::getKey, map);

Map<String, Entity> map = Maps.uniqueIndex(list, Entity::getKey);

import java.util.*;

public class ListToMapExample {

    public Map<String, List<Object>> convertListToMap(List<Object> list) {
        Map<String, List<Object>> map = new HashMap<>();
        
        for (Object obj : list) {
            String key = obj.getXXX(); // 获取指定字段的值，XXX为字段名称
            if (map.containsKey(key)) {
                List<Object> values = map.get(key);
                values.add(obj);
                map.put(key, values);
            } else {
                List<Object> values = new ArrayList<>();
                values.add(obj);
                map.put(key, values);
            }
        }
        
        return map;
    }

    public static void main(String[] args) {
        List<Object> list = new ArrayList<>();
        // 添加元素到List中
        
        ListToMapExample example = new ListToMapExample();
        Map<String, List<Object>> resultMap = example.convertListToMap(list);
        
        // 打印结果
        for (Map.Entry<String, List<Object>> entry : resultMap.entrySet()) {
            System.out.println("Key: " + entry.getKey());
            System.out.println("Value: " + entry.getValue());
        }
    }
}