Java8 Stream API中实现集合list转map的多种方式

public class QuotaDto{
	private Long id;
	private String quotaId;
	private String quotaName;
	private String reportDate;
	private String email;
}

List<QuotaDto>  quotaDtolists = new ArrayList<>();

List<String> quotaNames = quotaDtolists.stream()
        .map(QuotaDto::getQuotaName)
        .filter(Objects::nonNull) // 过滤null值
        .collect(Collectors.toList());

// 方式1：对象为Value（处理重复键）
Map<String, QuotaDto> quotaMap1 = quotaDtolists.stream()
        .collect(Collectors.toMap(
            QuotaDto::getQuotaId, 
            Function.identity(),
            (v1, v2) -> v2 // 键冲突时保留新值
        ));

// 方式2：提取字段为Value
Map<String, String> quotaMap2 = quotaDtolists.stream()
        .collect(Collectors.toMap(
            QuotaDto::getQuotaId,
            QuotaDto::getQuotaName,
            (oldVal, newVal) -> newVal
        ));

public enum QuotaEnum {
    TYPE_A("A"), TYPE_B("B");

    private final String value;

    QuotaEnum(String value) { this.value = value; }
    public String getValue() { return value; } // 正确方法名

    // 枚举转Map
    private static final Map<String, QuotaEnum> VALUE_MAP = Arrays.stream(values())
            .collect(Collectors.toMap(QuotaEnum::getValue, Function.identity()));
}

List<Map<String, String>> lists = new ArrayList<>(); // 统一泛型类型
//(v1,v2)->v2 冲突解决函数，当两个不同的map对象具有相同的“userid”键值。保留第二个遇到的值V2
Map<String, String> collect = lists.stream()
        .collect(Collectors.toMap(
            map -> map.getOrDefault("userid", ""),
            map -> map.getOrDefault("orgid", ""),
            (v1, v2) -> v2
        ));

//以某个字段为key
Map<String,List<QuotaDto>> quotaMap= quotaDtolists.stream().collect(Collectors.groupingBy(QuotaDto::getQuotaName));

//list<map> 转 map
Map<String,List<Map<String,String>>> quotaMap= maps.stream().collect(Collectors.groupingBy(map->map.get("userId")));

String emailAddress = quotaDtolists.stream()
        .map(quota -> Optional.ofNullable(quota.getEmail())
                .map(email -> email.toUpperCase() + "@qq.com")
                .orElse("")) // 处理null或空邮件
        .filter(str -> !str.isEmpty())
        .collect(Collectors.joining(";"));
        
//简单版
String emailAddress=quotaDtolists.stream().map(Quota-Quota.getEmail().toUpperCase()+"@qq.com").collect(Collectors.joining(";"));

List<Integer> list = Arrays.asList(1, 2, 3);
// 最大值（避免NoSuchElementException）
Integer maxValue = list.stream()
        .max(Integer::compare)
        .orElse(0); // 默认值

Integer maxValue2 = list.stream().collect(Collectors.collectingAndThen(Collectors.maxBy((a, b) -> a - b), Optional::get));

// 最小值
Integer minValue = list.stream()
        .min(Integer::compare)
        .orElse(0);
    
Integer minValue2 = list.stream().collect(Collectors.collectingAndThen(Collectors.minBy((a, b) -> a - b), Optional::get));

// 求和（直接使用sum()更高效）
int sumValue = list.stream().mapToInt(Integer::intValue).sum();

Integer sumValue2 = list.stream().collect(Collectors.summingInt(item -> item));

// 平均值
Double avg = list.stream()
        .mapToInt(Integer::intValue)
        .average()
        .orElse(0.0);

Double avg2 = list.stream().collect(Collectors.averagingDouble(x -> x));
    
// 求汇总数
long count= quotaDtolists.stream().filter(Quota-> Quota.getId()>1).collect(Collectors.counting());

ArrayList<QuotaDto> uniqueList = quotaDtolists.stream()
        .collect(Collectors.collectingAndThen(
            Collectors.toCollection(() -> new TreeSet<>(
                Comparator.comparing(QuotaDto::getId)
                          .thenComparing(QuotaDto::getQuotaName)
            )), 
            ArrayList::new
        ));


//方法二
ArrayList<QuotaDto> collect = quotaDtolists.stream().collect(Collectors.collectingAndThen(
            Collectors.toCollection(() -> new TreeSet<>(
                    Comparator.comparing(q -> q.getId()+";"+q.getQuotaName()))), ArrayList::new));