list.stream().collect例子

复制代码

// 已有股票ID列表
List<String> existList = Arrays.asList("001", "003", "005");

// 待处理的股票列表
List<Stock> stocks = Arrays.asList(
        new Stock("001", "腾讯", 350.0),
        new Stock("002", "阿里", 180.0),
        new Stock("003", "百度", 120.0),
        new Stock("004", "京东", 80.0),
        new Stock("005", "美团", 150.0)
);

// 分组：是否存在于 existList 中
Map<Boolean, List<Stock>> grouped = stocks.stream()
        .collect(Collectors.groupingBy(
                stock -> existList.contains(stock.getStkId())
        ));

System.out.println("已存在的股票: " + grouped.get(true));


List<Stock> stocks1 = grouped.get(true);
for(Stock stock : stocks1){
    System.out.println(stock.getStkId());//已存在的股票:  001   003 005
}
System.out.println("不存在的股票: " + grouped.get(false));
List<Stock> stocks2 = grouped.get(false);
for(Stock stock : stocks2){
    System.out.println(stock.getStkId());//不存在的股票:002  004
}

复制代码

  List<Person> people = Arrays.asList(
          new Person("Alice", 25, "IT"),
          new Person("Bob", 30, "HR"),
          new Person("Charlie", 25, "IT"),
          new Person("David", 30, "HR")
  );

  // 按部门分组
  Map<String, List<Person>> byDept = people.stream()
          .collect(Collectors.groupingBy(Person::getDepartment));
   // 结果: {IT=[Alice, Charlie], HR=[Bob, David]}
  System.out.println(byDept); //{HR=[Person.Person@7ab2bfe1, Person.Person@497470ed], IT=[Person.Person@63c12fb0, Person.Person@b1a58a3]}
  for(String dept : byDept.keySet()){
      System.out.println(dept);
      for(Person p : byDept.get(dept)){
          System.out.print(p.getName()+"   ");
      }
      System.out.println();
  }
 /* HR
  Bob   David
  IT
  Alice   Charlie*/


  // 分组后对值进行操作
  Map<String, Long> countByDept = people.stream()
          .collect(Collectors.groupingBy(
                  Person::getDepartment,
                  Collectors.counting()
          ));
  System.out.println(countByDept);//{HR=2, IT=2}

  // 分组后提取特定字段
  Map<String, Set<String>> namesByDept = people.stream()
          .collect(Collectors.groupingBy(
                  Person::getDepartment,
                  Collectors.mapping(Person::getName, Collectors.toSet())
          ));
  System.out.println(namesByDept);//{HR=[Bob, David], IT=[Alice, Charlie]}




  List<String> names = Arrays.asList( "David","Alice", "Charlie", "Bob","David","Alice","Aa");

  // 转换为 List
  List<String> list = names.stream()
          .filter(name -> name.length() > 3)
          .collect(Collectors.toList());
  System.out.println(list); // [David, Alice, Charlie, David, Alice]

// 使用 toCollection 指定具体实现
  LinkedList<String> linkedList = names.stream()
          .collect(Collectors.toCollection(LinkedList::new));
  System.out.println(linkedList);//[David, Alice, Charlie, Bob, David, Alice, Aa]

// 转换为 Set（自动去重）
  Set<String> set = names.stream()
          .collect(Collectors.toSet());
  System.out.println(set); // [Aa, Bob, Alice, Charlie, David]

 // 使用 toCollection 指定 TreeSet
  Set<String> treeSet = names.stream()
          .collect(Collectors.toCollection(TreeSet::new));
  System.out.println(treeSet);//[Aa, Alice, Bob, Charlie, David]


  // 简单连接
  String result1 = names.stream()
          .collect(Collectors.joining());
  System.out.println(result1); // DavidAliceCharlieBobDavidAliceAa


 // 带分隔符
  String result2 = names.stream()
          .collect(Collectors.joining(", "));
  System.out.println(result2); // David, Alice, Charlie, Bob, David, Alice, Aa

  // 带前缀和后缀
  String result3 = names.stream()
          .collect(Collectors.joining(", ", "[", "]"));
  System.out.println(result3); // [David, Alice, Charlie, Bob, David, Alice, Aa]