【Java8】常用lambda表达式

迷南。 2023-10-08 16:26 46阅读 0赞

### **交集 并集 差集** ###

交集（listA ∩ ListB）：
    List<Person> listC = listA.stream().filter(item -> listB.contains(item)).collect(Collectors.toList());
    listC中的元素有：属性name值为 aa, bb, cc 的对象。
     
    并集（listA ∪ listB）:
    //先合体
    listA.addAll(listB);
    //再去重
    List<Person> listC = listA.stream().distinct().collect(Collectors.toList());
    listC中的元素有：属性name值为 aa, bb, cc ,dd的对象。
     
    差集（listA - listB）:
    List<Person> listC = listA.stream().filter(item -> !listB.contains(item)).collect(Collectors.toList());
    listC中的元素有：属性name值为 dd的对象。

### **List集合的过滤之lambda表达式** ###

stmtList = stmtList.stream().filter(stmt -> !(stmt instanceof OracleInsertStatement) && stmt.getParent() == null).collect(Collectors.toList());

### **lambda表达式将List对象某个字段转换以逗号分隔的String类型** ###

// 获取 users 集合中的 id 集合
    List<Long> ids = users.stream().map(User::getId).collect(Collectors.toList());
    System.out.println("ids: " + ids);
    System.out.println();
    
    // 获取 users 集合中的 id 集合并转为字符串, 通过 , 拼接p
    String idsTxt = users.stream().map(User::getId).map(String::valueOf).collect(Collectors.joining(","));
    System.out.println("idsTxt: " + idsTxt);
    System.out.println();
    
    List<String> names = users.stream().map(User::getName).collect(Collectors.toList());
    System.out.println("names: " + names);

### **分组功能** ###

Collection<List<CheckTaskAndCompanyBindEntity>> collect2 = checkTaskAndCompanyBindByList.stream().
    filter(s -> s.getTaskType() == 2).collect(Collectors.groupingBy(CheckTaskAndCompanyBindEntity::getCompanyId)).values();

### **排序功能** ###

//升序 123
    List.sort(Comparator.comparing(CheckBjwaInputResp::getCheckNum));
    //降序 321
    List.sort(Comparator.comparing(CheckBjwaInputResp::getCheckNum).reversed());

### **Map相关操作** ###

// 获取map第一个元素
    Optional<Object> first = map.values().stream().findFirst();
    String s = first.get().toString();
    
    // 循环map
    cloumMap.forEach((k, v) -> {
        
        System.out.println("k:" + k + ",v:" + v);
    });

### **filter 过滤** ###

/**
     * filter 过滤，留下超过20岁的用户
     */
    List<UserInfo> userInfoResultList = userInfoList.stream().filter(user -> user.getAge() > 20).collect(Collectors.toList());
    userInfoResultList.forEach(a -> System.out.println(a.getUserName()));

### **forEach 遍历** ###

List.forEach(System.out::println);
    
    Map.forEach((k, v) -> System.out.println(k + ":\t" + v));

### **groupingBy分组** ###

Map<String, List<UserInfo>> result = originUserInfoList.stream()
    .collect(Collectors.groupingBy(UserInfo::getCity));

### **sorted+Comparator排序** ###

/**
     *  sorted + Comparator.comparing 排序列表，
     */
    userInfoList = userInfoList.stream().sorted(Comparator.comparing(UserInfo::getAge)).collect(Collectors.toList());
    userInfoList.forEach(a -> System.out.println(a.toString()));
    
    System.out.println("开始降序排序");
    
    /**
     * 如果想降序排序，则可以使用加reversed()
     */
    userInfoList = userInfoList.stream().sorted(Comparator.comparing(UserInfo::getAge).reversed()).collect(Collectors.toList());
    userInfoList.forEach(a -> System.out.println(a.toString()));

### **distinct去除重复的元素** ###

List<String> list = Arrays.asList("A", "B", "F", "A", "C");
    List<String> temp = list.stream().distinct().collect(Collectors.toList());
    temp.forEach(System.out::println);

### **map 转换** ###

List<String> list = Arrays.asList("jay", "tianluo");
    //转化为大写
    List<String> upperCaselist = list.stream().map(String::toUpperCase).collect(Collectors.toList());
    upperCaselist.forEach(System.out::println);

### **Max，Min 最大最小** ###

Optional<UserInfo> maxAgeUserInfoOpt = userInfoList.stream().max(Comparator.comparing(UserInfo::getAge));
    maxAgeUserInfoOpt.ifPresent(userInfo -> System.out.println("max age user:" + userInfo));
    
    Optional<UserInfo> minAgeUserInfoOpt = userInfoList.stream().min(Comparator.comparing(UserInfo::getAge));
    minAgeUserInfoOpt.ifPresent(userInfo -> System.out.println("min age user:" + userInfo));

### **count 统计** ###

long count = userInfoList.stream().filter(user -> user.getAge() > 18).count();
    System.out.println("大于20岁的用户:" + count);