Java8流常见用法

冷不防 2024-03-29 14:45 64阅读 0赞

## nonNull，isNotBlank，distinct ##

List<Person> list = new ArrayList<>();
    List<String> collect = list.stream()
            .filter(Objects::nonNull)
            .map(Person::getName)
            .filter(StringUtils::isNotBlank)
            .distinct()
            .collect(Collectors.toList());

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## 求和 ##

# 求和
    int[] ints = new int[]{
        1, 2, 3};
    int sum = Arrays.stream(ints).sum(); // 1+2+3=6
    
    # 求前3项和
    int sum1 = Stream.of(1, 2, 2, 4, 4).mapToInt(Integer::intValue).limit(3).sum(); // 1+2+2=5
    
    # 求3到5项的加和
    int sum2 = Stream.of(1, 2, 2, 4, 4).mapToInt(Integer::intValue).skip(2).limit(3).sum(); // 2+4+4=10

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## 统计个数 ##

# 统计符合条件的个数
     long count = Arrays.stream("a,b,c,a".split(",")).filter("a"::equals).count(); // 2

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## 最大值，最小值，平均值 ##

int[] ints = new int[]{
        1, 2, 3};
    
    # 求最大值
    int max = Arrays.stream(ints).max().getAsInt(); // 3
    
    # 最小值
    int min = Arrays.stream(ints).min().getAsInt(); // 1
    
    # 平均值
    double average = Arrays.stream(ints).average().getAsDouble(); // 2.0

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## 获取第1条数据 ##

# 获取第一条数据
    String str = list.stream().findFirst().orElse(null);

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## 获取第4条数据 ##

# 获取第四条数据
    Integer integer = list.stream().sorted(Comparator.reverseOrder()).skip(3).limit(1).findFirst().orElse(0); // 1
    
    list.stream().sorted(Comparator.reverseOrder()).skip(3).limit(1).forEach(System.out::println); // 1

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## 收集符合条件的值 ##

List<Integer> list22 = new ArrayList<>();
    Arrays.stream("1,2,3,4,5".split(",")).mapToInt(Integer::parseInt).max().ifPresent(list22::add); // [5]
            
    List<String> list33 = new ArrayList<>();
    Arrays.stream("a,b,c,a,a,a".split(",")).filter("a"::equals).findAny().ifPresent(list33::add); // [a]
            
    List<String> list44 = new ArrayList<>();
    Arrays.stream("a,b,c,a".split(",")).filter("a"::equa=-).findFirst().ifPresent(list44::add); // [a]
            
    List<Person> list = new ArrayList<>();
    list.add(new Person("John", 18, "1234567"));
    List<String> list55 = list.stream().filter(Objects::nonNull).map(Person::getName).collect(Collectors.toList()); // [John]

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## 最安全使用list.stream的方式 ##

List<String> collect = Optional.ofNullable(list).orElse(new ArrayList<>()).stream().map(Person::getName).collect(Collectors.toList());

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## 条件判断 ##

---------------------------------------------
    # 任意满足
    ---------------------------------------------
    List<Person> list = new ArrayList<>();
    boolean present2 = list.stream().anyMatch(a -> a.getAge() > 10);
    
    boolean present1 = list.stream().filter(a -> a.getAge() > 10).findAny().isPresent();
    ---------------------------------------------
    # 全部满足
    ---------------------------------------------
    boolean present3 = list.stream().allMatch(a -> a.getAge() > 10);
    
    Boolean high = true;
    Boolean rich = true;
    Boolean cool = true;
    boolean b = Stream.of(high, rich, cool).allMatch(BooleanUtils::isTrue);
    ---------------------------------------------
    # 全不满足
    ---------------------------------------------
    boolean present4 = list.stream().noneMatch(a -> a.getAge() > 10);

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## 集合合并 ##

// Map
    Map<String, List<Person>> map = new HashMap<>();
    List<Person> collect1 = map.values().stream().flatMap(Collection::stream).collect(Collectors.toList());
    
    // List
    List<Person> list1 = new ArrayList<>();
    List<Person> list2 = new ArrayList<>();
    List<Person> collect2 = Stream.of(list1, list2).flatMap(Collection::stream).collect(Collectors.toList());
    
    List<Person> collect3 = Stream.of(list1.stream(), list2.stream()).flatMap(Function.identity()).collect(Collectors.toList());

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## Map构造Model ##

# 素材
    Map<String, List<Person>> map = new HashMap<>();
    
    # 结果
    List<Person> res = new ArrayList<>();
    
    # 构造Model
    map.forEach((name, persons) -> {
        
        if (persons != null && persons.size() > 0) {
        
            persons.forEach(person -> {
        
                Person per = new Person();
                per.setName(name);
                per.setAge(person.getAge());
                per.setNumber(person.getNumber());
                res.add(per);
            });
        }
    });

**[展平嵌套数组][Link 1]**  
**[Bean列表根据Bean的某个属性去重][Bean_Bean]**  
**[List转Map][List_Map]**  
**[字符串，数组，集合之间相互转换总结][Link 2]**  
**[数组，List，Map排序总结][List_Map 1]**

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[Link 1]: https://blog.csdn.net/weixin_37646636/article/details/131948833
[Bean_Bean]: https://blog.csdn.net/weixin_37646636/article/details/131948944
[List_Map]: https://blog.csdn.net/weixin_37646636/article/details/130117427
[Link 2]: https://blog.csdn.net/weixin_37646636/article/details/128600562
[List_Map 1]: https://blog.csdn.net/weixin_37646636/article/details/128601266