java stream的常见操作

最小的帆也能远航2023-12-17 9:11

1.List<String> 转 List<Integer>

如果 strList 中全是数字字符串，可通过如下转换为 integerList

java 复制代码 
        List<String> strList = new ArrayList<>(Arrays.asList("7","8","9"));
        List<Integer> integerList =     
        strList.stream().map(Integer::valueOf).collect(Collectors.toList());

2.List<String> 转 Integer[ ]

如果 strList 中全是数字字符串，可通过如下转换为 intergerArry

java 复制代码 
        List<String> strList = new ArrayList<>(Arrays.asList("7","8","9"));
        Integer[] intergerArry = 
        strList.stream().map(Integer::valueOf).toArray(Integer[]::new);

3.String 转 List<Character>

java 复制代码 
        String str = "abcdef";
        List<Character> list = str.chars().mapToObj(x -> (char) 
       x).collect(Collectors.toList());

4.String[ ] 转 List<String>

java 复制代码 
        String[] array = new String[]{"aa","bb","cc"};
        List<String> strList = Arrays.stream(array)
        .collect(Collectors.toList());

5.String[ ] 转 Integer [ ]

java 复制代码 
        String[] strArry = new String[]{"5", "6", "1", "4", "9"};
        Integer[] integerArry = 
       Arrays.stream(strArry).map(Integer::parseInt).toArray(Integer[]::new);

6.List<Integer> 转 int [ ]

java 复制代码 
        List<Integer> integerList = new ArrayList<>(Arrays.asList(5,6,1,4,9));
        int[] arr = integerList.stream()
        .mapToInt(Integer::intValue).toArray();

7.int [ ] 转 List<Integer>

java 复制代码 
        int[] intArry = new int[]{5,6,1,4,9};
        List<Integer> integerList = 
        Arrays.stream(intArry).boxed().collect(Collectors.toList());

8.int [ ] 转 Integer [ ]

java 复制代码 
        int[] intArry = new int[]{5, 6, 1, 4, 9};
        Integer[] integerArry = Arrays.stream(intArry)
                               .boxed().toArray(Integer[]::new);

9.int [] 最大、最小值、平均值，求和

java 复制代码 
        int[] arr = new int[]{12,3,34,67,100,99};
        int maxValue = Arrays.stream(arr).max().getAsInt();
        int minValue = Arrays.stream(arr).min().getAsInt();
        double averValue = Arrays.stream(arr).average().getAsDouble();
        int sumValue = Arrays.stream(arr).sum();
        System.out.println("the max:" + maxValue);
        System.out.println("the min:" + minValue);
        System.out.println("the average:" + averValue);
        System.out.println("the sum:" + sumValue);

10.上面是流里面使用 max、min、average 、sum方法获取数组的最大、最小值、平均值，求和。

除了上面的方法还可以像下面这样获取最大值、最小值、平均值，求和。

java 复制代码 
        int[] intArray = {12,3,34,67,100,99};
        IntStream intStream = IntStream.of(intArray);
        IntSummaryStatistics statistics = intStream.summaryStatistics();
        System.out.println("the max:" + statistics.getMax());
        System.out.println("the min:" + statistics.getMin());
        System.out.println("the average:" + statistics.getAverage());
        System.out.println("the sum:" + statistics.getSum());
       //statistics.getCount()相当于获取数组大小

        System.out.println("the count:" + statistics.getCount());

11.String[] 对每个元素分割并转换为其他类型

java 复制代码 
        String[] array = {"a-1", "b-2", "c-3"};

        //"-分割后获取字母，转为新数组
        String[] strArray = Arrays.stream(array).map(v -> v.split("-") 
                           [0]).toArray(String[]::new);
        //"-分割后获取字母，转List
        List<String> list = Arrays.stream(array).map(v -> v.split("-") 
                            [0]).collect(Collectors.toList());
        //"-分割后获取数字，转List
        List<Integer> numberlist = Arrays.stream(array).map(v -> 
                                Integer.parseInt(v.split("-") 
                           [1])).collect(Collectors.toList());
        //"-分割后获取字母，用逗号拼接为字符串
        String str = Arrays.stream(array).map(v -> v.split("-") 
                      [0]).collect(Collectors.joining(","));

        System.out.println(Arrays.toString(strArray));//[a, b, c]
        System.out.println(list);//[a, b, c]
        System.out.println(numberlist);//[1, 2, 3]
        System.out.println(str);//a,b,c

12.List<String> 逗号拼接为一个字符串

java 复制代码 
        List<String> strList = new ArrayList<>(Arrays.asList("a","b","c"));
        String str = strList.stream().collect(Collectors.joining(","));
        System.out.println(str);//a,b,c

13.List<Integer> 逗号拼接为一个字符串

java 复制代码 
        List<Integer> integerList = Arrays.asList(7, 8, 9);
        String str = 
              integerList.stream().map(String::valueOf)
                .collect(Collectors.joining(","));
        System.out.println(str);//7,8,9

14.生成指定范围的数组或List

  • 生成[0,100)的 int[ ]
java 复制代码 
        int[] intArray = IntStream.range(0, 100).toArray();
  • 生成[0,100)的 List
java 复制代码 
		List<Integer> intList= IntStream.range(0, 100)
                .boxed()
                .collect(toList());
                
        List<String> strList = IntStream.range(0, 100)
                .boxed()
                .map(i -> i.toString())
                .collect(toList());
  • 生成[0,100]的 int[ ]
java 复制代码 
        int[] intArray1 = IntStream.rangeClosed(0, 100).toArray();
  • 生成[0,100]的 List
java 复制代码 
		List<Integer> intList= IntStream.rangeClosed(0, 100)
                .boxed()
                .collect(toList());
                
        List<String> strList = IntStream.rangeClosed(0, 100)
                .boxed()
                .map(i -> i.toString())
                .collect(toList());

15.判断数组中是否含有某一值

  • 字符串数组
java 复制代码 
        String[] values = {"AB","BC","CD","AE"};
        boolean contains = Arrays.stream(values).anyMatch("AE"::equals);
  • int数组
java 复制代码 
        int[] a = {1,2,3,4};
        boolean contains = IntStream.of(a).anyMatch(x -> x == 4);

16.数组或List求和

  • 数组求和
java 复制代码 
        int[] intArray = {11, 5, 3, 2, 1};
        int sum = Arrays.stream(intArray).reduce(0, Integer::sum);
  • List求和
java 复制代码 
        List<Integer> list = new ArrayList<>(Arrays.asList(5, 1, 7, 10));
        int sum = list.stream().reduce(0, Integer::sum);

17.两个数组合并为一个新的数组

  • 两个字符串数组合并为一个新的数组
java 复制代码 
        String[] a = {"a", "b", "c"};
        String[] b = {"1", "2", "3"};
        String[] c = Stream.of(a,b).flatMap(Stream::of)
                     .toArray(String[]::new);
  • 两个 int 型数组合并为一个新的 int 型数组
java 复制代码 
        int[] a = new int[]{1,3};
        int[] b = new int[]{2,4};
        int[] c =  IntStream.concat(Arrays.stream(a), 
                    Arrays.stream(b)).toArray();

18.两个数组合并为一个List

  • 两个String数组转List<String>
java 复制代码 
        String[] a = {"a", "b", "c"};
        String[] b = {"1", "2", "3"};
        List<String> strList = Stream.of(a, 
             b).flatMap(Stream::of).collect(Collectors.toList());
  • 两个int数组转List<Integer>
java 复制代码 
        int[] a = new int[]{1, 2};
        int[] b = new int[]{3, 4};
        List<Integer> integerList = Stream.of(IntStream.of(a).boxed(), 
          IntStream.of(b).boxed()).flatMap(s -> s)
          .collect(Collectors.toList());

19.两个List合并为一个新的List

java 复制代码 
        List<String> lis1 = new ArrayList<>(Arrays.asList("a", "b", "c"));
        List<String> list2 = new ArrayList<>(Arrays.asList("e", "f", "g"));
        List<String> newList = Stream.of(lis1, 
              list2).flatMap(Collection::stream)
           .collect(Collectors.toList());

20.List<Integer>求交集、并集、差集

java 复制代码 
List<Integer> list = new ArrayList<>(Arrays.asList(7, 8, 9));
List<Integer> list2 = new ArrayList<>(Arrays.asList(3,4, 9));

对 list 和 list2求交集、并集、差集

java 复制代码 
//交集
 List<Integer> beMixed = list.stream().filter(list2::contains)
                        .collect(Collectors.toList());
 System.out.println(beMixed);//[9]

//并集
List<Integer> aggregate = Stream.of(list, list2).flatMap(Collection::stream)
                         .distinct()
                         .collect(Collectors.toList());
System.out.println(aggregate);//[7, 8, 9, 3, 4]

//差集
List<Integer> subtraction = list.stream()
                            .filter(v->!list2.contains(v))
                            .collect(Collectors.toList());
System.out.println(subtraction);//[7, 8]

21.Map的value保存为List、Set

  • 保存为List
java 复制代码 
        Map<String, String> map = new HashMap<>();
        map.put("1", "a");
        map.put("2", "b");
        map.put("3", "c");
        List<String> values = map.values().stream()
                             .collect(Collectors.toList());
  • 保存为Set
java 复制代码 
        Map<String, String> map = new HashMap<>();
        map.put("1", "a");
        map.put("2", "b");
        map.put("3", "c");
        Set<String> values = map.values().stream()
                            .collect(Collectors.toSet());

22.Map对value求和

java 复制代码 
        Map<String, Integer> map = new HashMap<>();
        map.put("1", 11);
        map.put("2", 22);
        map.put("3", 100);
        Integer sum = map.values().stream()
                     .mapToInt(Integer::valueOf).sum();
        System.out.println(sum);

23.Map<String, List<Integer>>获取所有的values为一个List<Integer>

将map 里的所有value合并为一个List

java 复制代码 
       List<Integer> list1 = new ArrayList<>(Arrays.asList(1, 2, 3));
        List<Integer> list2 = new ArrayList<>(Arrays.asList(4, 5, 6));
        List<Integer> list3 = new ArrayList<>(Arrays.asList(7, 8, 9));

        Map<String, List<Integer>> map = new HashMap<>();
        map.put("a", list1);
        map.put("b", list2);
        map.put("c", list3);

        List<Integer> list = map.values().stream()
                            .flatMap(Collection::stream)
                            .collect(Collectors.toList());
        //下面的代码是一样的效果
        //List<Integer> list = map.entrySet().stream()
                              .map(e -> e.getValue())
                              .flatMap(Collection::stream)
                               .collect(Collectors.toList());
        System.out.println(list);//[1, 2, 3, 4, 5, 6, 7, 8, 9]

24.String[]中的元素转大写并转为List<String>

java 复制代码 
        String[] strArray= { "java", "react", "angular", "vue" };
        List<String> list = Stream.of(strArray)
                           .map(test -> test.toUpperCase())
                             .collect(Collectors.toList());

25.对String字符串中的数字求和

首先需要将该字符串转换为数组，接下来需要过滤掉非整数元素，最后，将该数组的剩余元素转换为数字并求和。

java 复制代码 
        String string = "Item1 10 Item2 25 Item3 30 Item4 45";

        Integer sum = Arrays.stream(string.split(" "))
                .filter((s) -> s.matches("\\d+"))
                .mapToInt(Integer::valueOf)
                .sum();

26.List<String>中统计首字母是j的个数

java 复制代码 
 List<String> list = Arrays.asList("java", "react", "angular", "javascript", "vue");
 long count = list.stream().filter(p -> p.startsWith("j")).count();

27.List<String>中获取第一个首字母是j的元素

java 复制代码 
List<String> list = Arrays.asList( "react","java", "angular", "javascript", "vue");
        //java
String firstJ = list.stream().filter(p -> p.startsWith("j")).findFirst().get();

28.List<String>统计各字符串出现的次数

java 复制代码 
List<String> items = Arrays.asList("apple", "apple", "orange", "orange", "orange", 
  "blueberry", "peach", "peach", "peach", "peach");
        Map<String, Long> result = 
         items.stream()
        .collect(Collectors.groupingBy(Function.identity(), 
         Collectors.counting()));
        //{orange=3, apple=2, blueberry=1, peach=4}
        System.out.println(result);

29.List去除重复元素

java 复制代码 
        List<String> list = Arrays.asList("aa", "bb", "cc", "bb");
        List<String> distinctList = list.stream()
                                   .distinct()
                                   .collect(Collectors.toList());

30.List<List<String>>转List<String>

java 复制代码 
        List<String> a = Arrays.asList("Virat", "Dhoni", "Jadeja");
        List<String> b = Arrays.asList("1", "2", "3");
        List<List<String>> list = new ArrayList<>();
        list.add(a);
        list.add(b);

        List<String> newList = list.stream()
                               .flatMap(v -> v.stream())
                               .collect(Collectors.toList());
        //[Virat, Dhoni, Jadeja, 1, 2, 3]
        System.out.println(newList);

31.多个List<String> 合并为一个List<String>

java 复制代码 
    public static void main(String[] args) {
        List<String> list1=new ArrayList<>(Arrays.asList("a","b","c"));
        List<String> list2=new ArrayList<>(Arrays.asList("d","e","f"));
        List<String> list3=new ArrayList<>(Arrays.asList("g","h","i"));
        List<String> list4=new ArrayList<>(Arrays.asList("1","2"));
        List<String> list5=new ArrayList<>(Arrays.asList("3","4","5"));

        List<String> result1 = join(list1, list2,list3);
        List<String> result2 = join(list4, list5);
        //[a, b, c, d, e, f, g, h, i]
        System.out.println(Arrays.toString(result1.toArray()));
        //[1, 2, 3, 4, 5]
        System.out.println(Arrays.toString(result2.toArray()));
    }

    @SafeVarargs
    public static  List<String> join(List<String>... lists) {
        return Arrays.stream(lists)
                     .flatMap(Collection::stream)
                     .collect(Collectors.toList());
    }

32.List<Object>转Map<String, Object>

假设有Student类，里面含有id、name、score等信息。现在想把 List<Student> 转化为Map<String,Student>，其中key是name。

java 复制代码 
        List<Student> list = new ArrayList<>();
        list.add(new Student(1, "张三", 85));
        list.add(new Student(2, "李四", 60));
        Map<String, Student> map = list.stream()
                                       .collect(Collectors
                                       .toMap(Student::getName, 
        Function.identity()));

33.List<Object>针对某一成员变量获取最大、小值的Object

假设有 Student 类，里面含有 id、name、score 等信息。现在想获取List<Student> 中 score 最大的 Student 和 score 最小的 Student。

java 复制代码 
        List<Student> list = new ArrayList<>();
        list.add(new Student(1, "张三", 85));
        list.add(new Student(2, "李四", 60));
        list.add(new Student(3, "刘一", 70));
        list.add(new Student(4, "李四", 99));
        //获取 score 最大的 Student
        Student maxStudent = list.stream()
                                 .max(Comparator
                                 .comparing(Student::getScore))
                                 .get();
        //获取 score 最小的 Student
        Student minxStudent = list.stream()
                                  .min(Comparator
                                  .comparing(Student::getScore))
                                  .get();

34.List<Object>获取某个成员变量最大、最小值、平均值，求和

假设有 Student 学生类，里面含有 id、name、score 等信息。现在想获取 List<Student> 中的 score 的最大值、最小值、平均值，求和：

java 复制代码 
        List<Student> list = new ArrayList<>();
        list.add(new Student(1, "小名", 17));
        list.add(new Student(2, "小红", 18));
        list.add(new Student(3, "小蓝", 19));
        list.add(new Student(4, "小灰", 20));
        list.add(new Student(5, "小黄", 21));
        list.add(new Student(6, "小白", 22));
        
        IntSummaryStatistics intSummary = list.stream() 
                           .collect(Collectors
                           .summarizingInt(Student::getScore));
        System.out.println(intSummary.getAverage());// 19.5
        System.out.println(intSummary.getMax());// 22
        System.out.println(intSummary.getMin());// 17
        System.out.println(intSummary.getSum());// 117

如果你只想求和可以像下面这样写：

java 复制代码 
        List<Student> list = new ArrayList<>();
        list.add(new Student(1, "小名", 17));
        list.add(new Student(2, "小红", 18));
        list.add(new Student(3, "小蓝", 19));
        list.add(new Student(4, "小灰", 20));
        list.add(new Student(5, "小黄", 21));
        list.add(new Student(6, "小白", 22));

        Integer sum = list.stream().mapToInt(Student::getScore).sum();//117
        //下面注释的两行代码和上一行的效果一样，都可以进行求和
        //Integer sum = list.stream().map(Student::getScore)
                                     .reduce(0, Integer::sum);
        //Integer sum = list.stream().map(Student::getScore)
                                     .mapToInt(Integer::intValue).sum();

35.List<Integer> 获取最大、最小值、平均值，求和

java 复制代码 
        List<Integer> list = Arrays.asList(12, 3, 34, 67, 100, 99);
        int maxValue = list.stream().max(Integer::compare).get();
        int minValue = list.stream().min(Integer::compare).get();
        double averValue = list.stream().mapToDouble(a -> a)
                                       .average().getAsDouble();
        int sumValue = list.stream().reduce(0, Integer::sum);
        System.out.println("the max:" + maxValue);
        System.out.println("the min:" + minValue);
        System.out.println("the average:" + averValue);
        System.out.println("the sum:" + sumValue);

上面是流里面使用 max、min、average 、reduce 方法获取数组的最大、最小值、平均值，求和。

除了上面的方法还可以像下面这样获取最大值、最小值、平均值，求和。

java 复制代码 
        List<Integer> list = new ArrayList<>(Arrays.asList(12,3,34,67,100,99));
        IntSummaryStatistics statistics = list.stream()
                                             .mapToInt(value -> value)
                                             .summaryStatistics();
        System.out.println("the max:" + statistics.getMax());
        System.out.println("the min:" + statistics.getMin());
        System.out.println("the average:" + statistics.getAverage());
        System.out.println("the sum:" + statistics.getSum());
        System.out.println("the count:" + statistics.getCount());//获取list大小

拓展：

除了anyMatch ，stream 中还有 allMatch、noneMatch ，可以参考

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