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
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
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>
String str = "abcdef";List<Character> list = str.chars().mapToObj(x -> (char) x).collect(Collectors.toList());
4.String[ ] 转 List<String>
String[] array = new String[]{"aa","bb","cc"};List<String> strList = Arrays.stream(array).collect(Collectors.toList());
5.String[ ] 转 Integer [ ]
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 [ ]
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>
int[] intArry = new int[]{5,6,1,4,9};List<Integer> integerList = Arrays.stream(intArry).boxed().collect(Collectors.toList());
8.int [ ] 转 Integer [ ]
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
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。
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
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ListList<String> list = Arrays.stream(array).map(v -> v.split("-") [0]).collect(Collectors.toList());//"-分割后获取数字,转ListList<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
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
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[ ]
int[] intArray = IntStream.range(0, 100).toArray();
- 生成[0,100)的 List
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[ ]
int[] intArray1 = IntStream.rangeClosed(0, 100).toArray();
- 生成[0,100]的 List
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
String[] values = {"AB","BC","CD","AE"};boolean contains = Arrays.stream(values).anyMatch("AE"::equals);
- int数组
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
int[] intArray = {11, 5, 3, 2, 1};int sum = Arrays.stream(intArray).reduce(0, Integer::sum);
- List求和
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
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
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>
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>
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
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
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
//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
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
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
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
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>
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。
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
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
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
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
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>
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>
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()));}@SafeVarargspublic 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。
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。
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 StudentStudent maxStudent = list.stream().max(Comparator.comparing(Student::getScore)).get();//获取 score 最小的 StudentStudent 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:
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());// 19.5System.out.println(intSummary.getMax());// 22System.out.println(intSummary.getMin());// 17System.out.println(intSummary.getSum());// 117
如果你只想求和可以像下面这样写:
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
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。
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