关于stream流的操作网上有很多教程,
比如流里的 filter、map、flatMap、collect、reduce、limit、distinct、skip、findFirst、count等常用操作。
如对基本概念不熟,可参考其他博客:
JAVA8 Stream接口,map操作,filter操作,flatMap操作
java8 stream流操作的flatMap(流的扁平化)
Java 8 Stream 的终极技巧——Collectors 操作
本文主要讲开发中常用的流操作,熟练使用可简化代码、使代码更简洁优雅。
目录:
List<String> 转 List<Integer>
List<String> 转 Integer[]
String[] 转 List<String>
String[] 转 Integer []
int [] 转 List<Integer>
int [] 转 Integer []
int [] 最大、最小值、平均值,求和
String[] 对每个元素分割并转换为其他类型
List<String> 逗号拼接为一个字符串
List<Integer> 逗号拼接为一个字符串
生成指定范围的数组
判断数组中是否含有某一值
数组或 List 求和
两个数组合并为一个新的数组
两个数组合并为一个 List
两个 List合 并为一个新的 List
List<Integer>求交集、并集、差集
Map 的 value 保存为 List、Set
Map<String, List<Integer>>获取所有的values为一个List<Integer>
String[] 中的元素转大写并转为 List<String>
List<String> 中统计首字母是j的个数
List<String> 中获取第一个首字母是j的元素
List<String> 统计各字符串出现的次数
List 去除重复元素
List<List<String>> 转List<String>
List<Object> 转 Map<String, Object>
List<Object> 针对某一成员变量获取最大、小值的 Object
List<Object> 获取某个成员变量最大、最小值、平均值,求和
List<Integer> 获取最大、最小值、平均值,求和
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(v -> Integer.valueOf(v)).collect(Collectors.toList());
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(v -> Integer.valueOf(v)).toArray(Integer[]::new);
String[ ] 转 List<String>
String[] array = new String[]{"aa","bb","cc"};
List<String> strList = Arrays.stream(array).collect(Collectors.toList());
String[ ] 转 Integer [ ]
String[] strArry = new String[]{"5", "6", "1", "4", "9"};
Integer[] integerArry = Arrays.stream(strArry).map(Integer::parseInt).toArray(Integer[]::new);
int [ ] 转 List<Integer>
int[] intArry = new int[]{5,6,1,4,9};
List<Integer> integerList = Arrays.stream(intArry).boxed().collect(Collectors.toList());
int [ ] 转 Integer [ ]
int[] intArry = new int[]{5, 6, 1, 4, 9};
Integer[] integerArry = Arrays.stream(intArry).boxed().toArray(Integer[]::new);
int [] 最大、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());
System.out.println("the count:" + statistics.getCount());
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]).flatMap(Stream::of).toArray(String[]::new);
//"-分割后获取字母,转List
List<String> list = Arrays.stream(array).map(v -> v.split("")[0]).flatMap(Stream::of).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]).flatMap(Stream::of).collect(Collectors.joining(","));
System.out.println(Arrays.toString(strArray));//[a, b, c]
System.out.println(list);//[a, b, c]
System.out.println(str);//a,b,c
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
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(v->String.valueOf(v)).collect(Collectors.joining(","));
System.out.println(str);//7,8,9
生成指定范围的数组
- 生成[0,100)的 int[ ]
int[] intArray = IntStream.range(0, 100).toArray();
- 生成[0,100]的 int[ ]
int[] intArray1 = IntStream.rangeClosed(0, 100).toArray();
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);
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);
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();
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());
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());
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]
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());
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.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]
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());
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();
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();
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);
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());
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);
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()));
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最大值
Optional<Student> maxOptStudent = list.stream().collect(Collectors.maxBy(Comparator.comparing(Student::getScore)));
Student maxStudent = maxOptStudent.get();
//获取score最小值
Optional<Student> mixOptStudent = list.stream().collect(Collectors.minBy(Comparator.comparing(Student::getScore)));
Student minxStudent = mixOptStudent.get();
List<Object>获取某个成员变量最大、最小值、平均值,求和
假设有 Student 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());// 19.5
System.out.println(intSummary.getMax());// 22
System.out.println(intSummary.getMin());// 17
System.out.println(intSummary.getSum());// 117
List 获取最大、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());
拓展:
除了anyMatch ,stream 中还有 allMatch、noneMatch ,可以参考 java8新特性Stream流中anyMatch和allMatch和noneMatch的区别详解
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