目录
前言
毕业入职工作后开始接触Stream流，觉得挺方便的，尽可能全面的记录一下Stream流的方法。
1. forEach遍历
forEach:该方法接收一个Consumer接口函数，将每一个流元素交给该函数处理
forEach方法：用来遍历流中的数据
注：是一个终结方法，遍历之后就不能继续调用Stream流中的其他方法
public class Stream_ForEach {
public static void main(String[] args) {
//获取一个Stream流
Stream<String>stream= Stream.of("张三","李四","王五","赵六");
//使用Stream流的方法forEach对stream流中的数据遍历
stream.forEach((String name)->{
System.out.println(name);
});
}
}
2. filter过滤
filter：用于对Stream流中的数据进行过滤
filter(Predicate<? super T>predicate)
filter方法的参数Predicate是一个函数式接口，可以使用lambda表达式
Predicate中的抽象方法
boolean test（T t）
public class Stream_filter {
public static void main(String[] args) {
//创建一个Stream流
Stream<String> stream = Stream.of("张三", "李四", "王五", "赵六1", "刘老七");
//单条件过滤
Stream<String> stream1 = stream.filter((String name) -> {
return name.startsWith("刘");
});
//多条件过滤
List<String> stream2 = stream.filter((String name) -> {
if(name.length()>=3 && name.equals("刘老七")){
return true;
}
return false;
}).collect(Collectors.toList);
//遍历stream1
stream1.forEach((name)-> System.out.println(name));
//输出stream2
System.out.println(stream2)
}
}
3. distinct去重
public class Stream_distinct {
public static void main(String[] args) {
//创建一个Stream流
Stream<String> stream = Stream.of("张三", "张三","张三","李四", "王五", "赵六1", "刘老七");
//去重
List<String> stream1 = stream().distinct().collect(Collectors.toList());
//输出stream1
System.out.println(stream1)
}
}
4. limit截取
limit:用于截取流中的元素
limit可以对流进行截取，只取用前n个
limit（long maxSize）;
参数是一个long型，如果集合当前长度大于参数则进行截取，否则不进行操作
limit是一个延迟方法，可以继续使用Stream流方法
public class Stream_limit {
public static void main(String[] args) {
//创建一个Stream流
Stream<String> stream = Stream.of("张三", "张三","张三","李四", "王五", "赵六1", "刘老七");
//去重
List<String> list = stream().distinct().collect(Collectors.toList());
//截取去重后的前2个元素
list = list.stream().limit(2).collect(Collectors.toList();
//输出stream1
System.out.println(list)
}
}
5. skip跳过
skip方法：用于跳过元素
skip(long n)
如果流的当前长度大于n，则跳过前n个，否则将会得到一个长度为0的空流
public class Stream_skip {
public static void main(String[] args) {
//创建一个Stream流
Stream<String> stream = Stream.of("张三", "张三","张三","李四", "王五", "赵六1", "刘老七");
//去重
List<String> list = stream().distinct().collect(Collectors.toList());
//跳过去重后的前2个元素
list = list.stream().skip(2).collect(Collectors.toList());
//输出stream1
System.out.println(list)
}
}
6.排序sorted
public class Stream_sorted {
public static void main(String[] args) {
List<Test> list = new ArrayList<>();
list.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
list.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
list.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
//根据年龄从大到小排序
list = list.stream()
.sorted(Comparator.comparing(Test::getAge).reversed())
.collect(Collectors.toList());
list.forEach(System.out::println);
}
}
7.最值max,min
public class Stream_max_min {
public static void main(String[] args) {
List<Test> list = new ArrayList<>();
list.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
list.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
list.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
//获取年龄最大的人
Test maxPeople = list.stream().max(Comparator.companing(Test::getAge)).get();
//获取年龄最小的人
Test minPeople = list.stream().min(Comparator.companing(Test::getAge)).get();
}
}
8. 统计reduce
public class Stream_reduce {
public static void main(String[] args) {
List<Test> testList = new ArrayList<Test>();
testList.add(new Test("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
testList.add(new Test("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
testList.add(new Test("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
//统计年龄总和
int totalAge =testList.stream().mapToInt(Test::getAge).sum();
//统计工资总和
BigDecimal totalSalary = testList.stream().map(Test::getSalary)
//统计工资乘以各自系数的总和(向上保留两位)
BigDecimal totalRatioSalary = testList.stream()
.map(s->s.getSalary()
.multiply(s.getRatio())
.setScale(2,BigDecimal.ROUND_HALF_UP))
.reduce(BigDecimal.ZERO,BigDecimal::add);
}
}
9. List结构转换Map结构
public class Stream_List_Map {
public static void main(String[] args) {
List<Test> testList = new ArrayList<Test>();
testList.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
testList.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
testList.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
//根据姓名转map,map的key为name
Map<String, Test> nameMap= testList.stream().collect(Collectors.toMap(Test::getName, Test -> Test);
System.out.println(map);
}
}
10. List<Object>对象转List<String>
public class Stream_object_string {
public static void main(String[] args) {
List<Test> testList = new ArrayList<Test>();
testList.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
testList.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
testList.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
//获取姓名集合
List<String> nameList = testList.stream().map(Test::getName()).collect(Collectors.toList());
System.out.println("value:"+nameList);
}
}11. List<Object>对象转List<Object>
public class Stream_object_object {
public static void main(String[] args) {
List<People> peopleList = new ArrayList<People>();
peopleList.add(new People("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
peopleList.add(new People("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
peopleList.add(new People("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
//对象转对象
List<Student> studentList = peopleList.stream().map(s->{
return new Student(s.getName(),s.getAge());
}).collect(Collectors.toList());
System.out.println("value:"+studentList);
}
}
总结
略