Java入门11---- Stream流

不可变集合

不可变就是不可被修改。
集合的数据在创建的时候提供，并且在整个生命周期中都不可改变。

List<Integer> list1=List.of(33,222,11);

Set<String> names=Set.of("Damon","Stefan","ELena");
Set<String> names=Set.of("Damon","Stefan","ELena"，"Damon");//如果有重复元素会报错。


Map<String ,Integer> maps=Map.of("fxw",1,"sw",3);

Stream流

用于简化集合和数组操作的API。

public class stream_test {
    
    
    public static void main(String[] args) {
    
    
        List<String > names=new ArrayList<>();
        Collections.addAll(names,"Damon","Stefan","ELena","Dell");;
        //找到D开头的名字
        //1。for遍历
        List<String > Dname=new ArrayList<>();
        for (String name : names) {
    
    
            if (name.startsWith("D") && name.length()==4){
    
    
                Dname.add(name);
            }
        }
        System.out.println(Dname);
        //2。使用Stream实现
        names.stream().filter(s -> s.startsWith("D")).filter(s -> s.length()==4).forEach(s -> System.out.println(s));

    }
}

Stream流的三类方法

获取Stream流

创建一条流水线，并把数据放到流水线上准备进行操作

public class Demo1 {
    
    
    public static void main(String[] args) {
    
    
        /**
         * Collection集合获取流
         */
        List<String> list=new ArrayList<>();
        Stream<String> stream=list.stream();
        /**
         * Map集合获取流
         */
        Map<String ,Integer> map=new HashMap<>();
        //键流
        Stream<String> keystream=map.keySet().stream();
        //值流
        Stream<Integer> valuestream=map.values().stream();
        //键值对流（拿整体）
        Stream<Map.Entry<String ,Integer>> kvstream =map.entrySet().stream();
        /**
         * 数组获取流
         */
        String[] names={
    
    "Damon","Stefan","ELena","Dell"};
        //方法1
        Stream<String> namestream1= Arrays.stream(names);
        //方法2
        Stream<String> namestream2=Stream.of(names);
    }
}

中间方法

public class Demo2 {
    
    
    public static void main(String[] args) {
    
    
        List<String > names=new ArrayList<>();
        Collections.addAll(names,"Damon","Stefan","ELena","Dell");
        //filter过滤
        names.stream().filter(s -> s.startsWith("D")).filter(s -> s.length()==4).forEach(s -> System.out.println(s));

        //map加工方法：第一个参数原材料 -> 第二个参数是加工后的结果
        //给集合元素的前面都加上 2020
        names.stream().map(s -> 2020+s).forEach(s -> System.out.println(s));

        //吧所有的名称都加工成一个学生对象
        names.stream().map(s -> new Student(s)).forEach(s -> System.out.println(s));

        //合并流
        Stream<String> stream1=names.stream();
        Stream<Integer> stream2=Stream.of(11,22);
        Stream<Object> stream3=Stream.concat(stream1,stream2);
        stream3.forEach(s-> System.out.println(s));
    }
}

中间方法也称为非中间方法，调用完成后返回新的Stream流可以继续使用，支持链式编程。
在Stream流中无法直接修改集合、数组中的数据。

Demo

//注意：流只能使用一次

public class Demo3 {
    
    
    public static double allmoney1;
    public static double allmoney2;
    public static double allmoney;

    public static void main(String[] args) {
    
    

        List<emp> one=new ArrayList<>();
        String[] p={
    
    "Damon","Stefan","ELena","Dell"};
        one.add(new emp("fxw1",'m',3000,2000,null));
        one.add(new emp("fxw2",'f',400,20,"摸鱼"));
        one.add(new emp("fxw3",'m',3000,1000,"韭菜"));
        one.add(new emp("fxw4",'m',1000,500,"韭菜"));
        List<emp> two=new ArrayList<>();
        two.add(new emp("wyx1",'m',323,21,"迟到"));
        two.add(new emp("wyx2",'f',222,223,"摸鱼"));
        two.add(new emp("wyx3",'m',500,100,null));
        two.add(new emp("wyx4",'f',555,30,"吃东西"));
        two.add(new emp("wyx5",'f',888,90,"摸鱼"));

        //一部的最高工资员工
        topemp t1=one.stream().max((emp o1, emp o2)-> Double.compare(o1.getSalary()+o1.getBouns(),o2.getSalary()+ o2.getBouns()) ).map(s-> new topemp(s.getName(),s.getSalary()+s.getBouns())).get();
        //一部的最高工资员工
        topemp t2=two.stream().max((emp o1, emp o2)-> Double.compare(o1.getSalary()+o1.getBouns(),o2.getSalary()+ o2.getBouns()) ).map(s-> new topemp(s.getName(),s.getSalary()+s.getBouns())).get();
        //统计平均工资，去除最高工资和最低工资
        one.stream().sorted((emp o1, emp o2)-> Double.compare(o1.getSalary()+o1.getBouns(),o2.getSalary()+o2.getBouns())).skip(1).limit(one.size()-2).forEach(s->allmoney1+=(s.getSalary()+s.getBouns()));
        System.out.println(allmoney1/(one.size()-2));
        two.stream().sorted((emp o1, emp o2)-> Double.compare(o1.getSalary()+o1.getBouns(),o2.getSalary()+o2.getBouns())).skip(1).limit(two.size()-2).forEach(s->allmoney2+=(s.getSalary()+s.getBouns()));
        System.out.println(allmoney2/(one.size()-2));

        //统计全部门的平均工资
        Stream<emp> s1=one.stream();
        Stream<emp> s2=two.stream();
        Stream<emp> s3=Stream.concat(s1,s2);
        s3.sorted((emp o1, emp o2)-> Double.compare(o1.getSalary()+o1.getBouns(),o2.getSalary()+o2.getBouns())).skip(1).limit(one.size()+two.size()-2).forEach(s->allmoney=(s.getSalary()+s.getBouns()));
        System.out.println(allmoney/(one.size()+two.size()-2));//可能会有精度问题
        //BigDecimal解决精度问题
        BigDecimal b1=BigDecimal.valueOf(allmoney);
        BigDecimal b2=BigDecimal.valueOf(one.size()+two.size()-2);
        System.out.println(b1.divide(b2,2, RoundingMode.HALF_UP));
    }
}

终结方法

Stream流的收集操作
收集Stream流的含义：就是把Stream流操作后的结果数据转回到集合或者数组中去。
Stream流：方便操作集合/数组的手段。

List<String > names=new ArrayList<>();
Set
Collections.addAll(names,"Damon","Stefan","ELena","Dell");
List<String> res1=names.stream().collect(Collectors.toList());
Set<String> res2=names.stream().collect(Collectors.toSet());
Object[] arr=names.stream().toArray();//转为数组