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Stream流简介

1.Stream流的获取

public class StreamDemo { 
   
    public static void main(String[] args) { 
   
        Collection<String> lis=new ArrayList<>();
        Stream<String> listStream=lis.stream();

        Map<String,Integer> map=new HashMap<>();
        /*获取map集合的key的stream流*/
        Stream<String> mapKeyStream=map.keySet().stream();
        /*获取map集合的values的stream流*/
        Stream<Integer> mapValuesStream=map.values().stream();
        /*获取map集合的键和值的Stream流*/
        Stream<Map.Entry<String,Integer>>keyAndValuesStream=map.entrySet().stream();

        /*数组的Stream流的获取方式*/
        String[] names={ 
   "张三","李四","王五"};
        Stream<String> array=Arrays.stream(names);
        Stream<String> arrays=Stream.of(names);
    }
}

2.Stream流常见的API

public class StreamDemo1 { 
   
    /*Stream流的实例*/
    public static void main(String[] args) { 
   
        List<String> list=new ArrayList<>();
        list.add("张十三");
        list.add("张三");
        list.add("李四四");
        list.add("王五");
        list.add("赵六");
        list.add("张无忌");
        list.add("张无忌");

        /*取出姓张的*/
        list.stream().filter(s->s.startsWith("张")).forEach(s-> System.out.println(s));
        System.out.println("------------------------------");

        /*取出长度为3的*/
        list.stream().filter(s->s.length()==3).forEach(s-> System.out.println(s));
        System.out.println("------------------------------");

        /*取出姓张的前两个*/
        list.stream().filter(s->s.startsWith("张")).limit(2).forEach(s-> System.out.println(s));
        System.out.println("------------------------------");

        /*取出姓张的并且跳过前两个*/
        list.stream().filter(s->s.startsWith("张")).skip(2).forEach(s-> System.out.println(s));
        System.out.println("------------------------------");

        /*map加工方法，可以给集合中的每一个元素进行加工*/
        list.stream().map(s->"电视剧："+s).forEach(s-> System.out.println(s));
        System.out.println("------------------------------");

        /*将集合中的员工加工成一个对象(得有这个类)*/
        list.stream().map(s->new Student(s)).forEach(s -> System.out.println(s));
        System.out.println("------------------------------");

        /*当然，如果其中拉姆达前后的参数一样也可以有另一种书写方式*/
        list.stream().map(Student::new).forEach(System.out::println);

        System.out.println("------------------------------");
        /*合并流*/
        Stream<String> s1=list.stream().filter(s->s.startsWith("张"));
        Stream<String> s2=Stream.of("javaSE","javaEE");
        Stream<String> s3=Stream.concat(s1,s2);
        s3.forEach(System.out::println);

        System.out.println("------------------------------");
        /*当然也可以合并不同数据类型的流，只不过新的流要使用Object来接收*/
        Stream<String> ss1=list.stream().filter(s->s.startsWith("张"));
        Stream<Integer>ss2=Stream.of(2022,4,15);
        Stream<Object> ss3=Stream.concat(ss1,ss2);
        ss3.forEach(System.out::println);

        System.out.println("------------------------------");
        /*去除Stream流中重复的元素依靠 hashCode() 和 equals() 方法*/
        list.stream().distinct().forEach(System.out::println);

    }
}

运行结果

3.Stream流案例

Employee类

public class Employee { 
   
    private String name;
    private char sex;
    private double salary;
    private double bonus;
    private String punish; // 处罚信息

    public Employee(){ 
   
    }
    public Employee(String name, char sex, double salary, double bonus, String punish) { 
   
        this.name = name;
        this.sex = sex;
        this.salary = salary;
        this.bonus = bonus;
        this.punish = punish;
    }

    public String getName() { 
   
        return name;
    }

    public void setName(String name) { 
   
        this.name = name;
    }

    public char getSex() { 
   
        return sex;
    }

    public void setSex(char sex) { 
   
        this.sex = sex;
    }

    public double getSalary() { 
   
        return salary;
    }

    public void setSalary(double salary) { 
   
        this.salary = salary;
    }

    public double getBonus() { 
   
        return bonus;
    }

    public void setBonus(double bonus) { 
   
        this.bonus = bonus;
    }

    public String getPunish() { 
   
        return punish;
    }

    public void setPunish(String punish) { 
   
        this.punish = punish;
    }

    public double getTotalSalay(){ 
   
        return salary * 12 + bonus;
    }

    @Override
    public String toString() { 
   
        return "Employee{" +
                "name='" + name + '\'' +
                ", sex=" + sex +
                ", salary=" + salary +
                ", bonus=" + bonus +
                ", punish='" + punish + '\'' +
                '}'+"\n";
    }
}

Topperformer类

public class Topperformer { 
   
    private String name;
    private double money; // 月薪

    public Topperformer() { 
   
    }

    public Topperformer(String name, double money) { 
   
        this.name = name;
        this.money = money;
    }

    public String getName() { 
   
        return name;
    }

    public void setName(String name) { 
   
        this.name = name;
    }

    public double getMoney() { 
   
        return money;
    }

    public void setMoney(double money) { 
   
        this.money = money;
    }

    @Override
    public String toString() { 
   
        return "Topperformer{" +
                "name='" + name + '\'' +
                ", money=" + money +
                '}';
    }
}

主要代码

public class StreamDemo3 { 
   
        public static double allMoney ;
        public static double allMoney2 ; // 2个部门去掉最高工资，最低工资的总和
        public static void main(String[] args) { 
   
            List<Employee> one = new ArrayList<>();//开发部一
            one.add(new Employee("张三",'男',30000 , 25000, null));
            one.add(new Employee("李四",'男',25000 , 1000, "顶撞上司"));
            one.add(new Employee("王五",'男',20000 , 20000, null));
            one.add(new Employee("赵六",'男',20000 , 25000, null));

            List<Employee> two = new ArrayList<>();//开发部二
            two.add(new Employee("田七",'男',15000 , 9000, null));
            two.add(new Employee("李逵",'男',20000 , 10000, null));
            two.add(new Employee("西门庆",'男',50000 , 100000, "被打"));
            two.add(new Employee("潘金莲",'女',3500 , 1000, "被打"));
            two.add(new Employee("武大郎",'女',20000 , 0, "下毒"));

            //开发一部员工最高
            Topperformer t=one.stream().max((e1, e2) -> Double.compare(e1.getSalary() + e1.getBonus(),  e2.getSalary() + e2.getBonus()))
              .map(e->new Topperformer(e.getName(),e.getSalary()+e.getBonus())).get();
            System.out.println(t);
            //开发二部员工最高
            Topperformer t1=two.stream().max((e1, e2) -> Double.compare(e1.getSalary() + e1.getBonus(),  e2.getSalary() + e2.getBonus()))
                    .map(e->new Topperformer(e.getName(),e.getSalary()+e.getBonus())).get();
            System.out.println(t1);
            // 2、统计平均工资，去掉最高工资和最低工资
            one.stream().sorted((e1, e2) -> Double.compare(e1.getSalary() + e1.getBonus(),  e2.getSalary() + e2.getBonus()))
                    .skip(1).limit(one.size() - 2).forEach(e -> { 
   
                        // 求出总和：剩余员工的工资总和
                        allMoney += (e.getSalary() + e.getBonus());
                    });
            System.out.println("开发一部的平均工资是：" + allMoney / (one.size() - 2));
            //开发二部的平均工资
            two.stream().sorted((e1, e2) -> Double.compare(e1.getSalary() + e1.getBonus(),  e2.getSalary() + e2.getBonus()))
                    .skip(1).limit(two.size() - 2).forEach(e -> { 
   
                        // 求出总和：剩余员工的工资总和
                        allMoney += (e.getSalary() + e.getBonus());
                    });
            System.out.println("开发二部的平均工资是：" + allMoney / (two.size() - 2));

            // 3、合并2个集合流，再统计
            Stream<Employee> s1 = one.stream();
            Stream<Employee> s2 = two.stream();
            Stream<Employee> s3 = Stream.concat(s1 , s2);
            s3.sorted((e1, e2) -> Double.compare(e1.getSalary() + e1.getBonus(),  e2.getSalary() + e2.getBonus()))
                    .skip(1).limit(one.size() + two.size() - 2).forEach(e -> { 
   
                        // 求出总和：剩余员工的工资总和
                        allMoney2 += (e.getSalary() + e.getBonus());
                    });

            // BigDecimal
            BigDecimal a = BigDecimal.valueOf(allMoney2);
            BigDecimal b = BigDecimal.valueOf(one.size()  + two.size() - 2);
            System.out.println("开发部的平均工资是：" + a.divide(b,2, RoundingMode.HALF_UP));
    }
}

运行截图

收集Stream流

public class StreamDemo05 { 
   
    public static void main(String[] args) { 
   
        List<String> list = new ArrayList<>();
        list.add("张无忌");
        list.add("周芷若");
        list.add("赵敏");
        list.add("张强");
        list.add("张三丰");
        list.add("张三丰");

        Stream<String> s1 = list.stream().filter(s -> s.startsWith("张"));
        List<String> zhangList = s1.collect(Collectors.toList()); // 可变集合
        zhangList.add("java1");
        System.out.println(zhangList);

        Stream<String> s2 = list.stream().filter(s -> s.startsWith("张"));
        Set<String> zhangSet = s2.collect(Collectors.toSet());
        System.out.println(zhangSet);

        Stream<String> s3 = list.stream().filter(s -> s.startsWith("张"));
        String[] arrs = s3.toArray(String[]::new);
        System.out.println("Arrays数组内容：" + Arrays.toString(arrs));

    }
}

截图