Java8新特性之Stream API详解

一、前言

streamapi在java8版本中使用，关注的是对数据的筛选、查找、存储等

它可以做的事情有：过滤、排序、映射、归约

二、使用流程

stream实例化中间操作（过滤、排序、映射、规约）终止操作（匹配查找、归约、收集）

三、案例演示

public class employeedata {
   
   public static list<employee> getemployees(){
      list<employee> list = new arraylist<>();
      
      list.add(new employee(1001, "马化腾", 34, 6000.38));
      list.add(new employee(1002, "马云", 12, 9876.12));
      list.add(new employee(1003, "刘强东", 33, 3000.82));
      list.add(new employee(1004, "雷军", 26, 7657.37));
      list.add(new employee(1005, "李彦宏", 65, 5555.32));
      list.add(new employee(1006, "比尔盖茨", 42, 9500.43));
      list.add(new employee(1007, "任正非", 26, 4333.32));
      list.add(new employee(1008, "扎克伯格", 35, 2500.32));
      
      return list;
   }
   
}

package jdk_8;

import org.junit.test;

import java.util.arrays;
import java.util.list;
import java.util.optional;
import java.util.stream.collectors;
import java.util.stream.stream;

public class streamapi {
    // 方式1：使用集合创建stream
    @test
    public void test1() {
        list<employee> list = employeedata.getemployees();
        // 返回一个顺序流
        stream<employee> stream = list.stream();
        // 返回一个并行流
        stream<employee> employeestream = list.parallelstream();
    }

    // 方式2：使用数组创建流对象
    @test
    public void test2() {
        employee e1 = new employee(1003, "wzy", 26, 3000.69);
        employee e2 = new employee(1007, "王紫玉", 25, 8888);

        employee[] list2 = new employee[]{e1, e2};
        stream<employee> stream = arrays.stream(list2);
        stream.foreach(system.out::println);

    }

    // 通过stream.of 创建
    @test
    public void test3() {
        stream<integer> stream = stream.of(1, 2, 3, 4, 5, 6);
        stream.foreach(system.out::println);
    }

    // stream.filter进行过滤
    @test
    public void test4() {
        list<employee> list = employeedata.getemployees();
        list.stream().filter(employee -> employee.getage() > 25).foreach(system.out::println);
    }

    // 使用streamapi筛选
    @test
    public void test5() {
        list<employee> list = employeedata.getemployees();
        list.add(new employee(1004, "wzy", 25, 18888));
        list.add(new employee(1004, "wzy", 25, 18888));
        list.add(new employee(1004, "wzy", 25, 18888));
        list.add(new employee(1004, "wzy", 25, 18888));

        list.stream().foreach(system.out::println);
        system.out.println();
        list.stream().distinct().foreach(system.out::println);
    }

    @test
    public void test6() {
        list<string> list = arrays.aslist("a", "b", "c");
//        map(function<? super t,? extends r> mapper)
//        返回由给定函数应用于此流的元素的结果组成的流。
        list.stream().map(s -> s.touppercase()).foreach(system.out::println);

    }

    // 使用streamapi筛选：工资大于6000的员工
    @test
    public void test7() {
        list<employee> list = employeedata.getemployees();
        list.stream().filter(s -> s.getsalary() > 6000).foreach(system.out::println);
    }

    // streamapi映射：map,接收参数，将参数转换为其他形式的信息；
    @test
    public void test8() {
        list<employee> list = employeedata.getemployees();
        list.stream().map(e -> e.getage()).foreach(system.out::println);

    }

    // streamapi排序,sorted帮助实现comparable接口进行对象比较
    @test
    public void test9() {
        list<employee> list = employeedata.getemployees();
        list.stream().sorted((e1, e2) -> {
            int agevalue = integer.compare(e1.getage(), e2.getage());
            if (agevalue != 0) {
                return agevalue;
            } else {
                return double.compare(e1.getsalary(), e2.getsalary());
            }
        }).foreach(system.out::println);
    }

    // streamapi匹配:所有元素是否满足下列条件
    @test
    public void test10() {
        list<employee> list = employeedata.getemployees();
        boolean allmatch = list.stream().allmatch(e -> e.getsalary() > 5000);
        system.out.println(allmatch);
    }

    // streamapi查找：返回第一个元素
    @test
    public void test11() {
        list<employee> list = employeedata.getemployees();
        optional<employee> first = list.stream().findfirst();
        system.out.println(first);
    }

    // streamapi查找：返回薪水数最大的员工
    @test
    public void test12() {
        list<employee> list = employeedata.getemployees();
        stream<double> salarystream = list.stream().map(e -> e.getsalary());
        optional<double> max = salarystream.max(double::compare);
        system.out.println(max);
    }

    // streamapi查找：薪水最小的员工对像
    @test
    public void test13() {
        list<employee> list = employeedata.getemployees();
        optional<employee> min = list.stream().min((e1, e2) -> double.compare(e1.getsalary(), e2.getsalary()));
        system.out.println(min);
    }

    // streamapi:归约
    @test
    public void test14() {
        list<employee> list = employeedata.getemployees();
        stream<double> salarystream = list.stream().map(e -> e.getsalary());
        optional<double> sum = salarystream.reduce((s1, s2) -> s1 + s2);
        system.out.println(sum.get());
    }

    // streamapi：收集
    @test
    public void test15(){
        list<employee> list = employeedata.getemployees();
        list<employee> employeelist = list.stream().filter(e -> e.getsalary() > 5000).collect(collectors.tolist());
        for (employee employee : employeelist) {
            system.out.println(employee);
        }
    }
}

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