java8 - stream操作

 

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Stream;

import org.junit.Test;

/*
 * 一、 Stream 的操作步骤
 * 
 * 1. 创建 Stream
 * 
 * 2. 中间操作
 * 
 * 3. 终止操作
 */
public class TestStreamAPI1 {
    
    List<Employee> emps = Arrays.asList(
            new Employee(102, "李四", 59, 6666.66),
            new Employee(101, "张三", 18, 9999.99),
            new Employee(103, "王五", 28, 3333.33),
            new Employee(104, "赵六", 8, 7777.77),
            new Employee(104, "赵六", 8, 7777.77),
            new Employee(104, "赵六", 8, 7777.77),
            new Employee(105, "田七", 38, 5555.55)
    );
    
    //2. 中间操作
    /*
        映射
        map——接收 Lambda ， 将元素转换成其他形式或提取信息。接收一个函数作为参数，该函数会被应用到每个元素上，并将其映射成一个新的元素。
        flatMap——接收一个函数作为参数，将流中的每个值都换成另一个流，然后把所有流连接成一个流
     */
    @Test
    public void test1(){
        Stream<String> str = emps.stream()
            .map((e) -> e.getName());
        
        System.out.println("-------------------------------------------");
        
        List<String> strList = Arrays.asList("aaa", "bbb", "ccc", "ddd", "eee");
        
        Stream<String> stream = strList.stream()
               .map(String::toUpperCase);
        
        stream.forEach(System.out::println);
        
        Stream<Stream<Character>> stream2 = strList.stream()
               .map(TestStreamAPI1::filterCharacter);
        
        stream2.forEach((sm) -> {
            sm.forEach(System.out::println);
        });
        
        System.out.println("---------------------------------------------");
        
        Stream<Character> stream3 = strList.stream()
               .flatMap(TestStreamAPI1::filterCharacter);
        
        stream3.forEach(System.out::println);
    }

    public static Stream<Character> filterCharacter(String str){
        List<Character> list = new ArrayList<>();
        
        for (Character ch : str.toCharArray()) {
            list.add(ch);
        }
        
        return list.stream();
    }
    
    /*
        sorted()——自然排序
        sorted(Comparator com)——定制排序
     */
    @Test
    public void test2(){
        emps.stream()
            .map(Employee::getName)
            .sorted()
            .forEach(System.out::println);
        
        System.out.println("------------------------------------");
        
        emps.stream()
            .sorted((x, y) -> {
                if(x.getAge() == y.getAge()){
                    return x.getName().compareTo(y.getName());
                }else{
                    return Integer.compare(x.getAge(), y.getAge());
                }
            }).forEach(System.out::println);
    }
}

 

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Stream;

import org.junit.Test;

import com.atguigu.java8.Employee.Status;

/*
 * 一、 Stream 的操作步骤
 * 
 * 1. 创建 Stream
 * 
 * 2. 中间操作
 * 
 * 3. 终止操作
 */
public class TestStreamAPI2 {
    
    List<Employee> emps = Arrays.asList(
            new Employee(102, "李四", 59, 6666.66, Status.BUSY),
            new Employee(101, "张三", 18, 9999.99, Status.FREE),
            new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
            new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
            new Employee(104, "赵六", 8, 7777.77, Status.FREE),
            new Employee(104, "赵六", 8, 7777.77, Status.FREE),
            new Employee(105, "田七", 38, 5555.55, Status.BUSY)
    );
    
    //3. 终止操作
    /*
        allMatch——检查是否匹配所有元素
        anyMatch——检查是否至少匹配一个元素
        noneMatch——检查是否没有匹配的元素
        findFirst——返回第一个元素
        findAny——返回当前流中的任意元素
        count——返回流中元素的总个数
        max——返回流中最大值
        min——返回流中最小值
     */
    @Test
    public void test1(){
            boolean bl = emps.stream()
                .allMatch((e) -> e.getStatus().equals(Status.BUSY));
            
            System.out.println(bl);
            
            boolean bl1 = emps.stream()
                .anyMatch((e) -> e.getStatus().equals(Status.BUSY));
            
            System.out.println(bl1);
            
            boolean bl2 = emps.stream()
                .noneMatch((e) -> e.getStatus().equals(Status.BUSY));
            
            System.out.println(bl2);
    }
    
    @Test
    public void test2(){
        Optional<Employee> op = emps.stream()
            .sorted((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()))
            .findFirst();
        
        System.out.println(op.get());
        
        System.out.println("--------------------------------");
        
        Optional<Employee> op2 = emps.parallelStream()
            .filter((e) -> e.getStatus().equals(Status.FREE))
            .findAny();
        
        System.out.println(op2.get());
    }
    
    @Test
    public void test3(){
        long count = emps.stream()
                         .filter((e) -> e.getStatus().equals(Status.FREE))
                         .count();
        
        System.out.println(count);
        
        Optional<Double> op = emps.stream()
            .map(Employee::getSalary)
            .max(Double::compare);
        
        System.out.println(op.get());
        
        Optional<Employee> op2 = emps.stream()
            .min((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()));
        
        System.out.println(op2.get());
    }
    
    //注意：流进行了终止操作后，不能再次使用
    @Test
    public void test4(){
        Stream<Employee> stream = emps.stream()
         .filter((e) -> e.getStatus().equals(Status.FREE));
        
        long count = stream.count();
        
        stream.map(Employee::getSalary)
            .max(Double::compare);
    }
}

 

import java.util.Arrays;
import java.util.DoubleSummaryStatistics;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Collectors;

import org.junit.Test;

import com.atguigu.java8.Employee.Status;

public class TestStreamAPI3 {
    
    List<Employee> emps = Arrays.asList(
            new Employee(102, "李四", 79, 6666.66, Status.BUSY),
            new Employee(101, "张三", 18, 9999.99, Status.FREE),
            new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
            new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
            new Employee(104, "赵六", 8, 7777.77, Status.FREE),
            new Employee(104, "赵六", 8, 7777.77, Status.FREE),
            new Employee(105, "田七", 38, 5555.55, Status.BUSY)
    );
    
    //3. 终止操作
    /*
        归约
        reduce(T identity, BinaryOperator) / reduce(BinaryOperator) ——可以将流中元素反复结合起来，得到一个值。
     */
    @Test
    public void test1(){
        List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        
        Integer sum = list.stream()
            .reduce(0, (x, y) -> x + y);
        
        System.out.println(sum);
        
        System.out.println("----------------------------------------");
        
        Optional<Double> op = emps.stream()
            .map(Employee::getSalary)
            .reduce(Double::sum);
        
        System.out.println(op.get());
    }
    
    //需求：搜索名字中 “六” 出现的次数
    @Test
    public void test2(){
        Optional<Integer> sum = emps.stream()
            .map(Employee::getName)
            .flatMap(TestStreamAPI1::filterCharacter)
            .map((ch) -> {
                if(ch.equals('六'))
                    return 1;
                else 
                    return 0;
            }).reduce(Integer::sum);
        
        System.out.println(sum.get());
    }
    
    //collect——将流转换为其他形式。接收一个 Collector接口的实现，用于给Stream中元素做汇总的方法
    @Test
    public void test3(){
        List<String> list = emps.stream()
            .map(Employee::getName)
            .collect(Collectors.toList());
        
        list.forEach(System.out::println);
        
        System.out.println("----------------------------------");
        
        Set<String> set = emps.stream()
            .map(Employee::getName)
            .collect(Collectors.toSet());
        
        set.forEach(System.out::println);

        System.out.println("----------------------------------");
        
        HashSet<String> hs = emps.stream()
            .map(Employee::getName)
            .collect(Collectors.toCollection(HashSet::new));
        
        hs.forEach(System.out::println);
    }
    
    @Test
    public void test4(){
        Optional<Double> max = emps.stream()
            .map(Employee::getSalary)
            .collect(Collectors.maxBy(Double::compare));
        
        System.out.println(max.get());
        
        Optional<Employee> op = emps.stream()
            .collect(Collectors.minBy((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary())));
        
        System.out.println(op.get());
        
        Double sum = emps.stream()
            .collect(Collectors.summingDouble(Employee::getSalary));
        
        System.out.println(sum);
        
        Double avg = emps.stream()
            .collect(Collectors.averagingDouble(Employee::getSalary));
        
        System.out.println(avg);
        
        Long count = emps.stream()
            .collect(Collectors.counting());
        
        System.out.println(count);
        
        System.out.println("--------------------------------------------");
        
        DoubleSummaryStatistics dss = emps.stream()
            .collect(Collectors.summarizingDouble(Employee::getSalary));
        
        System.out.println(dss.getMax());
    }
    
    //分组
    @Test
    public void test5(){
        Map<Status, List<Employee>> map = emps.stream()
            .collect(Collectors.groupingBy(Employee::getStatus));
        
        System.out.println(map);
    }
    
    //多级分组
    @Test
    public void test6(){
        Map<Status, Map<String, List<Employee>>> map = emps.stream()
            .collect(Collectors.groupingBy(Employee::getStatus, Collectors.groupingBy((e) -> {
                if(e.getAge() >= 60)
                    return "老年";
                else if(e.getAge() >= 35)
                    return "中年";
                else
                    return "成年";
            })));
        
        System.out.println(map);
    }
    
    //分区
    @Test
    public void test7(){
        Map<Boolean, List<Employee>> map = emps.stream()
            .collect(Collectors.partitioningBy((e) -> e.getSalary() >= 5000));
        
        System.out.println(map);
    }
    
    //
    @Test
    public void test8(){
        String str = emps.stream()
            .map(Employee::getName)
            .collect(Collectors.joining("," , "----", "----"));
        
        System.out.println(str);
    }
    
    @Test
    public void test9(){
        Optional<Double> sum = emps.stream()
            .map(Employee::getSalary)
            .collect(Collectors.reducing(Double::sum));
        
        System.out.println(sum.get());
    }
}