Java 流操作

public class JDK8Test {
    @Test
    public void test1() {
        System.err.println("======================集合排序======================");
        List<People> list = getPeopleList();
        list.sort((a,b) -> a.getAge()-b.getAge());
        System.out.println(list);
    }
    @Test
    public void test2() {
        System.err.println("======================数据重构======================");
        String longStr = "152,135,1561,121,131513,545,4566,1321,1321,151515";
        List<Integer> result1 = Arrays.stream(longStr.split(",")).map(s-> Integer.parseInt(s.trim())).collect(Collectors.toList());
        System.out.println(result1);
        List<People> list = getPeopleList();
        String result2 = list.stream().map(p->p.getName()).collect(Collectors.joining(",", "{", "}"));
        System.out.println(result2);
    }
    @Test
    public void test3() {
        System.err.println("======================求最大值======================");
        final DecimalFormat df = new DecimalFormat("0.00");//保留两位小数点
        List<People> list = getPeopleList();
        Optional<People> people = list.stream().max((a,b) -> a.getAge()-b.getAge());
        System.out.println(people.get());

        int max = list.stream().mapToInt(p->p.getAge()).max().getAsInt();
        System.out.println("最大值："+max);
        int min = list.stream().mapToInt(p->p.getAge()).min().getAsInt();
        System.out.println("最小值："+min);
        int sum = list.stream().mapToInt(p->p.getAge()).sum();
        System.out.println("求和值："+sum);
        double ave = list.stream().mapToDouble(p->p.getAge()).average().getAsDouble();
        System.out.println("平均值；"+df.format(ave));
    }
    @Test
    public void test4() {
        System.err.println("======================Map-操作======================");
        List<People> list = getPeopleList();
        List<Integer> collect1 = list.stream().map(People::getId).collect(Collectors.toList());
        List<Integer> collect2 = list.stream().map(a-> a.getId()).collect(Collectors.toList());

        List<Map<String,Object>> collect3 = list.stream().map(p->{
            HashMap<String, Object> map = new HashMap<String,Object>();
            map.put("id", p.getId());
            map.put("age", p.getAge());
            return map;
        }).collect(Collectors.toList());
    
        Map<String, Integer> collect4 = list.stream().collect(Collectors.toMap(People::getName, People::getAge));
        Map<String, Integer> collect5 = list.stream().collect(Collectors.toMap(key->key.getName(), value->{return value.getAge();}));
        System.out.println(collect5);
        
    }



    @Test
    public void test5() {
        System.err.println("======================数据分组======================");
        List<People> list = getPeopleList();
        Map<Integer, List<People>> list1 = list.stream().collect(Collectors.groupingBy(People::getAge));
        Set<Integer> set = list1.keySet();
        set.stream().forEach( s -> System.out.println(list1.get(s).toString()));
    }
    @Test
    public void test6() {
        System.err.println("======================数据过滤======================");
        List<People> list = getPeopleList();
        List<People> list1 = list.stream().filter(a -> a.getAge() == 18).filter(p -> p.getName().equals("小A")).collect(Collectors.toList());
        List<People> list2 = list.stream().filter(a->a.getAge() == 18 && a.getName().equals("小A")).collect(Collectors.toList());
        System.out.println(list1);
    }
    

    private List<People> getPeopleList(){
        List<People> list = new ArrayList<>();
        People p1 = new People(1,"小A",18);
        People p2 = new People(2,"小B",19);
        People p3 = new People(3,"小C",20);
        People p4 = new People(4,"小D",25);
        People p5 = new People(5,"小E",23);
        People p6 = new People(6,"小F",15);
        People p7 = new People(7,"小G",24);
        People p8 = new People(8,"小H",23);
        People p9 = new People(9,"小I",18);
        list = Arrays.asList(p1, p2, p3, p4, p5,p6, p7, p8, p9);
        return list;
    }
}

class People {
    private Integer id;
    private String name;
    private Integer age;
    public People(Integer id, String name, Integer age) {
        this.id = id;this.name = name;this.age = age;
    }
    @Override public String toString() {
        return "\n" + "People{" + "id=" + id + ", name='" + name + '\'' + ", age=" + age + '}' + "\n";
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
}

 

其它参考：https://blog.csdn.net/lu930124/article/details/77595585

reduce()求和：https://www.jb51.net/article/213142.htm