高度抽象的集合数据处理

1. 使用Stream流对List<Integer>进行去重、去偶数，然后查找最大值

1import java.util.Arrays;
2import java.util.Comparator;
3
4public class Stream1 {
5    public static void main(String[] args) {
6        int max = Arrays.asList(1,2,2,5,6,8,9)
7                .stream()
8                //去重
9                .distinct()
10                //去偶
11                .filter(n -> n%2 != 0)
12                //取最大
13                .max(Comparator.comparing(n -> n))
14                .get();
15        System.out.println(max);
16    }
17}
18

2. 使用Stream流对List<Integer>进行排序，然后取第一个值

1import java.util.Arrays;
2import java.util.Comparator;
3
4public class Stream2 {
5    public static void main(String[] args) {
6        int first = Arrays.asList(1,2,2,5,6,8,9)
7                .stream()
8                //排序-升序
9                .sorted(Comparator.comparing(n -> n))
10                //取第一
11                .findFirst()
12                .get();
13        System.out.println(first);
14        
15        int first_reversed = Arrays.asList(1,2,2,5,6,8,9)
16                .stream()
17                //排序-降序
18                .sorted(Comparator.comparing(n -> n,Comparator.reverseOrder()))
19                .findFirst()
20                .get();
21        System.out.println(first_reversed);
22
23    }
24}
25

3. 使用Stream流对List<Integer>进行求平均值、求和

1import java.util.Arrays;
2
3public class Stream3 {
4    public static void main(String[] args) {
5        //求平均值
6        double average = Arrays.asList(1,2,2,5,6,8,9)
7                .stream()
8                //对流中的元素执行一对一的转换
9                .mapToInt(n->n)
10                //返回流中所有元素的平均值
11                .average()
12                .getAsDouble();
13        System.out.println(average);
14        
15        //求和
16        int sum = Arrays.asList(1,2,2,5,6,8,9)
17                .stream()
18                .mapToInt(n->n)
19                .sum();
20        System.out.println(sum);
21    }
22}
23

4. 使用Stream流对List<Person>进行排序，按照age从大到小排序

1import java.util.Arrays;
2import java.util.Comparator;
3import java.util.List;
4import java.util.stream.Collectors;
5
6public class Stream4 {
7    static class Person{
8        int age;
9        String name;
10
11        public Person(int age, String name) {
12            this.age = age;
13            this.name = name;
14        }
15
16        public int getAge() {
17            return age;
18        }
19
20        public void setAge(int age) {
21            this.age = age;
22        }
23
24        public String getName() {
25            return name;
26        }
27
28        public void setName(String name) {
29            this.name = name;
30        }
31
32        @Override
33        public String toString() {
34            return "Person{" +
35                    "age=" + age +
36                    ", name='" + name + '\'' +
37                    '}';
38        }
39    }
40
41
42    static List<Person> list = Arrays.asList(
43            new Person(30,"王五"),
44            new Person(33,"张三"),
45            new Person(27,"李四")
46            );
47    public static void main(String[] args) {
48        System.out.println("排序前："+list);
49        List<Person> personList = list
50                .stream()
51                .sorted(Comparator.comparing(n->n.getAge()))
52                .collect(Collectors.toList());
53        System.out.println("排序后："+personList);
54    }
55}
56