Java 8 stream 流获取 list 中最大值、最小值、总和值、平均值-蒲公英云

Java 8 stream 流获取 list 中最大值、最小值、总和值、平均值

你的名字 2022-12-10 10:18 932阅读 0赞

模拟普通对象输出：

// list
static List<Employee> empList = new ArrayList<Employee>();
    private static void initEmp(){ 
        empList.add(new Employee("张三",30));
        empList.add(new Employee("张三1",96));
        empList.add(new Employee("张三2",23));
        empList.add(new Employee("张三3",69));
        empList.add(new Employee("张三4",85));
        empList.add(new Employee("张三5",62));
        empList.add(new Employee("张三6",12));
        empList.add(new Employee("张三7",99));
        empList.add(new Employee("张三8",11));
    }
    //获取员工年龄的最大、最小、总和、平均值
    public static void main(String[] args) { 
        initEmp();
        int sum = empList.stream().mapToInt(Employee->Employee.getAge()).sum();
        int max = empList.stream().mapToInt(Employee->Employee.getAge()).max().getAsInt();
        int min = empList.stream().mapToInt(Employee->Employee.getAge()).min().getAsInt();
        double avg = empList.stream().mapToInt(Employee->Employee.getAge()).average().getAsDouble();
        System.out.println("最大值："+max+"\n最小值："+min+"\n总和："+sum+"\n平均值："+avg);

模拟SQL输出：

import org.testng.collections.Lists;
import org.testng.collections.Maps;
import java.util.List;
import java.util.Map;
/** * @Class: TestStream * @Description: * @Author: Jerry(姜源) * @Create: 2020/9/25 16:14 */
public class TestStream { 
    public static void main(String[] args) { 
        List<Map> list = initData();
        // 取SQL中 number列 的最大值
        int max = list.stream().mapToInt(
                m -> m.get("number") != null ? Integer.parseInt(m.get("number").toString()) : 0
        ).max().getAsInt();
        System.err.println("max = " + max); //22
        // 取SQL中 number列 的最小值
        int min = list.stream().mapToInt(
                m -> m.get("number") != null ? Integer.parseInt(m.get("number").toString()) : 0
        ).min().getAsInt();
        System.err.println("min = " + min); //1
        // 取SQL中 number列 的总和值
        int sum = list.stream().mapToInt(
                m -> m.get("number") != null ? Integer.parseInt(m.get("number").toString()) : 0
        ).sum();
        System.err.println("sum = " + sum); //26
        // 取SQL中 number列 的平均值
        double avg = list.stream().mapToInt(
                m -> m.get("number") != null ? Integer.parseInt(m.get("number").toString()) : 0
        ).average().getAsDouble();
        System.err.println("avg = " + avg); //8.666666666666666
    }
    /** * 模拟 SQL 的输出 * test number * A 1 * B 22 * C 3 */
    public static List<Map> initData() { 
        List<Map> list = Lists.newLinkedList();
        Map map = Maps.newLinkedHashMap();
        map.put("test", "A");
        map.put("number", 1);
        list.add(map);
        Map map1 = Maps.newLinkedHashMap();
        map1.put("test", "B");
        map1.put("number", 22);
        list.add(map1);
        Map map2 = Maps.newLinkedHashMap();
        map2.put("test", "C");
        map2.put("number", 3);
        list.add(map2);
        System.err.println("sql result = " + list);
        return list;
    }
}