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 10:18 852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;
        }
    }