selection does not contain a main type的问题

selection does not contain a main type的问题import java.util.Scanner;public class Population {private String name;private int population;private double growthRate;public void readInput(){Scanner keyboard = new Scanner(System.in);System.out.println("what is the species' name ");name = keyboard.nextLine();System.out.print("what is the popution of lll");population = keyboard.nextInt();while (population < 0);{System.out.println("population can not be negatived");System.out.print("Reenter population:");population = keyboard.nextInt();}System.out.println("Enter growth rate:");growthRate = keyboard.nextDouble();}public void writeOutput(){System.out.println("Name = " + name);System.out.println("Population = " + population);System.out.println("Goth rate = " + growthRate + "%");}}Java文件也在src下面,为什么就是不能运行呢?写的其他程序就能,怎么解决啊,不要找什么更改目录的回答了