java – 为什么变量必须初始化？

为什么monthString必须初始化？

import java.util.Scanner;
public class SwitchClass {

public static void main(String[]args)
{
   Scanner input = new Scanner(System.in);
   System.out.printf(" when did u born ? ");
   int monthNumber = input.nextInt();
   String monthString ;

switch (monthNumber)
{
    case 1:
    monthString = "January ";
        break;
    case 2:
        monthString = "February ";
        break;
    case 3:
        monthString = "March ";
        break;
    case 4:
        monthString = "April ";
        break;
    case  5:
        monthString = "May";
        break;
    case  6:
        monthString = "June";
        break;
    case  7:
        monthString = "July";
        break;
    case  8:
        monthString = "August";
        break;
    case  9:
        monthString = "September";
        break;
    case  10:
        monthString = "October";
        break;
    case  11:
        monthString = "November";
        break;
    case  12:
        monthString = "December";
        break;
   }
System.out.println(monthString);       }

 }