Java – why must variables be initialized?

Here, I try to get the output month by entering the number of months, but why do I make a mistake

Why must monthstring be initialized?

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);       }

 }

Solution

What if the monthnumber is not between 1 and 12? In this case, monthstring will not be initialized You should give it some default values when declaring:

String monthString = null; // or ""