홍대 자바 수업 핵심:char [] arr=str.toCharArray(); String ->char [] 로

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import java.io.*; import java.util.*; class WrapperTest {     public static void main(String[] args)     {         /*Integer num01 =new Integer(10);         Integer num02 =new Integer("20");                   int n01=num01.intValue(); //기본형 데이터로 강제적 변환         int n02=num02.intValue();           int sum=n01+n02;         System.out.println(sum);                   System.out.println(Integer.toBinaryString(sum));         System.out.println(Integer.toOctalString(sum));         System.out.println(Integer.toHexString(sum));//다 문자열로 반환 시켜주는 메소드                   /*2번 int n01=10 ,n02;         Integer num01;         Integer num02 =new Integer(20); //객체생성         num01=n01; //오토 박싱         n02=num02;//오토 언박싱         System.out.println(num01+num02);         System.out.println(n01+n02);*/         /* 향상된 포문         char [] data ={'A','a','4',' ','#'};         for (char i :data)         {             if (Character.isUpperCase(i))             {                 System.out.println(i+"은 대문자입니다");             }             else if (Character.isLowerCase(i))               {                 System.out.println(i+"은 소문자입니다");             }             else if (Character.isSpace(i))               {                 System.out.println(i+"은 공백");             }             else if (Character.isDigit(i))               {                 System.out.println(i+"은 숫자입니다");             }              */           Scanner scan =new Scanner(System.in);               String str =scan.next();         char [] arr=str.toCharArray();         kk(arr);   }         public static  void kk(char [] arr){     for (char i:arr)         {             if (Character.isDigit(i))             {                                   System.out.print(i);                               }             else             {                 System.out.println("숫자만 입력해주세요 ");                                   break;             }         }     } }