[배운 내용 정리] 1111 자바 국비교육

2020년 11월 11일 9시 ~ 15시 30분 zoom으로 수업 진행

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배열

 같은 자료형의 변수를 하나의 묶음으로 다루는 것

 배열은 저장된 값마다 인덱스 번호가 0부터 시작한다.

 

배열의 선언과 할당

 - 배열 선언

   자료형[] 배열명;                   자료형 배열명[];

   ex) String[] args;                   int num[];

 

 - 배열 할당

   자료형[] 배열명 = new 자료형[배열크기];

   자료형 배열명[] = new 자료형[배열크기];

   ex) int[] nums = new int[3];

 

배열의 저장 구조

 배열은 참조변수로 heap 영역에 할당되며, 배열 공간의 주소를 저장한다,

 배열 공간의 주소를 이용해 인덱스를 참조하는 방식으로 값을 처리한다

 int[] arr = new int[4];

 

배열의 초기화

 - 인덱스를 이용한 초기화

arr[0] = 1; arr[1] = 2;

 - for 문을 이용한 초기화

for(int i=0; i< arr.length; ++i) {    arr[i] = i;  }

배열의 길이를 가져오려면 length를 이용하면 된다

 

 - 선언과 동시에 초기화

int[] arr = {1, 2, 3, 4}; int[] arr = new int[] {1,2,3,4}; String[] fruit = {“사과”, “포도”};

 

 

package com.test01;   public class Main {            public static void main(String[] args) {                      ArrayTest01 arrTest = new ArrayTest01();                      arrTest.test1();                                 }   }

package com.test01;   import java.util.Scanner;   public class ArrayTest01 {              public void test1() {                      /*                      int num1 = 10;                      int num2 = 10;                      int num3 = 10;                      int num4 = 10;                      int num5 = 10;                                           int sum = 0;                                           sum += num1;                      sum += num2;                      sum += num3;                      sum += num4;                      sum += num5;                      */                                           //배열 생성                      int[] arr = new int[5];                                           /*                      arr[0] = 10;                      arr[1] = 20;                      arr[3] = 30;                      arr[4] = 40;                      arr[5] = 50;                      */                                           for(int i=0; i<arr.length; ++i) {                                 arr[i] = i*10;                                 System.out.println(arr[i]);                                                      }                                 }              public void test2() {                      //배열의 선언                      //자료형[] 변수명;                      //자료형 변수명[]; //                   //                   int[] arr; //                   char carr[];                                           //new 연산자는 heap 영역에 공간을 할당하고                      //주소값을 반환하는 연산자                                           //배열을 할당 할 때는 배열의 크기를 지정해주어야한다                      /*                      arr = new int[5];                      carr = new char[10];                      */                                           //배열의 선언과 할당을 동시에!                      int[] iarr = new int[5];                      char carr[] = new char[10];                                           System.out.println("iarr = " + iarr);                      System.out.println("carr = " + carr);                                           System.out.println("iarr의 hashcode : " + iarr.hashCode());                      System.out.println("carr의 hashcode : " + carr.hashCode());                                           System.out.println("iarr의 길이 : " + iarr.length);                      System.out.println("carr의 길이 : " + carr.length);                                           //스캐너를 통해 입력받은 정수로 배열 생성                      Scanner sc = new Scanner(System.in);                      System.out.print("정수 하나 입력 : ");                      int size = sc.nextInt();                      double dArr[] = new double[size];                                           System.out.println("dArr의 hashcode : " + dArr.hashCode());                      System.out.println("dArr의 길이 : " + dArr.length);                                           //배열은 변경할 수 없다, 사이즈를 변경한게 아니라 새로 배열을 만들어 주소값을 덮어씌운것                      dArr = new double[30];                      System.out.println("수정 후 dArr의 hashcode : " + dArr.hashCode());                      System.out.println("수정 후 dArr의 길이 : " + dArr.length);                                           //배열을 비우고싶다면?                      dArr = null;                      //일정시간 지나면 알아서 사라진다. -> 힙 영역에서 old 영역으로 이동하고                      //가비지 컬렉터(GC)가 old 영역에 존재하는 친구를 삭제시킨다                      //System.out.println(dArr.length); -> NullPointerException 에러 뜬다                      //dArr이 가리키는 주소가 없기떄문!                                           //eden에서 생성되고 old 에서 삭제된다.                                 }              public void test3() {                      //heap영역에는 빈 공간이 존재할 수 없어서 기본적으로 초기화 할때 JVM이                      //각 타입별로 지정된 값으로 배열을 초기화 시켜버린다                                           int[] iarr = new int[5];                                           for(int i=0; i<iarr.length; ++i) {                                 System.out.println("iarr[" + i + "]의 값 : " + iarr[i]);                      }                                           int[] iarr2 = new int[] {11, 22, 33, 44, 55};                      int[] iarr3 = {11, 22, 33, 44, 55, 0};                                           for(int i=0; i<iarr2.length; ++i) {                                 System.out.println("iarr2[" + i + "]의 값 : " + iarr2[i]);                      }                                           for(int i=0; i<iarr3.length; ++i) {                                 System.out.println("iarr3[" + i + "]의 값 : " + iarr3[i]);                      }                      System.out.println("iarr3.length : " + iarr3.length);                                 } }

 

HashCode()

 해당 주소와 연결 되어있는 키 값을 반환하는 함수

 자바에서는 배열의 정확한 주소를 알 수 없지만 주소와 연결된 키는 알 수 있다.

 이 키를 통해 데이터에 연결된 주소 값을 찾아가므로 주소 값이라고 생각하면 편하다

 

length 와 length()

 length는 배열의 길이를 알고자 할 때 사용된다.

 length()는 문자열의 길이를 알고자 할 때 사용된다.

 

package com.test01;   public class ArrayTest02 {              public static void main(String[] args) {                      ///////////                      String[] sArr = {"apple", "banana", "orange"};                                           for(int i=0; i<sArr.length; ++i) {                                 System.out.println(sArr[i]);                      }                      //////////                                           //배열에 데이터 입력                      char[] ch = new char[26];                      for(int i=0; i<ch.length; ++i) {                                 ch[i] = (char)('a' + i);                      }                                           for(int i=0; i<ch.length; ++i) {                                 System.out.print(ch[i] + " ");                      }                      System.out.println("----");                      System.out.println("----");                      //1. a~z 일차원 배열 출력                      prn(ch);                      System.out.println("----");                      //2. ch를 거꾸로 출력                      reverse(ch);                      System.out.println("----");                      //3. ch를 대문자로 바꾸어서 출력                      upper(ch);                      System.out.println("----");            }                       public static void prn(char[] ch) {                      //a b c d e f                      //g h i j k l                      //m n o p q r                      //s t u v w x                      //y z                                           for(int i=0; i<ch.length; ++i) {                                 System.out.print(ch[i] + " ");                                 if(i%6 == 5) {                                            System.out.println("");                                 }                      }                      System.out.println("");            }                       public static void reverse(char[] ch) {                      //2. ch를 거꾸로 출력                      for(int i=ch.length-1; i>=0; --i) {                                 System.out.print(ch[i] + " ");                      }                      System.out.println("\n");                      for(int i=ch.length-1; i>=0; --i) {                                 System.out.print(ch[i] + " ");                                 if(  i%6 == 0) {                                            System.out.println("");                                 }                      }                      System.out.println("\n");                      int cnt=1;                      for(int i=ch.length-1; i>=0; --i, cnt++) {                                 System.out.print(ch[i] + " ");                                 if(cnt % 6 == 0) {                                            System.out.println("");                                 }                      }                      System.out.println("");                                           //배열을 하나 더 생성해서 역순으로 데이터 넘겨주기                      int tmp=0;                                           char re[] = new char[26];                      for(int i=ch.length-1; i>=0; --i, tmp++) {                                 re[tmp] = ch[i];                      }                                 }                       public static void upper(char[] ch) {                      for(int i=0; i<ch.length; ++i) {                                 System.out.print(Character.toUpperCase(ch[i]) + " ");                      }                      System.out.println("\n");                                           for(int i=0; i<ch.length; ++i) {                                 System.out.print(Character.toUpperCase(ch[i]) + " ");                                 if(i!=0 && (i+1)%6 == 0) {                                            System.out.println("");                                 }                      }                      System.out.println("\n");                      int cnt=1;                      for(int i=0; i<ch.length; ++i, cnt++) {                                 System.out.print(Character.toUpperCase(ch[i]) + " ");                                 if(cnt % 6 == 0) {                                            System.out.println("");                                 }                               }                      System.out.println("");                                           for(int i=0; i<ch.length; ++i) {                                 ch[i] = (char)((int)ch[i]-32);                      }                                           for(int i=0; i<ch.length; ++i) {                                 System.out.println(ch[i] + " " );                      }            } }

 

 

package com.test01;   public class ArrayTest03 {              public static void main(String[] args) {                   //test1();                     test2();            }                       public static void test1() {                      int num1 = 4, num2 = 5;                                           System.out.println("num1 = " + num1 + " num2 = " + num2);                                           num1 = num2;                      num2 = num1;                                           System.out.println("num1 = " + num1 + " num2 = " + num2);                        int tmp = 0, arr[] = new int[2];                                           arr[0] = 1;                      arr[1] = 2;                                           tmp = arr[0];                      arr[0] = arr[1];                      arr[1] = tmp;                                 }                       public static void test2() {                      //정렬                      int[] arr = {2, 5, 4, 6, 1, 3};                      for(int k = 0; k<arr.length; ++k) {                                 System.out.print(arr[k] + " ");                      }                      System.out.println("\n정렬해요!~");                      for(int i=0; i<arr.length; ++i) {                                 //i인덱스가 증가 할 때마다 처음부터 해당하는 인덱스까지 비교하는 방법                                 for(int j=0; j<i; ++j) {                                            if(arr[i] < arr[j]) {                                                      System.out.println("교환발생");                                                      int tmp;                                                      tmp = arr[i];                                                      arr[i] = arr[j];                                                      arr[j] = tmp;                                            }                                                                            }                      }                      System.out.println("정렬 완료!~");                      for(int k = 0; k<arr.length; ++k) {                                 System.out.print(arr[k] + " ");                      }                                 } }

 

 

package com.test01;   public class ArrayTest04 {              public static void main(String[] args) {                      ArrayTest04 arrTest = new ArrayTest04();                      System.out.println("얕은 복사");                      arrTest.shallowCopy();                      System.out.println("------------------");                      System.out.println("깊은 복사");                      arrTest.deppCopy1();                      System.out.println("------------------");                      System.out.println("arraycopy 복사");                      arrTest.deppCopy2();                      System.out.println("------------------");                      System.out.println("clone 복사");                      arrTest.deppCopy3();            }                       public void shallowCopy() {                      //얕은 복사는 주소만 복사                      int[] originArr = {1, 2, 3, 4, 5};                      int[] copyArr = originArr;                                           //원본 배열 출력                      for(int i=0; i<originArr.length; ++i) {                                 System.out.print(originArr[i]+" ");                      }                      System.out.println("");                      //복사한 배열 출력                      for(int i=0; i<copyArr.length; ++i) {                                 System.out.print(copyArr[i]+" ");                      }                                                    System.out.println("");                                           //원본배열 값 변경                      originArr[0] = 99;                      //복사 배열 값 변경                      copyArr[2] = 22;                                           //원본 배열 출력                      for(int i=0; i<originArr.length; ++i) {                                 System.out.print(originArr[i]+" ");                      }                      System.out.println("");                      //복사한 배열 출력                      for(int i=0; i<copyArr.length; ++i) {                                 System.out.print(copyArr[i]+" ");                      }                                                    System.out.println("");                        //주소 값 확인 -> 둘이 같은 주소가 나온당                      System.out.println("originArr의 hashCode : "+ originArr.hashCode());                      System.out.println("copyArr의 hashCode : "+ copyArr.hashCode());                           }                       public void deppCopy1() {                      //깊은 복사                      int[] originArr = {1, 2, 3, 4, 5};                      int[] copyArr = new int[5];                                           //for문을 이용해 값 복사                      for(int i=0; i<originArr.length; ++i) {                                 copyArr[i] = originArr[i];                      }                                           //원본 배열 출력                      for(int i=0; i<originArr.length; ++i) {                                 System.out.print(originArr[i]+" ");                      }                      System.out.println("");                      //복사한 배열 출력                      for(int i=0; i<copyArr.length; ++i) {                                 System.out.print(copyArr[i]+" ");                      }                                                    System.out.println("");                                           //원본배열 값 변경                      originArr[0] = 99;                      //복사 배열 값 변경                      copyArr[2] = 22;                                           //원본 배열 출력                      for(int i=0; i<originArr.length; ++i) {                                 System.out.print(originArr[i]+" ");                      }                      System.out.println("");                      //복사한 배열 출력                      for(int i=0; i<copyArr.length; ++i) {                                 System.out.print(copyArr[i]+" ");                      }                                                    System.out.println("");                        //주소 값 확인 -> 둘이 같은 주소가 나온당                      System.out.println("originArr의 hashCode : "+ originArr.hashCode());                      System.out.println("copyArr의 hashCode : "+ copyArr.hashCode());                                                }                       public void deppCopy2() {                      //arraycopy()                      int[] originArr = {1, 2, 3, 4, 5};                      int[] copyArr = new int[10];                                           //arraycopy(Object src, int srcPos, Object dest, int destPos, length)                      //src : 원본 배열, srcPos : 복사 시작 위치                      //dest : 복사 배열, destPos : 붙여넣기 시작 위치                      //length : 복사될 길이                                           System.arraycopy(originArr, 0, copyArr, 0, originArr.length);                                           //원본 배열 출력                      for(int i=0; i<originArr.length; ++i) {                                 System.out.print(originArr[i]+" ");                      }                      System.out.println("");                      //복사한 배열 출력                      for(int i=0; i<copyArr.length; ++i) {                                 System.out.print(copyArr[i]+" ");                      }                                                    System.out.println("");            }                       public void deppCopy3() {                      //clone()                      int[] originArr = {1, 2, 3, 4, 5};                      int[] copyArr = new int[10];                                           copyArr = originArr.clone();                                           //원본 배열 출력                      for(int i=0; i<originArr.length; ++i) {                                 System.out.print(originArr[i]+" ");                      }                      System.out.println("");                      //복사한 배열 출력                      for(int i=0; i<copyArr.length; ++i) {                                 System.out.print(copyArr[i]+" ");                      }                                                    System.out.println("");            }              }