2021 Java Programming I

//pass value type by value (값형식을 값에 의한 전달) – copy of value 전달

static void square1(int x) {
    x *= x;
    System.out.printf("The value inside square1: %d\n", x);
}

public static void main(String[] args) {
    int i = 5; 
    System.out.println("Before i=" + i);
    square1(i);
    System.out.println("After i=" + i + "\n\n");
}

//Before i=5
//The value inside square1: 25
//After i=5

//pass reference type by value (참조형식을 값에 의한 전달) – copy of reference 전달

static void square2(IntValue value) {
    value.x *= value.x;
    System.out.printf("The value inside square2: %d\n", value.x);
}

public static void main(String[] args) {
    int i = 5;
    IntValue value = new IntValue(i);
    System.out.println("Before value.x=" + value.x);
    square2(value);
    System.out.println("After value.x=" + value.x + "\n\n");
}

//Before value.x=5
//The value inside square2: 25
//After value.x=25

static void changeArray1(int[] arr) {
    arr[0]=888; // arr -> myArray이므로 원본 배열의 첫번째 값은 888로 변경
    System.out.println("changeArray1 arr=" + Arrays.stream(arr).mapToObj(String::valueOf).collect(Collectors.joining(",")));
    arr = new int[] {-3, -1, -2, -3, -4}; // local 변수로 새롭게 할당하여 지정 그러나 원본 배열 변경 안됨
    System.out.println("changeArray1 arr=" + Arrays.stream(arr).mapToObj(String::valueOf).collect(Collectors.joining(",")));
}

public static void main(String[] args) {
    int[] myArray = {1, 4, 5};
    System.out.println("Before myArray=" + Arrays.stream(myArray).mapToObj(String::valueOf).collect(Collectors.joining(",")));
    changeArray1(myArray);
    System.out.println("After changeArray1 myArray=" + Arrays.stream(myArray).mapToObj(String::valueOf).collect(Collectors.joining(",")));
}

//Before myArray=1, 4, 5
//changeArray1 arr=888, 4, 5
//changeArray1 arr=-3, -1, -2, -3, -4
//After myArray=888, 4, 5

static void changeArray2(IntValue[] arr) {
    arr[0].x=888; // 원본 배열의 첫번째 값은 888로 변경
    System.out.println("changeArray2 arr=" + Arrays.stream(arr).map(Object::toString).collect(Collectors.joining(",")));
    arr = new IntValue[] { new IntValue(-3), new IntValue(-1), new IntValue(-2), new IntValue(-3), new IntValue(-4)};  // local 변수로 새롭게 할당하여 지정 그러나 원본 배열 변경 안됨
    System.out.println("changeArray2 arr=" + Arrays.stream(arr).map(Object::toString).collect(Collectors.joining(",")));
}

public static void main(String[] args) {
    IntValue[] myArray2 = { new IntValue(1), new IntValue(4), new IntValue(5)};
    System.out.println("Before myArray2=" + Arrays.stream(myArray2).map(Object::toString).collect(Collectors.joining(",")));
    changeArray2(myArray2);
    System.out.println("After myArray2=" + Arrays.stream(myArray2).map(Object::toString).collect(Collectors.joining(",")));
}

// Before myArray2=1, 4, 5
// changeArray2 arr=888, 4, 5
// changeArray2 arr=-3, -1, -2, -3, -4
// After myArray2=888,4,5

// returning an array from a method vs passing an array as parameter to a method

// passing an array as parameter to a method
public static int sumArray(int[] arr) {
	int sum = 0;
	for (int i = 0; i < arr.length; i++) {
		sum += arr[i];		
	}
	return sum;
}

// returning an array from a method
public static int[] assignArray(int length) {
	int[] arr = new int[length];
	for (int i = 0; i < arr.length; i++) {
		arr[i] = i;
	}
	return arr;
}

////////////////////////////////////// in main
// returning an array from a method
int[] arr1 = assignArray(3);
int[] arr2 = assignArray(5);
for (int e : arr1)	System.out.println(e); // 0 1 2
for (int e : arr2)	System.out.println(e); // 0 1 2 3 4

// passing an array as parameter to a method
int[] array1 = {100, 200, 300};
int[] array2 = {50, 60, 70, 80, 90};
int sum1 = sumArray(array1);
int sum2 = sumArray(array2);
System.out.println("array1 sum=" + sum1); // 100 + 200 + 300 = 600
System.out.println("array2 sum=" + sum2); // 50 + 60 + 70 + 80 + 90 = 350

TripleCalculator – TripleCalculator calculates MIN/MAX/MEDIAN/MEAN/SUM of 3 numbers.

PersonTest-Gender – Person has name, age, gender

EnumTest – Weekday

// break a loop if 'q'-key is pressed
static Scanner scan = new Scanner(System.in);
public static boolean getUserInputExitKey() {
    System.out.println("Please enter key to continue or the q-key to exit.");
    String input = scan.nextLine();
    if (input == null || input.contentEquals("")) return false;
    if (input.contentEquals("q")) return true;
    else return false;
}

//////////////////////////////////////////////////////////////////////////////////////////////

do {

// do whatever..

} while(!getUserInputExitKey());

// 2D array
char[] ops = {'+', '-', '*', '/'};
int[][] numbers = {{60,50}, {40,30}, {20,10}};
for (char op : ops) {
    for (int i = 0; i < numbers.length; i++) {
        int result = calc(numbers[i][0], numbers[i][1], op);
        System.out.printf("result = %d %s %d = %d\n", numbers[i][0], op, numbers[i][1], result);
    }
}

result = 60 + 50 = 110
result = 40 + 30 = 70
result = 20 + 10 = 30
result = 60 – 50 = 10
result = 40 – 30 = 10
result = 20 – 10 = 10
result = 60 * 50 = 3000
result = 40 * 30 = 1200
result = 20 * 10 = 200
result = 60 / 50 = 1
result = 40 / 30 = 1
result = 20 / 10 = 2

Person[] pArray = new Person[5];
// 만약 Person 객체를 하나만 생성한 후 for문에서 공유해 사용할 경우
// 마지막으로 입력된 데이터로 모든 데이터값이 치환됨
Person p = new Person();
for (int i = 0; i < 5; i++) {
    System.out.print("\n\nEnter Person name : ");
    Scanner input = new Scanner(System.in);
    p.setName(input.nextLine()); // 입력정보
    System.out.print("\n\nEnter Person age [int] : ");
    p.setAge(input.nextInt()); // 입력정보
    p.print();
    pArray[i] = p; // 리스트에 들어간 모든 원소는 동일한 p
}
System.out.println("pArray : " + Arrays.toString(pArray));

Person[] pArray = new Person[5];
// 아래와 같이 for문 안에 Person p = new Person()와같이
// 새로운 객체를 생성해야 각자 다르게 입력된 정보가 들어가게 됨
for (int i = 0; i < 5; i++) {
    Person p = new Person();
    System.out.print("\n\nEnter Person name : ");
    Scanner input = new Scanner(System.in);
    p.setName(input.nextLine()); // 입력정보
    System.out.print("\n\nEnter Person age [int] : ");
    p.setAge(input.nextInt()); // 입력정보
    p.print();
    pArray[i] = p; // 이때 p는 새로운 Person객체
}
System.out.println("pArray2 : " + Arrays.toString(pArray2));

int[] integerArray = new int[3];
integerArray[0] = 1;
integerArray[1] = 2;
integerArray[2] = 3;

for (int i : integerArray) 
    System.out.println(i);

for (int j = 0; j < integerArray.length; j++) 
    System.out.println(integerArray[ j ]);

int k = 0;
while (k < integerArray.length) 
    System.out.println(integerArray[k++]);

// int array
// 만약 int value1를 for문에서 공유해 사용한다해도 
// intArray1[i]에 값이 들어가므로 각각 다른 입력값이 들어가게 됨
int value1 = 0;
int[] intArray1 = new int[3];
for (int i = 0; i < intArray1.length; i++) {
	System.out.print("Please enter int number: ");
	value1 = scan.nextInt(); // 10 20 30
	intArray1[i] = value1;
}
for (int v : intArray1)	System.out.println(v); // 10 20 30

// int value2를 for문에서 내부에서 매번 생성해서 사용한다해도 
// intArray2[i]에 값이 들어가므로 각각 다른 입력값이 들어가게 됨
int[] intArray2 = new int[3];
for (int i = 0; i < intArray1.length; i++) {
	System.out.print("Please enter int number: ");
	int value2 = scan.nextInt(); // 100 200 300
	intArray2[i] = value2;
}
for (int v : intArray2)	System.out.println(v); // 100 200 300

import java.util.Scanner;

// for, foreach, if/else-if, switch, array, 2D array
public class BasicCalculation {

    public static Scanner scan = new Scanner(System.in);

    // calc (using if/else-if)
    public static int calc(int x, int y, char op) {
        if (op == '+') return x + y;
        else if (op == '-') return x - y;
        else if (op == '*') return x * y;
        else if (op == '/') return x / y;
        else {
            System.out.println("op not found");
            return 0;
        }
    }

    // calc2 (using switch)
    public static int calc2(int x, int y, char op) {
        switch(op) {
            case '+': return x + y;
            case '-': return x - y;
            case '*': return x * y;
            case '/': return x / y;
            default:
                System.out.println("op not found");
                return 0;
        }
    }

    // get user input op[+,-,*,/]
    static public char getUserInputOp() {
        char value = '\0';
        do {
            System.out.printf("Please enter the operator [+,-,*,/]: ");
            try {
                value = scan.next().charAt(0);
                System.out.println("value=" + value);
            }
            catch (Exception e) {
                System.out.printf("Error! Please re-enter!\n");
                scan.next();
                continue;
            }
        } while (value != '+' && value != '-' && value != '*' && value != '/');
        return value;
    }

    public static void main(String[] args) {
        // for
        for (int i = 0; i < 5; i++) {
            System.out.println("i=" + i);
        }
        // while
        int j = 0;
        while (j < 5) {
            System.out.println("j=" + j);
            j++;
        }
        // do-while
        int k = 0;
        do {
            System.out.println("k=" + k);
            k++;
        } while (k < 5);
        // break (0~4)
        int x = 0;
        while (true) {
            if (x >= 5) break;
            System.out.println("x=" + x);
            x++;
        }
        // continue (0~4)
        int y = 0;
        while (y < 5) {
            y++; // (1~5)
            if (y % 2 == 1) // 홀수
                continue;
            System.out.println("y=" + y); // 짝수만 출력
        }
        // for-each
        System.out.print("Please enter two numbers: ");
        x = scan.nextInt();
        y = scan.nextInt();
        char[] ops = {'+', '-', '*', '/', '^'}; // char array
        for (char op : ops) {
            int z = calc(x, y, op);
            System.out.printf("z = %d %s %d = %d\n", x, op, y, z);
        }
        // 2D array
        int[][] numbers = {{49, 57}, {36, 29}, {88, 66}};
        for (char op: ops) {
            for (int i = 0; i < numbers.length; i++) {
               int r = calc2(numbers[i][0], numbers[i][1], op);
               System.out.printf("r = %d %c %d = %d\n", 
                       numbers[i][0], op, numbers[i][1], r);
            }
        }
        // user input
        System.out.print("Please enter two numbers: ");
        x = scan.nextInt();
        y = scan.nextInt();
        char op = getUserInputOp();
        int w = calc2(x, y, op);
        System.out.printf("w = %d %s %d = %d\n", x, op, y, w)
    }
}