13-2: JAVA polymorphism

1. is a 관계와 has a 관계란?

• IS-A relationship: When child class is a kind of parent class, then the child class IS-A parent class.
• HAS-A relationship: When A class has an object of another class, then the class HAS-A another class.

2.다형성(polymorphism)이란 무엇인가?

• The multiple results from the inheritance between parent class and child class.
• The mutual memory address of parent class and child class makes the results.

3. 아래가 되지 않는 이유에 대하여 메모리 그림으로 설명하시오.

//MobilePhone(부모) SmartPhone(자식)
//
SmartPhone ph2 = new MobilePhone();

4.코딩 연습 문제:

- 2차원 배열 4 * 3 을 선언하고, 각각의 방을 랜덤으로 값을 넣은뒤 모두 출력하시오.

//
// 코드
//
class ArrRandom {
    private static final int INDEX_FIRST = 4;
    private static final int INDEX_SECOND = 3;
//    
    private static double random() {
        return Math.random();
    }
    public static void main(String[] args) {
        double[][] arrRandom = new double[INDEX_FIRST][INDEX_SECOND];
//        
        StringBuilder print = new StringBuilder();
//        
        for (int i = 0; i < INDEX_FIRST; i++) {
            for (int j = 0; j < INDEX_SECOND; j++) {
                arrRandom[i][j] = random();
//                
                print.append(arrRandom[i][j]);
                print.append(" ");
            };
            print.append("\n");
        };
        System.out.print(print);
    }
}

//
// print
//
0.8241809097221745 0.8345717176081866 0.4344396833101133
0.10558151266515803 0.3667412400610325 0.7009359343275011
0.6384126867871407 0.7514006782698701 0.4817299032014979
0.798995958701251 0.18613992971104754 0.8878757634798718