17: JAVA Exception

1. Exception(예외 처리)에 대하여 설명하시오.

• When an exception occurs in Java, JVM prints the exception class and die.
• Try and catch keywords handle exceptions in Java for safer code.

2.예외처리의 기본적인 매커니즘은?

• JVM prints the exception class and return from the main class. So the app dies.

3. 아래의 객체를 예를 들어 설명하시오.

-ClassCastException
-NullPointerException
-ArrayIndexOutOfBoundsException

//
// code
//
class Parent {
    Parent() {
    }
}
class Child extends Parent {
    Child() {
    }
}
class Print {
    Print() {
    }
    private static StringBuilder print = new StringBuilder();
    static void printClassCastExceptionOccur() {
        print.append("ClassCastException occurs.");
        printlnAndReset();
    }
    static void printNullPointerExceptionOccur() {
        print.append("NullPointerException occurs.");
        printlnAndReset();
    }
    static void printArrayIndexOutOfBoundsExceptionOccur() {
        print.append("ArrayIndexOutOfBoundsException occurs.");
        printlnAndReset();
    }
    private static void printlnAndReset() {
        System.out.println(print);
        print.setLength(0);
    }
}
class Exceptions {
    public static void main(String[] args) {
        try {
            Parent parent = new Parent();
            Child childFromParent = (Child)parent;
        } catch(ClassCastException e) {
            Print.printClassCastExceptionOccur();
        }
        try {
            String str = null;
            str.length();
        } catch(NullPointerException e) {
            Print.printNullPointerExceptionOccur();
        }
        try {
            boolean[] booleans = new boolean[0];
            booleans[0] = true;
        } catch(ArrayIndexOutOfBoundsException e) {
            Print.printArrayIndexOutOfBoundsExceptionOccur();
        }
    }
}

//
//print
//
ClassCastException occurs.
NullPointerException occurs.
ArrayIndexOutOfBoundsException occurs.

4. 아래의 결과를 나타내는 사칙연산 계산기를 완성하시오.

//문제
interface ICalculator {
	int add(int x, int y);
	int sub(int x, int y);
	int mul(int x, int y);
	int div(int x, int y);
}
public class CalculatorMain {
//
	public static void main(String[] args) {
		Calculator cal = new Calculator();
		System.out.println(cal.add(6, 5));
		System.out.println(cal.sub(6, 5));
		System.out.println(cal.mul(6, 5));
		System.out.println(cal.div(6, 0));
//
	}
//
}
//========================================
/* 출력값
================================================
11
1
30
java.lang.ArithmeticException: / by zero
                    at Calculator.div(CalculatorMain.java:32)
                    at CalculatorMain.main(CalculatorMain.java:45)
계산기 종료
*/

//
//code
//
class Print {
    Print() {
    }
    private static StringBuilder print = new StringBuilder();
    static void exitCalculator() {
        print.append("계산기 종료");
        System.out.println(print);
        print.setLength(0);
    }
}
class Calculator implements ICalculator {
    Calculator() {
    }
    @Override
    public int add(int x, int y) {
        return (x + y);
    }
    @Override
    public int sub(int x, int y) {
        return (x - y);
    }
    @Override
    public int mul(int x, int y) {
        return (x * y);
    }
    @Override
    public int div(int x, int y) {
        int result = -1;
        try {
            result = (x / y);
        } catch(ArithmeticException e) {
            e.printStackTrace();
            Print.exitCalculator();
            System.exit(0);
        }
        return result;
    }
}

//
//print
//
11
1
30
java.lang.ArithmeticException: / by zero
        at Calculator.div(CalculatorMain.java:34)
        at CalculatorMain.main(CalculatorMain.java:51)
계산기 종료