15: JAVA Override toString()

1. Object 클래스란?

• Default parent class

2. 아래의 소스코드에 대하여 아래와 같이 출력되는 이유는?

출력
//
A@28a418fc
//
==============

class A{
//
}
public class Test {
    public static void main(String[] args) {
        A a = new A();
        System.out.println(a); 
    }
}

//from java.io.PrintStream
    public void println(Object x) {
        String s = String.valueOf(x);
        synchronized (this) {
            print(s);
            newLine();
        }
    }

//from java.lang.String
    public static String valueOf(Object obj) {
        return (obj == null) ? "null" : obj.toString();
    }

//from java.lang.Object
    public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }

• "class Object" is parent class of "class A"
• So "a" went to "println(Object x)" as a parameter Object.
• println(a)
• "println(a)" does "print(s)".
• s = String.valueOf(a)
• a != null
• So "a.toString()".
• "a.toString()" does "return" A@28a418fc

3. class이름 및 함수에서 final의 의미는?

• final class {}: The class does not inherit.
• final method(): The method is not overridden.

4. @Override 에 대하여 설명하시오.

• Annotation to refer the method overrides other same name method in parent class.

5.아래의 결과가 나오도록 프로그래밍 하시오. (개별진척도 1번문제)

	public static void main(String[] args) {
//
		Object obj = new Circle(10);
		System.out.println(obj);
	}
//
//출력: 넓이는 314.134 입니다. 

//
// code
//
class Const {
    Const() {
    }
    static final double PI = 3.14134;
}
class Print {
    private static StringBuilder print = new StringBuilder();
    Print() {
    }
    static void area(double area) {
        print.append("넓이는 ");
        print.append(area);
        print.append(" 입니다.");
    }
    static StringBuilder getPrint() {
        return print;
    }
}
class Circle {
    double radius;
    Circle(int radius) {
        super();
        this.radius = (double)radius;
    }
    Circle(double radius) {
        super();
        this.radius = radius;
    }
    private double getArea() {
        return (Const.PI * Math.pow(this.radius, 2));
    }
    @Override
    public String toString() {
        Print.area(getArea());
        return Print.getPrint().toString();
    }
}
class CircleMain {
    CircleMain() {
    }
	public static void main(String[] args) {
		Object obj = new Circle(10);
		System.out.println(obj);
	}
}

//
// print
//
넓이는 314.134 입니다.