[자바초보] 조건문 관련 너무 이해가 안되요...

class Ch5Circle {
 
	public static final int INVALID_DIMENSION = 666;
 
	private double radius;
 
	public Ch5Circle(double r) {
		setRadius(r);
	}
 
	public double getArea() {
		double result = INVALID_DIMENSION;
 
		if (isRadiusValid()) {
			result = Math.PI*radius*radius;
		}
		return result;
	}
 
	public double getCircumference() {
 
		double result = INVALID_DIMENSION;
 
		if (isRadiusValid()) {
 
			result = 2.0*Math.PI*radius;
		}
 
		return result;
	}
 
	public double getDiameter() {
 
		double diameter = INVALID_DIMENSION;
 
		if (isRadiusValid()) {
 
			diameter = 2.0*radius;
		}
		return diameter;
	}
 
	public double getRadius() {
 
		return radius;
	}
 
	public void setDiameter(double d) {
 
		if ( d > 0 ) {
			setRadius(d/2.0);
		} else {
			setRadius(INVALID_DIMENSION);
		}
	}
 
	public void setRadius(double r) {
 
		if (r>0) {
			radius = r;
		} else {
			radius = INVALID_DIMENSION;
		}
	}
 
	private boolean isRadiusValid() {
		return radius != INVALID_DIMENSION;
	}
}