Question regarding java inheritance

As I know of, we need to declare an object first to use a method from its parent class.

However, I just noticed that just using method without any declaration of an object wouldn't cause an error

So here's a sample code below

//Parent class

package transport;

public class Vehicle

{

private int nWheels;

private double xPosition, yPosition;

public Vehicle(int nWheels)

{

this.nWheels = nWheels;

}

public void setPosition(double xPosition, double yPosition)

{

this.xPosition = xPosition;

this.yPosition = yPosition;

}

public double getXPosition()

{

return xPosition;

}

public double getYPosition()

{

return yPosition;

}

public void changePositionBy(double xDelta, double yDelta)

{

xPosition += xDelta;

yPosition += yDelta;

}

public static void main(String[] args)

{

MarsRover mr = new MarsRover();

}

}

---------------------------------

As my understading, we need to do like below

childClass child = new childClass();

childClass.setPosition(0, 0);

childClass.changePositionBy(x, y);

but I just noticed that below won't cause an error

setPisition(0, 0);

changePositionBy(x, y);

would both ways work as the same?

if so, is there any reason why we need to declare an object?

I mean, is it somehow better coding?