Why doesn't the method swap1 swap the radiuses even though the reference of the objects were passed? If it doesn't work out why is swap1 not a compile error?

 public class Test { public static void main(String[] args) { Circle circle1 = new Circle(1); Circle circle2 = new Circle(2); swap1(circle1, circle2); System.out.println("After swap1: circle1 = " + circle1.radius + " circle2 = " + circle2.radius); swap2(circle1, circle2); System.out.println("After swap2: circle1 = " + circle1.radius + " circle2 = " + circle2.radius); } public static void swap1(Circle x, Circle y) { Circle temp = x; x = y; y = temp; } public static void swap2(Circle x, Circle y) { double temp = x.radius; x.radius = y.radius; y.radius = temp; } } class Circle { double radius; Circle(double newRadius) { radius = newRadius; } } 

After swap1: circle1 = 1.0 circle2 = 2.0 After swap2: circle1 = 2.0 circle2 = 1.0