jls05_reference_assignment.java

public class Point { int x, y; }
public class Point3D extends Point { int z; }
public interface Colorable {
	void setColor(int color);
}
public class ColoredPoint extends Point implements Colorable 
{
	int color;
	public void setColor(int color) { this.color = color; }
}
class Test {
	public static void main(String[] args) {
		// Assignments to variables of class type:
		Point p = new Point();
		p = new Point3D();		// ok: because Point3D is a
						// subclass of Point

		Point3D p3d = p;		// error: will require a cast because a 
						// Point might not be a Point3D
						// (even though it is, dynamically,
						// in this example.)

		// Assignments to variables of type Object:
		Object o = p;			// ok: any object to Object
		int[] a = new int[3];
		Object o2 = a;			// ok: an array to Object

		// Assignments to variables of interface type:
		ColoredPoint cp = new ColoredPoint();
		Colorable c = cp;		// ok: ColoredPoint implements
						// Colorable

		// Assignments to variables of array type:
		byte[] b = new byte[4];
		a = b;				// error: these are not arrays
						// of the same primitive type
		Point3D[] p3da = new Point3D[3];
		Point[] pa = p3da;		// ok: since we can assign a
						// Point3D to a Point
		p3da = pa;			// error: (cast needed) since a Point
						// can't be assigned to a Point3D
	}

}