default access modifier class..

[Tausif Khanooni]

Hi,
I have a small doubt. Why java compiler allows a class to have public method while class itself is a default access modifier.
Thnks in advance!!
[ February 16, 2003: Message edited by: Tausif Khanooni ]

[Archana Annamaneni]

I hope that is to give the access to other classes in the same package
Archana

[Dan Culache]

8.2.1.4 Accessing Members of Inaccessible Classes
Even though a class might not be declared public, instances of the class might be available at run time to code outside the package in which it is declared if it has a public superclass or superinterface. An instance of the class can be assigned to a variable of such a public type. An invocation of a public method of the object referred to by such a variable may invoke a method of the class if it implements or overrides a method of the public superclass or superinterface. (In this situation, the method is necessarily declared public, even though it is declared in a class that is not public.)
Consider the compilation unit:

package points;
public class Point {
public int x, y;
public void move(int dx, int dy) {
x += dx; y += dy;
}
}
//and another compilation unit of another package:
package morePoints;
class Point3d extends points.Point {
public int z;
public void move(int dx, int dy, int dz) {
super.move(dx, dy); z += dz;
}
public void move(int dx, int dy) {
move(dx, dy, 0);
}
}
public class OnePoint {
public static points.Point getOne() { 
return new Point3d(); 
}
}

An invocation morePoints.OnePoint.getOne() in yet a third package would return a Point3d that can be used as a Point, even though the type Point3d is not available outside the package morePoints. The two argument version of method move could then be invoked for that object, which is permissible because method move of Point3d is public (as it must be, for any method that overrides a public method must itself be public, precisely so that situations such as this will work out correctly). The fields x and y of that object could also be accessed from such a third package.
While the field z of class Point3d is public, it is not possible to access this field from code outside the package morePoints, given only a reference to an instance of class Point3d in a variable p of type Point. This is because the expression p.z is not correct, as p has type Point and class Point has no field named z; also, the expression ((Point3d)p).z is not correct, because the class type Point3d cannot be referred to outside package morePoints.
The declaration of the field z as public is not useless, however. If there were to be, in package morePoints, a public subclass Point4d of the class Point3d:

package morePoints;
public class Point4d extends Point3d {
public int w;
public void move(int dx, int dy, int dz, int dw) {
super.move(dx, dy, dz); w += dw;
}
}

then class Point4d would inherit the field z, which, being public, could then be accessed by code in packages other than morePoints, through variables and expressions of the public type Point4d.
[ February 17, 2003: Message edited by: Dan Culache ]