SE450: Object Classes and Static Classes [2015/09/15]

SE450: Basics: Object Classes and Static Classes [27/63]

Let's distinguish two types of classes:

object classes are instantiable -- they are templates for stamping objects
static classes are not instantiable -- they are collections of functions

file:Main1.java [source] [doc-public] [doc-private]

package basics.objectclass;
// A static class
public class Main1 {
  private Main1() {}
  static public void main (String[] args) {
    //stdlib.Trace.graphvizShowSteps (true); stdlib.Trace.run ();
    Circle1 c = new Circle1(1);
    String s = ((c==null) ? "null" : c.toString());
    System.out.println(s);
  }
}
// An object class
final class Circle1 extends Object {
  final int radius;
  public Circle1(int radius) { super(); this.radius = radius; }
  public String toString() { return "Circle(" + radius + ")"; }
}

Java allows some shorthand.

The following declaration is equivalent to the one above.

file:Main2.java [source] [doc-public] [doc-private]

package basics.objectclass;
// A static class
public class Main2 {
  private Main2() {}
  static public void main (String[] args) {
    //stdlib.Trace.graphvizShowSteps (true); stdlib.Trace.run ();
    System.out.println(new Circle2 (1));
  }
}
// An object class
final class Circle2 extends Object {
  int radius;
  public Circle2(int radius) { this.radius = radius; }
  public String toString() { return "Circle(" + radius + ")"; }
}

Important: compilers add/remove local variables as they wish (that are not parameters). Some locals get mapped to addresses on the stack and some to registers.

Do not optimize local variables for performance!