Use of abstract interface

Question

I have a series of questions....... Go through.
What is the use of abstract interface in java ?
What is the use of static object in java ?
What is the use of abstract key word to variables ?
Object reference passed to method are final. Can ' t we change it ?

Answer 1

class Testing {
     int i;
public class RefFinal {
   public static void f ( Testing t1, Testing t2) {
       Testing temp;
       temp=t1;
       t1=t2;
       t2=temp;
       System.out.println("value of i in t1 =" +t1.i);
       System.out.println("value of i in t2 =" +t2.i);
   public static void main( String args[]) {
      Testing t1 = new Testing();    
      Testing t2 = new Testing();    
      t1.i=10;
      t2.i=20;
      f(t1,t2); 
      System.out.println("value of i in t1 =" +t1.i);
      System.out.println("value of i in t2 =" +t2.i);
}      Here the code explain why?