How do I make a static reference to a method? Sample Code Included

Question

Why doesn't this work? How do I use the method add(int a, int b)?
ERROR - "Can't make static reference to method int add(int, int) in testClass"
interface testInterface{
    static String sString = "TESTING";
    int add(int a, int b);
class testClass implements testInterface{
    public int add(int a, int b){
        return a+b;  
    public static void main(String argv[]){
        int sum = add(3,4);    // here's the error
        System.out.println("test");
        System.out.println( sum );
}

Answer 1

Why doesn't this work?Because you can't call a non-static method like add (which operates on the object called this) from a static method like main (for which there is no this).
There are two ways to fix this:
(1) In main, create a TestClass object, and call add() for that object:
class TestClass implements TestInterface {
   public int add(int a, int b) {
      return a+b;
   public static void main(String[] args) {
      TestClass testObject = new TestClass();
      int sum = testObject.add(3, 4);
      System.out.println("test");
      System.out.println(sum);
}(2) Make add() static. This is the preferred approach, because add() doesn't really need a TestClass object.
class TestClass implements TestInterface {
   public static int add(int a, int b) {
      return a+b;
   // main is same as the original
}