About Inheritance and Overriding Java

Question

Why the "a" variable is null in "user.view" when i called "tulis" method from different object in program below ? Please help me :)

import java.io.*;
import java.util.InputMismatchException;

class kelas1 {

public String a;

static BufferedReader input = new BufferedReader (new InputStreamReader(System.in));

public void tulis () throws IOException{

a = "9";

}

}

class kelas2 extends kelas1 {

@Override
public void tulis () throws IOException{

a = "10";

}

}

public class tr extends kelas2 {

public void view (){

System.out.println(a);

}

public static void main (String [] args) throws Exception{

tr user = new tr ();
kelas1 obj = new kelas1 ();
kelas2 obj2 = new kelas2 ();

String in = input.readLine ();

switch (in){

case "1" :

obj.tulis();
user.view ();

break;

case "2" :

obj2.tulis();
user.view ();

break;
}
}
}

Answer 1

inheritance is a mechanism that allows a new class to be created from an existing class