Confusing Java extend

Question

I am trying to understand the usage of Java extend.... I created a sample testing code to under how it works....

public class Parent {

    public String Msg="Original";

    public String getMsg() {
        return Msg;
    }
    public void setMsg(String msg) {
        Msg = msg;
    }

    public void printing(){
        System.out.println(Msg);    
    }
}

---

public class Child extends Parent{

    public HashMap<String, String> Msg2;

    public Integer Msg3;

    public HashMap<String, String> getMsg2() {
        return Msg2;
    }
    public void setMsg2(HashMap<String, String> msg2) {
        Msg2 = msg2;
    }

    public void printing(){
        System.out.println("1 : " + Msg);
        System.out.println( Msg2 );
    }


    public static void main(String[] args) {

        Child a = new Child();

        System.out.println(a.Msg.getClass());  // able to detect variable from parent

        System.out.println(a.Msg2.getClass()); // Not able to detected, even variable from                  
                               // same instance object child    
        System.out.println(a.Msg3.getClass()); // Not able to detected, even variable from                  
                               // same instance object child


        a.printing();

    }

}

I getting confuse why Msg variable from parent object can detected easy.

While Msg2 and Msg 3 coming from the same instance Child -> a can't recognize it's own variable.

The error message getting from Msg2 or Msg 3 is, Exception in thread "main" java.lang.NullPointerException

Is there anyone able to explain why java behave in such way ?

Thank you....

Answer 1

You have not initialized Msg2 and Msg3. You need to use the new keyword to initialize them, so that they are not null.

add these two statements.

Msg2 = new HashMap<String,String>();

Msg3 = new Integer();

Answer 2

Because Msg has been initialised to a non-null value (the string "Original") but the other fields have not, so they're null and you get an exception trying to call a method (getClass) on a null reference. If you just tried to print out Msg2 rather than Msg2.getClass() then you'd see the value null with no exception.

Answer 3

first set a value to your msg in your child class so its not null by default.

 Int Msg3 = 5;

and display it this way

System.out.println("Here is a msg:" + child.Msg4);