explain me this code

[anamika henry]

class Mixer{
Mixer(){}
Mixer(Mixer m){m1=m;}
Mixer m1;
public static void main(String[] args){
Mixer m2= new Mixer();
Mixer m3 = new Mixer(m2); m3.go();
Mixer m4= m3.m1; m4.go();
Mixer m5 = m2.m1; m5.go();

}
void go() {System.out.print("hi");}
}

[Abdullah Mamun]

This will throw NullPointerException because of the line m5.go();

May I ask, what actually you want to know?

[anamika henry]

can you explain me the line

Mixer m5 = m2.m1; m5.go();

what is m2.m1 and m5.go(); means

[Abdullah Mamun]

First, lets have a look at the Mixer class. It has one instance variable m1 of type Mixer and go() method.

 Mixer m2= new Mixer(); // line 1
Mixer m3 = new Mixer(m2); m3.go(); // line 2
Mixer m4= m3.m1; m4.go(); // line 3
Mixer m5 = m2.m1; m5.go(); //line 4

In line 4, m2 is a mixer object created at line 1 and that time m1 is initialized with null. Since m1 is also a Mixer, it can be assigned in m5 but as m1 is null m5 is also null. So, when you are calling the go() method on a null object(m5) JVM is giving a NullPointerException.

Hope, I could make you understand...

[anamika henry]

thank you moon, i could understand now

[tirapa reddy]

hi
m1 is assigned with m how it becomes null ?

[David Grant]

m1 is instantiated using the no-arg constructor, Mixer(), which leaves the m instance variable unchanged from its initial value of null.

[Niranjan Deshpande]

yes it will throw a null pointer exception..