Please Explain

[Thirumalai Muthu]

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;//line1
m4.go();
Mixer m5= m2.m1;
m5.go();//line4
}
void go() { System.out.print("hi");
}
}

Can anyone Please explain the lines from 1 to 4.Thanks

[Robert Elbourn]

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;//line1 - m4 is created using m1 as a parameter (not null)
m4.go(); - this is invoked
Mixer m5= m2.m1; - m5 is created using m2.m1 which is null!!! as m2 was created using a direct call without passing a parameter as m2.m1 returns a null value then m5 is null
m5.go();//line4 - any method on a null returns my favourite exception (nullpointer exception)
}
void go() { System.out.print("hi");
}
}