java - Why my class field is still pointing null after the initialisation? -

i have java nullpointerexception question.

for instance: have 3 classes a, b , c.

public class {     public void methoda1()     {    } } ////////////////////////////// public class b {     private a;     private c c;      public void methodb1()     {         = c.methodc1;     }      public void methodb2()     {         a.methoda1();     } } ///////////////////////////// public class c {    public methodc1()   {        return new a();   } } 

as can see code above, main issues in class b. question is, if field in class b initilised in methodb1, why cannot use again in methodb2? compiler giving me nullpointerexception using code a.methodea1(). , cannot create new again need results methodb1(), namely in methodb1(). help, please.

you create object of c class forget assign new object

public class b {     private a;     private c c;      public void methodb1()     {         c = new c(); // added          = c.methodc1(); // problem here       }      public void methodb2()     {         a.methoda1();     } } 

you may want read nullpointerexception.

inside constructor must firstly call methodb1() :

    public b(){     this.methodb1();     this.methodb2();     }      public static void main(string...args){       new b();      } 

because of avoiding nullpointerexception of a, see inside methodb1 assign new object ot a.