java - NullPointer exception when using second non-static reference to static object -

-

i have static object in class, class writer, , instances of class need refer 1 of static objects. avoid code duplication (where i'd have write same code multiple times, each difference in of static objects used). solution have static writer called writer1 , static writer call writer7, , have non-static writer called otherwriter, in constructor of either writer1 or writer7 points other writer.
however, keep getting nullpointer exceptions when access otherwriter. error , code below - ideas? sorry, code not neat - it's hacky fiddeling @ stage. thanks

error:

java.lang.nullpointerexception     @ popl.poplformative.generalwrite(poplformative.java:108)     @ popl.poplformative.run(poplformative.java:51)  package popl;  import java.util.arrays;  public class poplformative extends thread {      int currentindex = 0;     int lastindexwritten = -1;      static int data[];     int writerid;      static poplformative writer1;     static poplformative writer7;      poplformative otherwriter;      public static void main(string[] args) {         data = new int[10];         writer1 = new poplformative(1);         writer7 = new poplformative(7);         writer1.start();         writer7.start();         try {             thread.sleep(600);         } catch (interruptedexception e) {             // todo auto-generated catch block             e.printstacktrace();         }         system.out.println(arrays.tostring(data));     }      public poplformative(int writer) {         this.writerid = writer;         if (writerid == 1) {             this.otherwriter = poplformative.writer7;         }         else if (writerid == 7) {             this.otherwriter = poplformative.writer1;         }         else {             system.out.println("big nasty error occurred");             while (true) {             }         }     }      public void run() {         try {             generalwrite();         } catch (interruptedexception e) {             // todo auto-generated catch block             system.out.println("exception thrown");             e.printstacktrace();         }     }      public void generalwrite() throws interruptedexception {         while (currentindex < 5) {             //system.out.println(writerid + " writer has currentindex; " + currentindex);             if (data[math.max(lastindexwritten,0)] == writerid || (lastindexwritten == -1 && writerid == 7)) {                 write();                 synchronized (this) {                     notify();                 }                 synchronized (otherwriter) {                     otherwriter.wait();                 }             }             else {                 synchronized (otherwriter) {                     otherwriter.wait();                     write();                 }                 synchronized (this) {                     notify();                 }             }             lastindexwritten += 1;             currentindex += 1;         }         system.out.println(writerid + " has completed");     }      public void write() {         system.out.println("writer: " + writerid + " currentindex: " + currentindex + " lastindex: " + lastindexwritten);         data[currentindex] = writerid;     }  }

in main, have 

writer1 = new poplformative(1);

which invokes constructor

public poplformative(int writer) {     this.writerid = writer;     if (writerid == 1) {         this.otherwriter = poplformative.writer7;     }     else if (writerid == 7) {         this.otherwriter = poplformative.writer1;     }     else {         system.out.println("big nasty error occurred");         while (true) {         }     } }

in case, code executed

if (writerid == 1) {     this.otherwriter = poplformative.writer7; }

but poplformative.writer7 null, hasn't been initialized yet, otherwriter becomes null.

if need these circular references, consider using setters.


Comments

Post a Comment

Popular posts from this blog

c# - Unity IoC Lifetime per HttpRequest for UserStore -

-
i'm trying clean default implementation of accountcontroller.cs comes out of box in new mvc5/owin security implementation. have modified constructor this: private usermanager<applicationuser> usermanager; public accountcontroller(usermanager<applicationuser> usermanager) { this.usermanager = usermanager; } also, have created lifetime manager unity looks this: public class httpcontextlifetimemanager<t> : lifetimemanager, idisposable { private httpcontextbase _context = null; public httpcontextlifetimemanager() { _context = new httpcontextwrapper(httpcontext.current); } public httpcontextlifetimemanager(httpcontextbase context) { if (context == null) throw new argumentnullexception("context"); _context = context; } public void dispose() { this.removevalue(); } public override object get...
Read more

Change the color of an oval at click in Java AWT -

-
i have draw ovals in java, , @ click change color. beginning tried change color after 20 ms, doesn't work. my code is: public class mycomponentnew extends frame { public graphics2d g2d; public mycomponentnew(string title) { super(title); setsize(400, 550); } @override public void paint(graphics g) { this.g2d = (graphics2d) g; this.g2d.setcolor(color.red); this.g2d.filloval(10, 55, 50, 100); } public void changecolor () { this.g2d.setcolor(color.blue); this.g2d.filloval(10, 55, 50, 100); } } and in class main method have: mycomponentnew m; m = new mycomponentnew("fereastra cu baloane"); m.setvisible(true); m.addwindowlistener(new windowadapter() { @override public void windowclosing(windowevent we) { system.exit(0); } }); try { thread.sleep(20); } catch(interruptedexception e) {} m.changecolor(); the color of oval remains red. ...
Read more

I am trying to solve the error message 'incompatible ranks 0 and 1 in assignment' in a fortran 95 program. -

-
i writing program in fortran find velocity of parachuting person in relation time. keep getting error can't fix. new programming , appreciated. the error is v(i+1)=v(i)+[32-((c*v(i)*v(i))/m)]*(h) 1 error: incompatible ranks 0 , 1 in assignment @ (1) and program is program para integer :: real :: v(11) !velocity real :: q !initial velocity real :: h !time step real :: c !drag coefficient real :: m !mass ! gravity equal 32 ft/s^2 write (*,*)'enter time step' read(*,*)h write(*,*)'enter initial velocity' read(*,*)q write(*,*)'enter drag coefficient' read(*,*)c write(*,*)'enter mass' read(*,*)m i=1,10 ! 1 10, 1 being interval. end v(i+1)=v(i)+[32-((c*v(i)*v(i))/m)]*(h) q=v(1) end program you cannot use [] normal parenthesis in expressions. array constructor, [ items ] means array items elements. end do should after line.
Read more