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

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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.


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