java - NullPointerException in put method of HashMap -

i know been asked many times before still cant understand mistake..

this normal code writing counting number of repetitions in array( :) might long method doing this, kindly suggest smaller way if can think of)

public int find(int[] sequence) {     arrays.sort(sequence);      int temp=0,count=0,j=0;      hashmap<integer,integer> data = new hashmap<integer,integer>();          for(int i:sequence){         integer c = new integer(count);          integer d = new integer(j);          if(i!=temp) {          if(count!=0) data.put(c,d);         count++;          j=1;          temp=i;           }           else j++;           }          count++;//this 1 causes error         //system.out.println(count);         integer c = new integer(count);          integer d = new integer(j);          data.put(c,d);              long ans =  theswapsdivtwo.factorial(sequence.length);            for(int i=1;i<=data.size();i++){                  ans /=  theswapsdivtwo.factorial(data.get(i).intvalue());                 system.out.println(data.get(i));                 }                  return (int)ans;  }  public static long factorial(int n) {             long fact = 1; //  result             (long = 1; <= n; i++) {                 fact *= i;             }             return fact; }        

the put method doesn't produce errors in for loop implementation outside loop.

error this:

java.lang.nullpointerexception @ theswapsdivtwo.find(theswapsdivtwo.java:54) @ sun.reflect.nativemethodaccessorimpl.invoke0(native method) @ sun.reflect.nativemethodaccessorimpl.invoke(nativemethodaccessorimpl.java:39) @ sun.reflect.delegatingmethodaccessorimpl.invoke(delegatingmethodaccessorimpl.java:25) @ java.lang.reflect.method.invoke(method.java:585) @ com.topcoder.services.tester.java.testprocess$runner.run(testprocess.java:386) 

p.s. count++ caused error... foolish really.. loop going out of bounds..

this line should giving npe:

theswapsdivtwo.factorial(data.get(i).intvalue()); 

i.e. data.get(i) not present (1<=i<=data), either have null check on each get value or make sure in logic i present map require refer.

a test putting null:

map<integer, integer> m = new hashmap<integer, integer>(); m.put(null, null); system.out.println(m); 

it prints:

{null=null}