System.out.println("\nThere are " + polyInstance.signChanges(tempCoefsArray,  noCoefs) + " positive real roots in the equation you entered\n");

......other non important code here.....

public int signChanges(String tempCoefsArray[], int noCoefs)
	{

		int signChanges=0;           
     
			for(int i=0; i<=tempCoefsArray.length-2; i++)
			{
                               if(tempCoefsArray[i].charAt(0) != tempCoefsArray[i+1].charAt(0))//null ptr here
				 {

					 signChanges++;
				 }
				
			}
		return signChanges;
	}