Java ArrayList with objects

Question

So I have this class of terms

public class Term {
private int coefficient ;
private int exponent ;
public Term(int coefficient, int exponent)
{
 this.coefficient = coefficient ;
 this.exponent = exponent ;
}
public int getCo()
{
 return coefficient ;
}
public int getEx()
{
 return exponent ;
}
}

And I have a class called Polynomial that creates an arraylist of type term with a method called delete that isn't working the way I'd like it to and I can't figure out why:

public class Polynomial{
public Polynomial()
{
 poly = new ArrayList<>() ;
}
public void delete (int coeff, int expo)
{
 for(int i = 0; i < poly.size(); i++) 
 {
 System.out.println("**Delete Loop works") ; //This does not print
 if( (poly.get(i).getCo() == coeff) && (poly.get(i).getEx() == expo) ) 
 {
 poly.remove(i) ;
 removed = true ; 
 break ;
 }
 }
}
}

It keeps throwing an exception every time I run the delete method and I'm not sure why?? I am using input from a text file (and the input is working fine), I think it might be going wrong where I use : poly.get(i).getCo() == coeff is that properly written?

Thanks for any help you can provide!

• Szymon: What is the exception?
• David Baez: where I wrote //this does not print it doesn't even print if I write a loop that is always true. It is strange.
• Rahul: If that SOP does not print, then it means the poly arraylist is empty and has nothing to be deleted from it.
• polis: I think your ArrayList is empty.
• David Baez: at Polynomial.delete(Polynomial.java:72) at PolynomialTest.main(PolynomialTest.java:49) Java Result: 1

Answer 1

In your Polynomial constructor, you have defined poly = new ArrayList<>() ; which creates ArrayList with not a single object in it.

That means poly.size() = 0 that's why it will not enter in loop and your syso statement will not executed.

• David Baez: I'm using another class with a main method in it to populate the list. :<
• David Baez: speedy.sh/cwcqs/Polynomial.java speedyshare.com/848FJ/PolynomialTest.java //Main Method speedy.sh/q9qp2/Term.java speedy.sh/FAFZe/operations.txt
• Sage: -1 The OP specifically said he is having an exception. If size is 0 there no reason for occurring an exception.

Answer 2

It keeps throwing an exception every time I run the delete method and I'm not sure why??

You are probably having ConcurrentModificationException. The exception is thrown because you are iterating over the array list and removing an element at the same time. Try using Iterator<T>iterator instead and iterator.remove() method.

 Iterator<Poly>iterator = arrayList.iterator();
 while(iterator.hasNext())
 {
 Poly poly = iterator.next();
 if( (poly.getCo() == coeff) && (poly.getEx() == expo) ) 
 iterator.remove();
 }