Comparator in Arraylist

Question

I have a List of my user defined class.

class Customer{
  Integer age;
  String name;
  //getter & setter
}


Collections.sort(customerList, new Comparator <Customer>() {        
  public int compare(Customer o1, Customer o2) {
    // TODO Auto-generated method stub              
    if(o1.getAge()!=null && o2.getAge() != null)
        return o1.getDistance().compareTo(o2.getDistance());
    else
      return 1;
  }
});

Now my age variable may have a null value or the age of the customer . All null values should be appended at the end and remaining values should sorted in ascending or descending order(anything is ok)?

But this code is throwing an exception :

java.lang.IllegalArgumentException: Comparison method violates its general contract!

Please tell me what to do? Thanks in advance.

Answer 1

The ordering imposed by a comparator c on a set of elements S is said to be consistent with equals if and only if c.compare(e1, e2)==0 has the same boolean value as e1.equals(e2) for >every e1 and e2 in S.

Ref - link

Element set must be consistent. Inconsistent equals will behave straggly.

Answer 2

You are not handling the cases correctly where only one of the customers has an age and the other has not.

Also, returning 1 in the else-situation seems to be incorrect. If there is no difference according to the age, then return 0 instead of 1.