List contains object

All i have my list, and when i download new data i want to check if the list already has a certain object.

So if i do the following it does not work.

if(list.contains(newobject){
System.out.println("List already has this object in it.");
} else {
System.out.println("LIst does not have this object");
}

Is there another way to test the list to check if it has an object ?