ignore fields while comparing two lists - collections

Question

I have two lists of Person objects.
Person class has the below attributes
String Name,
Integer Age,
String Department,
Date CreatedTime,
String CreatedBy
When I compare my lists for equality, I would like not to compare the CreatedTime and CreatedBy fields.
How can I compare, using Java 8, the two lists for equality and also ignore the CreatedTime, CreatedBy fields for comparison?

Answer 1

You don't even need features from Java-8 to do this, simply override the equals & hashCode like this:
class Person {
String name;
Integer age;
String department;
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Person person = (Person) o;
if (name != null ? !name.equals(person.name) : person.name != null) return false;
if (age != null ? !age.equals(person.age) : person.age != null) return false;
return department != null ? department.equals(person.department) : person.department == null;
}
#Override
public int hashCode() {
int result = name != null ? name.hashCode() : 0;
result = 31 * result + (age != null ? age.hashCode() : 0);
result = 31 * result + (department != null ? department.hashCode() : 0);
return result;
}
}
Then you can compare two given lists like so:
boolean result = firstList.equals(secondList);
edit:
following your comment:
I need this form of comparison only for test purposeses . In my
production code I would like to retain equals and hashcode to compare
all fields
you can define a custom equal method like so:
public static boolean areEqual(List<Person> first, List<Person> second) {
Objects.requireNonNull(first, "first list must not be null");
Objects.requireNonNull(second, "second list must not be null");
return first.size() == second.size() &&
IntStream.range(0, first.size()).allMatch(index ->
customCompare(first.get(index), second.get(index)));
}
or if you want to allow null to be passed to the areEqual method then a little change will suffice:
public static boolean areEqual(List<Person> first, List<Person> second){
if (first == null && second == null)
return true;
if(first == null || second == null ||
first.size() != second.size()) return false;
return IntStream.range(0, first.size())
.allMatch(index ->
customCompare(first.get(index), second.get(index)));
}
then along with it the method that will determine if two given person objects are equal:
static boolean customCompare(Person firstPerson, Person secondPerson){
if (firstPerson == secondPerson) return true;
if (firstPerson.getName() != null
? !firstPerson.getName().equals(secondPerson.getName()) : secondPerson.getName() != null)
return false;
return (firstPerson.getAge() != null ? firstPerson.getAge().equals(secondPerson.getAge()) : secondPerson.getAge() == null)
&& (firstPerson.getDepartment() != null
? firstPerson.getDepartment().equals(secondPerson.getDepartment())
: secondPerson.getDepartment() == null);
}
Then call it like so:
boolean result = areEqual(firstList, secondList);

Answer 2

You can use Streams.zip() from Guava for a more functional solution. Ignoring null checks for brevity:
public static boolean comparePersonLists(List<Person> list1, List<Person> list2) {
if (list1.size() != list2.size()) {
return false;
}
return Streams.zip(list1.stream(), list2.stream(), (a, b) ->
a == b || (a.name.equals(b.name) && a.age.equals(b.age) && a.department.equals(b.department))
).allMatch(t -> t);
}