Java HashSet重复比较

Question

I have a class Person which contains String firstName, lastName . I want to insert instances of this class into a List , but I don't want to insert duplicates.

How do I use a HashSet such that it uses something like firstName+lastName to figure out duplicates?

Answer 1

You need an equals() and a hashCode() method in your Person class.

equals() is straightforward, and for hashCode() the easiest solution is:

public int hashCode() {
  return Arrays.hashCode( new Object[] { firstName, lastName } );
}

Although if your Person object is immutable (as it should be, if you're putting it in a HashSet), you should cache this value.

Answer 2

You need to make your .equals() method return true for two Person s with the same first and last name. You need to also implement the .hashcode() method to ensure that two equal object have the same hashcode.

Your question refers to using a List, and then mentions HashSet. If preserving insertion order is important then a HashSet is not what you want, you should use LinkedHashSet .

Answer 3

Important -> You need to implement equals AND hashcode

Always implement both. And they must be consistent-> if 2 object are equals they must have the same hashcode.

THIS IS VERY IMPORTANT. READ AGAIN. :)

HashSets and HashMaps use hashcode and equals to compare elements. Diferent elements may have the same hashcode, but they are not equals

Answer 4

You should just use a Set instead of a List . If you care about insertion order, use a LinkedHashSet .