Question

I have a List<Person> objects where person is defined like so

public Person {
    private firstName;
    private lastName;
    //getter and setter methods
    public boolean equals(Object obj) {
        return lastName.equals(obj.toString());
    }
}

Now I want to see if this List<Person> contains a certain last name.

if(myList.contains("Smith"))
    System.out.println("yay!");

However the way the contains method is specified is that is returns true when (o==null ? e==null : o.equals(e)). So in this instance it is using String.equals(Person) instead of Person.equals(String). Is there a simple way of fixing this or do I have to write my own logic for contains?

Answer 1

Is there a simple way of fixing this…

No, not using Collection.contains.

By the contract of equals the implementation must be symmetric:

for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.

This means that returning any thing else than false for somePerson.equals(someString) is a direct violation of the contract.

…or do I have to write my own logic for contains?

Yes. Use a for loop:

for (Person p : myList)
    if (p.getLastName().equals("Smith"))
        return true;
return false;

or, if you’re using Java 8,

return myList.stream().anyMatch(p -> p.getLastName().equals("Smith"));