如何重写子类的equals方法

Question

I am trying to sync users between two different locations, therefore I keep existing users in a list, and hence do a comparison at a set time interval to see if the user should be added (new) or just updated.

I have a class User that is the subclass to Principal.

However my compare on the list does not work; I googled a bit and found that you have to override the equals method, and I do - but that code does not seem to be executed, it goes into ArrayList.class (primitive) and executes the contains method there.

Is this because my class already extends the superclass Principal? What are my options if I want to execute the equals that I defined in User class?

public class User extends Principal
{
    // some protected properties
    ...

        @Override
        public boolean equals(Object obj) {
            return (this.getAlias().equals(((User) obj).getAlias())
                && this.getEmailAddress().equals(((User) obj).getEmailAddress()) && this.getCellNumber().equals(((User) obj).getCellNumber()));
    } 
}

The Principal class does not override the equals method, and more importantly, the properties I check for equality, is only contained in the subclass - User. Therefore it makes sense to check it here.

So in short, I have an ArrayList of Users, and I would like to check whether a certain User already exists or not. I call compare on the list, but it always fails, indicative that the method equals is not overrided properly in my code. Any suggestions?

Answer 1

Overriding equals is not as evident as it looks

• equals with null must return false
• equals with an object of a different class must return false because of symetry a.equals(b) <=> b.equals(a)

java

@Override
public boolean equals(Object obj) {
    if (obj == this) {
        return true;
    }
    if (obj == null || obj.getClass()!=getClass()) {
        return false;
    }
    return Object.equals(this.getAlias(),((User) obj).getAlias())
        && Object.equals(this.getEmailAddress(),((User) obj).getEmailAddress())
        && Object.equals(this.getCellNumber(),((User) obj).getCellNumber()));
} 

Also if object is used in hash collections it must override hashCode so that two objects that are equals must return the same hashCode, the contrary is not true.

Answer 2

You should not implement equals() (and hashcode() ) in a super class.

The reason is that when equals() returns true hashcode() must return same value

Imagine you have class Point2D and class Point3D extending the other.

Shall a point2D be equal to a point3D with same area coordinates? If so then point3D must return the same hashcode as the "equal" point2D and that means that you cannot not store more that one poin3d with same area coordinates in a Hash bases collection (eg.: as keys in a HashMap).

Answer 3

The problem probably comes from you instantiating a List<Person> . The compiler can't know if every subclasses of Person override equals . To correct this, you should promise your compiler you'll override this method, which you can do by changing your Person class to an abstract class.

public abstract class Person {
    @Override
    public abstract boolean equals(Object o);
}

public class User extends Person {
    // Some stuff...
    @Override
    public boolean equals(Object o) {
        if (o == null || ! (o instanceof User))
            return false;
        // etc
    }
}

Answer 4

According to the book Effective Java.If you have override the equals method,then you must override the hashcode method. some advice when you override the equals method: 1. equals with null return false. 2. !(obj instanceof this) return false. 3. cast obj to this class and compare the parameters in the obj and this class. return the result in the end

Answer 5

You should use the contains methode of the arrayList

https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html