Java should provide interface level equality

While working to a business domain model in a pure Object Oriented approach, one thing I miss in Java is the concept, hence the implementation, of type equality, where for type I mean interface.

Let me explain with a simple example.

Say you have a Customer entity, that in your domain model is an interface, that is, a type:

public interface Customer {
  
  public String getCustomerCode();

  public String getSurname();

  public Date getBirthdate();
}

You have different concrete Customer implementations, i.e. StandardCustomer and PremiumCustomer, but customer equality is the same for all customers, because defined by the customer code: so, you want to define equality for the Customer type.
Too bad, in Java the only way to go is to implement a base class for all customers, defining, among other methods, the equals() one:

public abstract class BaseCustomer implements Customer {
  
  public String getCustomerCode() { ... }

  public String getSurname() { ... }

  public Date getBirthdate() { ... }

  public boolean equals(Object obj) {
    if ((obj == null) || (!(obj instanceof Customer))) return false;

    Customer other = (Customer) obj;

    return this.getCustomerCode().equals(other.getCustomerCode());
  }
}


public class StandardCustomer extends BaseCustomer {
  ...
}


public class PremiumCustomer extends BaseCustomer {
  ...
}

However, this is not correct, nor safe, because you could implement another Customer directly implementing the interface, or even extending another base class: doing so, you would break the equals contract.
For example:

public class CustomerOfTheMonth implements Customer {
  
  public String getCustomerCode() { ... }

  public String getSurname() { ... }

  public Date getBirthdate() { ... }

  // You forget to implement equals(), or maybe implement a different one ....
}

Now, a StandardCustomer is equal to a CustomerOfTheMonth, but a CustomerOfTheMonth is not equal to a StandardCustomer!
This is because BaseCustomer defines equality for all Customers, but CustomerOfTheMonth, while being a Customer, is not a base one!
So, the symmetry of the equals() method is broken!

The problem would be solved if we could implement the equals() method into the Customer interface, rather than into the BaseCustomer concrete class: if you had then to implement equality in a different way for some concrete class, you'd be also free to overwrite it later!

Here we come back to the beginning of my post: why doesn't Java provide interface level equality?