Can one class extend two classes?

user_604388 0
Viewed 326523

My class should extend two classes at the same time:

public class Preferences extends AbstractBillingActivity {

public class Preferences extends PreferenceActivity {

How to do so?

Upd. Since this is not possible, how should I use that AbstractBillingActivity with Preferences then?

Upd2. If I go with interfaces, should I create:

  1. BillingInterface

    public interface BillingInterface extends PreferenceActivity, AbstractBillingActivity {

}

  2. PreferenceActivity

    public interface PreferenceActivity {

}

  3. AbstractBillingActivity

    public interface AbstractBillingActivity {

        void onCreate(Bundle savedInstanceState);

}

and then 

public class Preferences implements BillingInterface {
multiple-inheritance android java
13 Answers

I can think of a workaround that can help if the classes you want to extend include only methods.

Write these classes as interfaces. In Java, you can implements any number of interfaces, and implement the methods as default methods in the interfaces.

https://www.geeksforgeeks.org/default-methods-java/

user_54026180

Also, instead of inner classes, you can use your 2 or more classes as fields.

For example:

Class Man{
private Phone ownPhone;
private DeviceInfo info;
//sets; gets
}
Class Phone{
private String phoneType;
private Long phoneNumber;
//sets; gets
}

Class DeviceInfo{

String phoneModel;
String cellPhoneOs;
String osVersion;
String phoneRam;
//sets; gets
}

So, here you have a man who can have some Phone with its number and type, also you have DeviceInfo for that Phone.

Also, it's possible is better to use DeviceInfo as a field into Phone class, like

class Phone {
DeviceInfo info;
String phoneNumber;
String phoneType;
//sets; gets
}
user_97193370

If you are interested in using methods from multiple classes, the best way to approach to it is to use Composition instead of Inheritence

user_113379840

In Groovy, you can use trait instead of class. As they act similar to abstract classes (in the way that you can specify abstract methods, but you can still implement others), you can do something like:

trait EmployeeTrait {
    int getId() {
         return 1000 //Default value
    }
    abstract String getName() //Required
}

trait CustomerTrait {
    String getCompany() {
        return "Internal" // Default value
    }
    abstract String getAddress()
}

class InternalCustomer implements EmployeeTrait, CustomerTrait {
    String getName() { ... }
    String getAddress() { ... }
}

def internalCustomer = new InternalCustomer()
println internalCustomer.id // 1000
println internalCustomer.company //Internal

Just to point out, its not exactly the same as extending two classes, but in some cases (like the above example), it can solve the situation. I strongly suggest to analyze your design before jumping into using traits, usually they are not required and you won't be able to nicely implement inheritance (for example, you can't use protected methods in traits). Follow the accepted answer's recommendation if possible.

user_1965070
Related