如何在另一个列表对象内添加对象

Question

I have an object class A, and a list object for class B

A a = a.findById(aId);
List<B> b = b.findById(bId);

Class B has A class

public class B {
   private int id;
   private A a;
   ...getters setters
}

Is it possible to add object a in object list b ?

I get error when I try to add a in object list. b.add(a); This tells me to change object a as B a...

How can I add object a in the list object b ?

Answer 1

You can add object a if you wrap it in object B . Otherwise, you will not be able to add object a of type A to List<B> of B objects because of the type mismatch: you are trying to add object of type A to the list of B objects which is not allowed in Java.

Answer 2

Is it possible to add object a in object list b ?

Simply, no because they're not the same types.

How can I add object a in the list object b?

in order to add both instances of class A and class B into one collection, one approach you can take is to make an interface in which both class A and class B implement.

Example:

public interface ISomeInterface {
    // define some methods in which both class `A` and `B` share 
}

Then make the classes implement the ISomeInterface interface:

public class A implements ISomeInterface {

}

public class B implements ISomeInterface {
    private int id;
    private A a;
}

then you can do something like this:

List<ISomeInterface> b = new ArrayList<>();
b.add(new A());
b.add(new B());
// note you can only call methods through which the interface defines
// another solution is to cast the types, that way you can invoke the methods that belong to each type
// make use of instanceof keyword to prevent exception

or simply without the need of making interfaces:

List<Object> b = new ArrayList<>();
b.add(new A());  //if you want to call the methods of A or B you need to cast it first, remember to use instanceof keyword prior to casting
b.add(new B());

Answer 3

The syntax to add a A to B would be first get B, then add A to B, such as

A a = new A();
B b = new B();

b.add(A);

if you wish to add both to the same List<> then they need to implement the same or inherit from the same class, example interface IAB

List<IAB> list = new ArrayList<>();

list.add(new A());
list.add(new B());

given that

public interface IAB{}
public class A implements IAB{}
public class B implements IAB();

note that in this case you can only call the methods that are available on the Interface unless you cast it to the class (but if you mix A and B that might be complicated to handle)