当数据类型为抽象时，如何从ArrayList中删除项目？

Question

I'm new to Java so first of all sorry if this question is too stupid or something. I have an ArrayList of an abstract class. I added a few objects to the list. Now I need to remove one of them by finding it by one of its attributes. The thing is that the abstract class has two concrete classes and both of them were added to the list. The attribute is inherited from the abstract class so when I do a foreach I do it with the abstract class, but I don't know how to tell it that the object it needs to remove is this concrete class and not the other one.

    public void removeFruit (Integer fruitCode) {
Apple lostFruit = null;
Banana lostFruit2 =null;
    for (Fruit fruit1 : fruitList) {
        if (fruit1.getFruitCode().equals(fruitCode) && fruit1 == Apple) {
            lostFruit = (Apple) fruit1;
            fruitList.remove(lostFruit);
        }else {
            lostFruit2 = (Banana) fruit1;
            fruitList.remove(lostFruit2);
        }

    }
    System.out.println(fruitCode + "has been removed from the list");

}

Answer 1

You need an Iterator to accomplish that (which the for-each loop hides ). As the Iterator.remove() javadoc notes the behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method.

Iterator<Fruit> iter = fruitList.iterator();
while (iter.hasNext()) {
    Fruit f = iter.next();
    if (f.getFruitCode().equals(fruitCode)) {
        if (f instanceof Apple) {
            Apple a = (Apple) f;
            // ...
        } else if (f instanceof Banana) {
            Banana b = (Banana) f;
            // ...
        }
        iter.remove();
        System.out.println(fruitCode + " has been removed from the list");
    }
}

Answer 2

No need to cast

The attribute is inherited from the abstract class

No need to cast. When handling a Fruit , we do not care about Apple or Banana .

If the abstract class has what you need, you do not care about the concrete subclass. That is the point of polymorphism , not caring about the specific types when the more general type suffices.

public Fruit removeFruit (Integer fruitCode , List<Fruit> fruitList ) {

    for (Fruit fruit : fruitList ) {
        if ( fruit.getFruitCode().equals( fruitCode ) {
            fruitList.remove( fruit );
            return fruit ;
        }
    }
    return null ;  // In case you fruit code was not found.
}

Example usage:

List<Fruit> fruits = … ;
Integer fruitCode = … ;
Fruit fruitRemoved = this.removeFruit( fruitCode , fruits ) ;
System.out.println(
    "You deleted fruit code: " + fruitCode + " of type: " + fruitRemoved.getClass().getName() ;
)

You deleted fruit code: 42 of type: Apple

---

In the example above, I would actually return Optional<Fruit> rather than Fruit . But that is a whole other discussion.

Answer 3

Check if fruit1 is instance of class Apple

if (fruit1 instanceof Apple) {
// your code
}