Java向下转换ClassCastException错误

Question

I got tasked with a bug and got stuck with a problem with I guess downcasting in Java. Here's the code:

public class ComItem {

private String someValue1;
private String someValue2;

public void copy(ComItem temp) {
    this.someValue = temp.someValue1;
    this.someValue2 = temp.someValue2;
}

... getters & setters & etc omitted
}


public class ItemA extends ComItem {

private ThingA thingA;

@Override
public void copy(ComItem temp) {
    super.copy(temp);
    this.thingA = ((ItemA) temp).thingA; // <- **ERROR**
}

... getters & setters & etc omitted
}

The program throws ClassCastException when I try to use ItemA's copy(..) here:

if (form.getTempObject() instanceof ComItem) {
    ...
    ComItem temp = (ComItem) form.getTempObject();
    ComItem itemToEdit = (ComItem) form.getCurrentEditedObject(); // currentEditedObject is ItemA class
    itemToEdit.copy(temp);
    ...
} 

There are many classes like ItemA similar in structure, each with its own Thingx fields.

Could anyone please help me with a solution to how to change it to make it work?

Answer 1

In your code you checked types in the class that use the hierarchy

if (form.getTempObject() instanceof ComItem) {

but the tempObject could be any subclass of ComItem (or even ComItem if not abstract).

So in your copy method you supose that always use ItemA (and haven't checked it type), and not always is true.

I don't like instanceOf because is not so OO, but in this case you may use it.

@Override
public void copy(ComItem temp) {
    super.copy(temp);
    if (form.getTempObject() instanceof ItemA) {
        this.thingA = ((ItemA) temp).thingA; // <- **NOW NO ERROR**
    }
}

Or another alterative, its that ComItem implements an interface Copyable

public interface Copyable {

copyFrom(ItemA toCopy);
copyFrom(ItemB toCopy);
copyFrom(ItemC toCopy);
....
}

and you can define in any class the way they have to copy from (it's more polimorfic and OO).

Answer 2

Is form.getTempObject() also returning an instance of ItemA class?
That is the main requirement for you to be able to pass it to ItemA.copy() method and be able to cast it inside, to an ItemA .

Just because itemToEdit is an instance of ItemA does not guarantee the argument passed to copy() method is also an ItemA .
itemToEdit being an instance of ItemA causes the ItemA.copy() method to be called instead of ComItem.copy() .