ClassCastException：无法将ArrayList强制转换为

Question

This is the error that I am getting:

java.lang.ClassCastException: java.util.ArrayList cannot be cast to MenuComponent

Here is the code that is causing me trouble:

import java.util.ArrayList;
import java.util.Iterator;
public class Menu extends MenuComponent{
  ArrayList menuComponents = new ArrayList();
  String name;
  String description;

  public Menu() {    
  }

  public Menu(String name, String description) {
    super();
    this.name = name;
    this.description = description;
  }

  public void add(MenuComponent menuComponent) {
    menuComponents.add(menuComponents);
  }

  public void remove(MenuComponent menuComponent) {
    menuComponents.remove(menuComponents);
  }

  public MenuComponent getChild(int i) {
    return (MenuComponent) menuComponents.get(i);
  }

  public String getName() {
    return name;
  }

  public String getDescription() {
    return description;
  }

  public void print() {
    System.out.println(getName());
    System.out.print("---" + getDescription());
    System.out.println("---");

    Iterator iterator = menuComponents.iterator();
    while(iterator.hasNext()) {  
      MenuComponent menuComponent = (MenuComponent) iterator.next();
      menuComponent.print();
    }
  }

  public Iterator createIterator() {    
    return new CompositeIterator(menuComponents.iterator());
  }
}

The problem that is causing the error is in this line:

MenuComponent menuComponent = (MenuComponent) iterator.next();

I've tried messing with it, but am having no luck. Any tips are greatly appreciated.

Answer 1

That´s the disadvantage of using the raw type of the ArrayList , you don´t notice if you do a simple mistake. You should rather declare the ArrayList as ArrayList<MenuComponent> menuComponents . Then you would have noticed, that in your method add(MenuComponent menuComponent) you are allways adding the arrayList to the arrayList, because you made a simple typo.

It should be menuComponents.add(menuComponent); instead of menuComponents.add(menuComponents); . The same problem will occur in your remove method.

Answer 2

the problem is here:

public void add(MenuComponent menuComponent) {
    menuComponents.add(menuComponents);//you add menuComponentS to the list!
}

public void remove(MenuComponent menuComponent) {
    menuComponents.remove(menuComponents);//you remove menuComponentS to the list!
}

Answer 3

The problem ist you add Arraylists in your Arraylist at this point

public void add(MenuComponent menuComponent) {
    menuComponents.add(menuComponents);
}

it has to be

public void add(MenuComponent menuComponent) {
    menuComponents.add(menuComponent);
}

you can simply avoid this by using typed Arraylist like this ArrayList<MenuComponent> menuComponents = new ArrayList<>();

Answer 4

First, do not use raw collections, like this:

ArrayList menuComponents = new ArrayList();

The proper way, is like this:

ArrayList<MenuComponent> menuComponents = new ArrayList<MenuComponent>();

Or better:

List<MenuComponent> menuComponents = new ArrayList<MenuComponent>();

This will allow you to eliminate the casts, like this:

return (MenuComponent) menuComponents.get(i);

and instead just do this:

return menuComponents.get(i);

This will allow Java to tell you if you are sticking the wrong things in the array, as in here:

menuComponents.add(menuComponents);

Java lets you get away with this because it has no idea what is supposed to be in menuComponents . When you say new ArrayList<MenuComponent>() , you are telling Java that the ArrayList can only have MenuComponent objects in it. Then, the above code will fail to compile, because Java knows you are doing the wrong thing and can tell you.