Java downcasting

Question

Hi have one class like this

import java.util.ArrayList;

public class MobilePhone {

    private String number;
    private ArrayList<Message> messages;



    public MobilePhone(String n) {
        this.number = n;
        this.messages = new ArrayList<Message>();
    }

    public String getNumber() {
        return number;
    }

    public void setMessages(Message messages) {
        this.messages.add(messages);
    }

    public ArrayList<Message> getMessages() {
        return messages;
    }

}

And then a Message class

public class Message {

    protected String sender;
    protected String receiver;
    protected String subject;
    protected String bodyText;
    protected int tipo;

    protected Message() {
        this.sender = this.receiver = this.subject =
        this.bodyText = "";
    }

    protected Message(String s, String r, String sbj, String b, int t ) {
        this.sender = s;
        this.receiver = r;
        this.subject = sbj;
        this.bodyText = b;
        this.tipo = t;
    }

    public String getSender() {
        return sender;
    }

    public String getSubject() {
        return subject;
    }

    public String getBodyText() {
        return bodyText;
    }

    public int getTipo() {
        return tipo;
    }


}

And one subclass

public class SMS extends Message {
    static int maxBodySize = 160;


    public void showMessage(){
        System.out.println("SMS");
        System.out.println("Subject: " + super.subject);
        System.out.println("Text: " + super.bodyText);
    }
}

On my code I have this:

    for (MobilePhone item : listaTelefones) {
         for (Message item2: item.getMessages()){
             ((SMS) item2).showMessage();
         }
    }

And it gives me this error:

Exception in thread "main" java.lang.ClassCastException: Message cannot be cast to SMS

Can't I downcast Message to SMS so I can use the SMS showMessage() method?

Answer 1

Some of the items in the list are of class Message but not of class SMS. Therefore, you can not cast them to class SMS.

Add something like this to ensure you are dealing with SMS:

if (item2 instanceof SMS) {
    ((SMS) item2).showMessage();
}

Answer 2

You need to check if the Message is of type SMS before you do the cast as not all Messages are SMS's.

if(item2 instanceof SMS) {
    ((SMS) item2).showMessage();
}

This will make sure you don't try and cast messages which are not of SMS type to a SMS type.

Answer 3

You must be putting a Message in your list. Either:

Test for type:

if (item2 instanceof SMS) {
    ((SMS) item2).showMessage();
} else {
    // ?
}

If you know they're SMS, type your list to SMS:

private ArrayList<SMS> messages;

public ArrayList<SMS> getMessages() {
    return messages;
}