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;
}