adding objects in an arrayList

Question

I am a student taking a course in object oriented programming. I am struggling to understand objects anybody here knows about a book or webside where I can learn it?

I have been giving this exercise, and I have no clue why I can't add a message to my arraylist. What am I doing wrong?

import java.util.ArrayList;

public class MessagingService {
    ArrayList<Message> messages;


    public MessagingService(){
        this.messages = new ArrayList();
    }
    public void add(Message message){
        if(message.length() >=280 && !(this.messages.contains(message))){
            this.messages.add(message);
        }
    }
    public ArrayList<Message> getMessages(){
        ArrayList<Message> mess = new ArrayList();
        for(Message m:messages){
            mess.add(m);
        }
        return mess;
    }
    public static void main(String[] args) {

        // Try out your class here
        MessagingService ms = new MessagingService();
        System.out.println(ms.getMessages());
        Message m = new Message("sender", "message");
        ms.add(asdf);
        System.out.println(ms.getMessages());
    }

}

What error are you getting? or does it compile but give you the wrong output? Have you defined a message class? — fedoraHacker
– fedoraHacker, Commented Apr 4, 2020 at 10:30

Withel · Accepted Answer · 2020-04-04 10:40:44Z

Hello if you're asking about good articles to read I can recommend you this: https://www.freecodecamp.org/news/object-oriented-programming-concepts-21bb035f7260/

also I would recommend you to read about Collections in Java, because what you are doing here:

!(this.messages.contains(message))

is just preventing for adding duplicates to your list, so maybe you should reconsider and simply use Set Collection? Unless order in which objects are added is important to you (ArrayList is ordered, Set isn't). As you can see in this simple example there are many important features here and sometimes choosing correct Collection can simplify process of creating the code.

Previous answers recommend you some books. I personally find it boring when you are beginner so instead of buying book maybe just find good course on Udemy and learn in more interactive way?

https://www.udemy.com/course/java-the-complete-java-developer-course/

This is the one I think is very good and fun for beginners.

Cheers and wish you best.

afadhitya · Accepted Answer · 2020-04-04 10:21:41Z

I think the problem is when you will add Message object to your ArrayList the message must contain greater than or equal 280, and of course your m message will not added to your ArrayList because it contain less than 280 character.

You should change this code

public void add(Message message){
    if(message.length() >= 280 && !(this.messages.contains(message))){
        this.messages.add(message);
    }
}

Or you can try using the String with more than 280 character.

For learning java language it's very lot of resources out there, just type "Java Introduction" on Google and you'll find a lot of resources you to learn Java. And for book I will recommend you to read Book "Head First Java" By O'Reilly

Dina Bogdan · Accepted Answer · 2020-04-04 10:35:00Z

First of all your code it's not compiling (I bet that you already notice this).

Second, you must understand that when you want to add a message to your ArrayList, the message that you are trying to add must be compliant with the conditions that you are checking for in the if statement, in your case: the message should have a length of more than 280 characters and the messages ArrayList to not already contained the message that you are trying to add.

If you will replace this piece of code:

Message m = new Message("sender", "message");
ms.add(asdf);

with this one:

Message m = new Message("sender", "biUHobB2xW4Aed9jc5ZicRtgO1R2MUQGJeVHE3CBr0BxuSImM9E4ytkyeAKR22bEheJJIJ7PxzIsUjKYwsBKdYJeDA8VgKduTNJ0ofH66x3v20j3JnXZanWP0aRuxJmBatfmuEsBur8ald0eW6XwGirumGTloq9QD65KD5M8e1dsY6ep6jBmn7enL37Xn6IFgiI7KcQfHvABuWv6A2O62s7gtmEQ0HnchqXbknA5Xs5OxiQgTJU5bKNwetWsJOxCOxZKXZPSX5iN1DotXIg3mbyvwtBRR8y4qO
");
ms.add(m);

To have a better understanding of how Java works, you can check the following book:

https://www.amazon.com/Head-First-Java-Kathy-Sierra/dp/0596009208

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