Value overridden in hashmap

Question

I'm using a HashMap to store a list of tags that are required in a certain message, for multiple messages, ie a HashMap<String,ArrayList<TagObject>>. Each TagObject has attributes, the main one is whether that tag is required or not in a certain message. Now, if a tag is required in one message, and not in another, the final required value ends up being the one that was processed last. I am also creating a HashMap<String,TagObject> to store all the tags processed. Maybe that is causing the issue? I think the issue is the same variable gets overridden repeatedly, how do I make separate copies for each message to store in the main HashMap of messages?

I've looked up HashMap overridden by last value, but I am creating a new Field everytime, so this doesn't seem to be the reason.

for(TagObject o:allTags) {
    if(o instanceof Field){
        fieldResult=new Field();
        Field field = (Field) o;
        String required=field.getRequired(); //this is the value found in the file where the specifications are, ie whether the field should be required or no in this message. this value is received correctly
        if(required == null || required.equalsIgnoreCase("yes")) {
            required="true";
        }else{
            required="false";
        }
        else {
            fieldResult=ctd.searchFieldMap(field.name); //find the already created Field object
            fieldResult.setRequired(required); //set the required now as got from the specifications
        }
        fieldsInMessage.add(fieldResult); //add this field to list of fields to be there in the message
        //while being added, I can see the value of the required tag go as in the specs, however when I later print the hashmap of all messages, the tag which was required in one message, but set as not required in another, has changed value to the value of required of the last appearance of the field in the specifications
    }
}

I was hoping that a new copy of the field will be created for each message, but seems like the same object is used in all. How do I make sure each message can have a separate copy of the tag, and hence unique attributes?

Why do you have a data structure to store a list of tags required in a certain message if you already have in that concrete tag if the tag is required or not in a certain message? — Daniel Campos Olivares, May 17 '19 at 14:06

score 1 · Answer 1 · answered May 17 '19 at 14:18

I guess that you're adding the tags incorrectly to the HashMap.

For example, if I would like to do a structure with messages and tags I would do as the following snippet:

public static void main(String[] args) {
    MessageTagsMap map = new MessageTagsMap();

    map.addNewMessage("Hello World!", "Snippet");
    map.addNewMessage("StackOverflow", "Webpage", "Interesting");

    map.paintMap();

    map.addTagsToMessage("Hello World!", "Deprecated");

    map.paintMap();

}

public class MessageTagsMap {

    HashMap<String, ArrayList<String>> content;

    public MessageTagsMap() {
        content = new HashMap<>();
    }

    void addNewMessage(String message, String... tags) {
        ArrayList<String> messageTags = new ArrayList<>();
        for (String tag : tags) {
            messageTags.add(tag);
        }
        this.content.put(message, messageTags);
    }

    void addTagsToMessage(String message, String... tags) {
        ArrayList<String> messageTags = this.content.get(message);
        for (String tag : tags) {
            messageTags.add(tag);
        }
    }

    void paintMap() {
        for (Map.Entry<String, ArrayList<String>> entry : content.entrySet()) {
            System.out.print("Message: " + entry.getKey() + " tags: {");
            for (String tag : entry.getValue()) {
                System.out.print(tag + " ");
            }
            System.out.println("}");
        }
    }

}

As you can see, it's not the same to add more information to a previous Map.Entry than to add a new Map.Entry to your HashMap.

Of course, if you create a new instance of an object each time that you want to add information to your Map, and you put that information with the same key that already exists, you're overriding the value in the Map.

Hi, while this was not the solution I really needed, probably since my question itself was quite ambiguous, but this did help in clearing up the concept, so thank you! — user1677804, May 22 '19 at 08:49

score 0 · Accepted Answer · answered May 22 '19 at 08:46

Figured out what I really needed. I wanted to create a copy of a value in a HashMap, what I had been doing was copying over the reference to the object, which is why it kept getting overridden. What I really needed was a copy constructor. This helped: https://stackoverflow.com/a/33547834/1677804

Value overridden in hashmap

2 Answers2