1

The idea is simple I don't need to add objects that are already on the list. I guess the problem is that it creates a completely new object and it doesn't find a match with other identical objects that are already on the list(I might be completely wrong).

import java.util.Scanner;
import java.util.ArrayList;

public class archive {
    
    private String indentifier;
    private String name;
    
    public archive(String indentifier,String name) {
        this.name = name;
        this.indentifier = indentifier;
    }
    public String toString() {
        return this.indentifier + ": "+this.name;
    }

    public static void main(String[] args) {
        ArrayList<archive> arhivs = new ArrayList<>();
        Scanner scan = new Scanner(System.in);
        String indentifier, name;
        while(true) {
            System.out.println("Indentifier? (empty will stop)");
            indentifier = scan.nextLine();
            if(indentifier.isEmpty()) {
                break;
            }
            System.out.println("Name?(empty will stop)");
            name = scan.nextLine();
            if(name.isEmpty()) {
                break;
            }
            archive newAr = new archive(indentifier,name);
            
            if(arhivs.contains(newAr)) {
                System.out.println("Item is on the list");
            }else {
                arhivs.add(newAr);
            }
            
            
        }
        System.out.println("==Items==");
        for(archive arhivi:arhivs) {
            System.out.println(arhivi);
        }
        scan.close();

    }

}
user16731988
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  • Class names in Java should start with an uppercase letter. – Basil Bourque Jan 16 '22 at 22:05
  • Well yes, if you construct a new `archive` it will be a unique object and not equal to anything in your ArrayList. You can instead loop through and compare the `indentifier` and/or `name` values to check for duplicates. – skara9 Jan 16 '22 at 22:10
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    the contains method uses the equals() method to check if objects are equal. Since your `archive` class does not overwrite `equals`, this doesn't do what you expect. – f1sh Jan 16 '22 at 22:11
  • So I have to make my own equals method and loop through each object? – user16731988 Jan 16 '22 at 22:13
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    @user16731988 not a loop, just override the `equals` method to compare whichever values you want – skara9 Jan 16 '22 at 22:15

1 Answers1

2

Override equals (& hashCode)

You neglected to provide overrides of the inherited Object#equals and Object#hashCode methods.

The default implementation of equals uses identity to compare, that is, "Do the two object references being compared share the same implicit memory address?".

Instead, you want to compare the contents of the two objects. So you need to write that content-comparison code in your own equals method. And whenever overriding equals, it is generally best to override hashCode using the same logic.

Your IDE will assist you in writing that code. For example, in IntelliJ, see this documentation and this discussion.

This issue has been covered many times on Stack Overflow. So search to learn more.

Records

By the way, you can more briefly define your class as a record.

record Archive ( String identifier , String name ) {}

A record is appropriate when the main purpose of your class is to communicate data transparently and immutably.

The compiler implicitly creates the constructor, getters, equals & hashCode, and toString methods.

So using a record makes your problem moot, assuming you want to compare each and every member field on the class. The equals & hashCode overrides are provided for you.

Basil Bourque
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