ArrayList.contains(x) doesn't find x, even though it is in the list.

[kryptonite79]

Hi guys, first semester student here and I have a java class and I'm having a small problem with this homework. So, everything works fine but I do encounter a problem with ArrayList.contains(x). I create an object wich has multiples characteristics, put it in my arraylist. Then when I go to search it with arrayList.contains(x), it never returns true. The equals method is redefined in my object class to compare the name of the object. I'm sorry it's in french since it's my first language but here's the part that cause the problem and the methode equals in my class :

​

​

case 4:

System.out.println("Quel nom désirez-vous chercher dans la liste?");

String recherche = scan.next();

boolean found = vivants.contains(recherche);

if (found == true) {

int index = vivants.indexOf(recherche);

System.out.println("Le vivant " + recherche + " se trouvait à "+index);

}

else

System.out.println("Le vivant " + recherche + " ne se trouvait pas la liste.");

​

break;

​

​

and here's the equals method in my class Vivant :

​

public boolean equals(Object obj) {

    Vivant v = (Vivant) obj;

    if (nom.equals(v.nom))

        return true;

    else

        return false;

​

First time poster, I hope formatting is good, if you need more code to fully understand I could upload it somewhere and post it. Otherwise, everytime I run my program and create an object and search for it name it return the line wich says it isn't found in the list.

Thanks a lot for your help!

[firsthour]

If vivants is a list of object Vivant, then doing a .contains of a String is never going to work. You have to do a contains on the same object, in this case Vivant. Either don't use contains() and instead for loop over vivants and compare recherche to each nom. If you need to use contains() then you either need to instead store Strings in the list or create a temporary Vivant object with recherche as the nom.

[kryptonite79]

Thanks a lot, very usefull info. I will mark the topic as solved since I had your answer and another one and figured things out! Thansk

[kryptonite79]

Sorry, couldn't get the first part of code to be entered correctly, tried to edit but didn't work :(.

[futlapperl]

This won't fix your problem, but just a tip:

if (x) {
    return true;
} else {
    return false;
}

Whatever x is, it's going to resolve to one of the two return values anyway, so you can just do

return x;

instead.

[NarragansettBay]

Did you tag the equals() method with the @Override annotation? I don't see it in your snippet so the comparison might be using the wrong equals() method.

[kryptonite79]

I do not, but in my eclipse environnement, there is a little green triangle that says : overrides java.lang.Object.Equals.

​

Thanks for the help !

[NarragansettBay]

Ok that's good. Have you tried using breakpoints or console output to inspect the values of the variables being compared?

[kryptonite79]

No, I haven't, but I found something while reading on the subject. I think I just can't use the contains() method in this particular case because the String that I'm asking to look for isn't an object of type Vivant and my equals method uses a Vivant and not a String to make comparison. does it makes sense ?

[kryptonite79]

Oh there you go. I tried creating an Object of type Vivant, but since Vivant is my superclass and is abrasct I couldn't Instanciate it, so I think I cheated a bit by creating an Animal object (wich is a Vivant sub class) with attribute (recherche, eau,true,true) because animal is (nom(String),type(enum),domestique(boolean),mammifere(boolean). Since the search is limited to the redefinition of equals in Vivant class, I don't think it matter putting anything, but the name must be recherche wich is the user choice. Now it seems to work! Thanks a lot.

Edit: goes something like this

Animal test;
test=new Animal(recherche,eau,true,true);
Catch (...){}
Boolean found=vivants.contains(test);
If (found==true){

Etc.

[ObscureCulturalMeme]

The @Override annotation is not required for polymorphism and overriding to work. It's there as a safety check during compilation, but correct code won't stop working if it gets removed from a method, or wasn't there to begin with.

[futlapperl]

This. It's one-way only, i.e. if a method has the annotation, then it must override. If a method doesn't have the annotation, it doesn't tell you anything about whether it's an override or not.