Adding elements to an ArrayList

[ArcherSam]

Problem

    
    /**
     * Calculates the gross waste emission
     */
    public void calcGrossWasteEmission()
    {
        // gross waste emission equals CO2 generated by individual in household times number of people
        household.add(1018 * household.get(0)); // ERROR: Operator * cannot be applied to int, java.lang.Object
    }

Desired solution is to be able to add an element to ArrayList household! :)

Sam

[JosAH]

    
    /**
     * Calculates the gross waste emission
     */
    public void calcGrossWasteEmission()
    {
        // gross waste emission equals CO2 generated by individual in household times number of people
        household.add(1018 * household.get(0)); // ERROR: Operator * cannot be applied to int, java.lang.Object
    }

Sam

What is the type of variable 'household'? Is it a raw list? If so each of its elements is of type Object which explains the error message. You should change it to a type List<Integer> ...

kind regards,

Jos

[ArcherSam]

The type of household is an ArrayList object.

private ArrayList household;

Also changing the ArrayList to Integer won't allow me to add bool type to the list. I'm required to use an array with int and bool elements. Is another solution possible?

Sam

[JosAH]

The type of household is ArrayList object.

private ArrayList household;

Sam
Please don't be afraid to correct my text. I'm not sure if i'm using the correct terms.

You household variable stores elements of type Object (hence the error message); change its type to ArrayList<Integer> and the compiler and autoboxing will do the rest.

kind regards,

Jos

[ArcherSam]

Thank You!

Sam

[JosAH]

Also changing the ArrayList to Integer won't allow me to add bool type to the list. I'm required to use an array with int and bool elements. Is another solution possible?

(this remark was added to the reply by the OP later)

You can also create a List<?> and you can store every type of object in it. You do have to check the actual type of the element(s) when you get them from the list (use the 'instanceof' operator).

kind regards,

Jos

[golan2]

AFAIK you can’t do such a thing!
You can have your ArrayList generic with no specific type but what you insert into the list MUST be something which is valid for the “*” operator.
The Boolean is not!

You could have tried changing the code to:
household.add(1018 * (Integer) household.get(0));
You basically tell the compiler: “I know what I am doing just cast the Object to Integer and you can have my word for it”.
But during runtime if the household.get(0) contains a Boolean the cast will fail and a ClassCastException will be thrown.

Now the question that should be asked is: “What are you trying to do? Adding Booleans to Integers is not something we do every day”.
Would you like Boolean.False to be considered as “0”? What about Boolean.True? “1”? “-1”?
Think of what you want to do and post back.

[JosAH]

AFAIK you can’t do such a thing!
You can have your ArrayList generic with no specific type but what you insert into the list MUST be something which is valid for the “*” operator.
The Boolean is not!

You could have tried changing the code to:
household.add(1018 * (Integer) household.get(0));
You basically tell the compiler: “I know what I am doing just cast the Object to Integer and you can have my word for it”.
But during runtime if the household.get(0) contains a Boolean the cast will fail and a ClassCastException will be thrown.

Of course you can't do that but it's not the fault of the List<?> and that's what the OP wanted: a List that is capable of storing Booleans and Integers. That's the reason I wrote to use the 'instanceof' operator before casting an element from that list and manipulate it.

kind regards,

Jos