JAVA cast to Object to ArrayList doesn't work

Question

About problem

Eventually when i try cast Object to ArrayList i've problem when i use method from ArrayList like .size(), .get():

That the error returned to me:

error: cannot find symbol

Source

public String parseValueParam(Object value) throws InvalidAttribute{

        if(value instanceof ArrayList){
            value =  (ArrayList) value;

            String result = ""; 

            for(int i = 0, _len = value.size(); i < _len; i ++){
                result += this.parseValueParam(value.get(i));
                if (i < _len - 1)   result += " , ";
            };
            return result;

        } else if(helper.isString(value)){
        
            return value.toString().replaceAll("'", "");

        } else if (helper.isInteger(value) || helper.isDouble(value)){
        
            return value.toString();

        } else {
        
            throw new InvalidAttribute("Invalid attribute type");
        }
    }

I've tried to declare couple of methods with same name one using Object to accept String, Int and Double and another only accept ArrayList. However when i run my code execute only the use Object.

public String parseValueParam(ArrayList value) throws InvalidAttribute{

    String result = ""; 
    for(int i = 0, _len = value.size(); i < _len; i ++){
        result += this.parseValueParam(value.get(i));
        if (i < _len - 1)   result += " , ";
    };
    return result;
}

Is there a reason that you couldn't just change the signature to accept List if you know you're going to be dealing with lists anyway? — Makoto
– Makoto, Commented Oct 19, 2014 at 1:40
My method accept many kind of values and turn it to String. List is only to send many values of same parameter of SQL Query. So this resource i'll use only in sometimes. — Guilherme Soares
– Guilherme Soares, Commented Oct 19, 2014 at 1:49
So...why not a List<?>? Besides, you don't care what's in the list, you only care that it is a List. — Makoto
– Makoto, Commented Oct 19, 2014 at 1:51

afzalex · Accepted Answer · 2014-10-19 01:43:26Z

3

value is declared to be of Object type.
when you explicitly parsed it to ArrayList object, you still have assigned it to value variable. The type of value is still Object, it cannot be changed.
Use another variable of ArrayList, and use that variable instead.

ArrayList value2;
if(value instanceof ArrayList){
    value2 =  (ArrayList) value;
    String result = ""; 

    for(int i = 0, _len = value2.size(); i < _len; i ++){
     ....

answered Oct 19, 2014 at 1:43

afzalex

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5 Comments

Guilherme Soares Over a year ago

I tried this but i think it is bad pratice to resolve this problem.

nmore Over a year ago

Why is it bad practice? It is a necessary extra pointer (only 4 bytes).

afzalex Over a year ago

No, it is not. why do you think so? You are needed to create a local variable and use this variable. And if you need to use an Object which is an instance of other type, you can call the required method just when you casted it : ((ArrayList)myObject).size()

Stephen Buttolph Over a year ago

I wouldn't consider this bad practice, however if you really don't want to use another variable, you could cast it when you need the object to be an ArrayList. That being said, I would use the extra variable.

Guilherme Soares Over a year ago

Okay i understood. I thought when created another variable to cast. I wil duplicate use of memory

Mitch Talmadge · Accepted Answer · 2014-10-19 01:46:56Z

0

The problem is that even though you are casting value to ArrayList here:

value =  (ArrayList) value;

The variable value is still declared to be an Object, so casting really did nothing for you.

You need to make a new variable, like this:

ArrayList list = (ArrayList) value;

Then just use the new variable, list.

answered Oct 19, 2014 at 1:46

Mitch Talmadge

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