Cast object to unknown data type

Question

I am working on my project where I need to check if a specific value is equal or not to a value on a dataset.

This is the function I'm creating:

public void setSingleLimit(String columnName, Object limit){

}

where columnName is the column of my dataset I will be applying the function to and limit is the value used in the equality condition check.

So basically, this function will take the limit value and check if all the values in columnName are equal or not that value.

However, I do not know before hand the type of the column. I can, however, get the data type of the column in the function.

dataset.first().get(columnName).getClass() gives me the class of the values in a column (we can safely and securely assume all the values in a column will belong to the same class/data type)

I would like to cast the limit object received in the function to the columns class type in order to be able to do a comparison.

I KNOW I can override the function so it can accept different values (int, long, Boolean, String, etc.) and that will certainly work. However, I want to create a single method for less complexity and for easy understanding to my end user. We can also securely assume that the user will enter a correct value on the function, meaning that if the column is a column of Strings, the user will NOT put a value that is not a String.

Any idea of how I can cast an object to a class that is not known before hand?

Thank you.

I think you will have to use java reflection, like [this] (https://stackoverflow.com/questions/14319760/java-reflection-casting-to-an-unknown-object?rq=1) but I'm not able to to help you further more — Bruno Zamengo, Jul 27 '17 at 14:42
You say you want to check for equality. Why can't you just use `equals(Object o)`? It is declared on `Object` and accepts an `Object` as parameter. The implementation will be bound dynamically at runtime. — André Stannek, Jul 27 '17 at 14:44
do you know subset of possible types which can go inside? If so then you can create private methods with such parameters and in public method just use `if (limit instanceof )` or `if (.getClass().isAssignableFrom(limit.getClass()))`. I know it is so ugly, but you can live with it if there isn't too much types of limit. — bilak, Jul 27 '17 at 14:48

score 2 · Accepted Answer · answered Jul 27 '17 at 15:11

You say you want to check for equality. Just use equals(Object o). It is declared on Object and accepts an Object as parameter. The implementation will be bound dynamically at runtime.

public void setSingleLimit(String columnName, Object limit){
    Object columnValue = getValue(columnName);
    limit.equals(columnValue); // If limit can be null, swap variables
}

Example: If limit is a String, equals() from String will be used. It doesn't matter if columnValue is not a String because it will just return false in that case.

Just make sure the different equals() implmentations do what you want. If you use custom classes you can overwrite it yourself.

score 1 · Answer 2 · answered Jul 27 '17 at 14:49

1

Without using cast, I would do like that :

if(limit instanceof anyDataset) {
//Compare
}

answered Jul 27 '17 at 14:49

Logan Wlv

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score 1 · Answer 3 · answered Jul 27 '17 at 14:51

You can use the instanceof comparison to ensure type safety when comparing and casting your object.

For example:

if (limit instanceof Boolean) { // do something }

This will avoid ClassCastException and ensures you're doing exactly what you want with the exact data-type you're expecting.

Cast object to unknown data type

3 Answers3