generic method

[Slawomir Wieczorek]

Hi I need a solution how to create a generic method....
If it is an Integer ArrayList - it adds a random int variable into it and when it is a Double - it adds a random double...
So how to check whether ArrayList<T> is an instanceof Integer or Double and then how to add next random int/double into it...

[Paul Clapham]

Slawomir Wieczorek wrote:Hi I need a solution how to create a generic method....
If it is an Integer ArrayList - it adds a random int variable into it and when it is a Double - it adds a random double...

Why would you need to do that?

So how to check whether ArrayList<T> is an instanceof Integer or Double and then how to add next random int/double into it...

You can't. At run-time they are both just ArrayList objects.

Of course there's probably some way to implement your actual requirement, but not the one you asked about here.

[Slawomir Wieczorek]

Paul Clapham wrote:
Why would you need to do that?

There is no need for that, just a training purpose
Let me be more precise ..
Suppose I have a class that stores objects of type Integer or Double (depending which one you choose) in the ArrayList:

public class TheMostImportantClassEver<T extends Number> {
	private ArrayList<T> data;	
	
	public TheMostImportantClassEver(int size) {
		int initCapacity = size > 0 ? size : 10; //check if size positive 
		data = new ArrayList<T>(initCapacity);
		
		//if the type-parameter is Integer
			//add let's say 10 randomly created int variables (for example from 0 to 99)
		//if the type-parameter is Double
			//add 10 randomly created double variables (for example from 0.0 to 9.0)		
		
	} // end TheMostImportantClassEver constructor	
}// end class TheMostImportantClassEver

So when instantiate such a class:

public class TheMostImportantClassEverTest {
	public static void main(String[] args) {
		TheMostImportantClassEver<Integer> tMICE = new TheMostImportantClassEver<Integer>(10);                
	}// end method main(args)
}// end class TheMostImportantClassEverTest

I will get an object that has an ArrayList<Integer> with 10 random numbers.....
Does it make any sense or rather there's something wrong with me....

[Paul Clapham]

The usual way for a generic class to know about one of its parameterized types is for the class to ask to be given that type at run-time. Like this:

public class SomeUninterestingClass<T extends Number> {  
    private ArrayList<T> data;      
      
    public SomeUninterestingClass(int size, Class<? extends T> clazz) {  
        int initCapacity = size > 0 ? size : 10; //check if size positive   
        data = new ArrayList<T>(initCapacity);
        if (clazz == Integer.class) {
          // do the thing with integers
        }
        // and so on
    }
}

Then to create one of those things:

SomeUninterestingClass<Double> hoHum = new SomeUninterestingClass<Double>(42, Double.class);

[Slawomir Wieczorek]

Thank you Paul

[Campbell Ritchie]

Those if (clazz == Foo.class) tests look very unlike object-oriented programming to me. It suggests you need to goback and reconsider your original design.