Question

I am creating a library for image loading and caching. I want the user programmer to select the file extension between .png and .jpeg that is want to it to be selected from this 2 only and if programmer types another extension then it must show error. How can I do so?

Answer 1

If i understood your question right, you want to create a library, where you handle images, and when the programmer, who uses your library wants to instantiate your image handling class/classes, you want him/her to choose between jpg and png?

If so, your best option is to create an enum:

public enum ImageType
{
    TYPE_JPG, TYPE_PNG;
} 

And in your actual image-manipulating class:

public class MyImageHandler
{
    public MyImageHandler(ImageType type)
    {
        //You still need a nullcheck
        if(type == null) 
            throw new NullPointerException("null is not accepted!");

        if(type == ImageType.TYPE_JPG)
        {
            //JPG chosen
        }
        else if(type == ImageType.TYPE_PNG)
        {
            //PNG chosen
        }
    }
}

When the user-programmer uses it:

MyImageHandler handler = new MyImageHandler(ImageType.PNG); //correct
MyImageHandler handler = new MyImageHandler(ImageType.JPG); //correct
MyImageHandler handler = new MyImageHandler(null); //runtime error