Question

What are the best practices if you have a class which accepts some parameters but none of them are allowed to be null?

The following is obvious but the exception is a little unspecific:

public class SomeClass
{
     public SomeClass(Object one, Object two)
     {
        if (one == null || two == null)
        {
            throw new IllegalArgumentException("Parameters can't be null");
        }
        //...
     }
}

Here the exceptions let you know which parameter is null, but the constructor is now pretty ugly:

public class SomeClass
{
     public SomeClass(Object one, Object two)
     {
        if (one == null)
        {
            throw new IllegalArgumentException("one can't be null");
        }           
        if (two == null)
        {
            throw new IllegalArgumentException("two can't be null");
        }
        //...
  }

Here the constructor is neater, but now the constructor code isn’t really in the constructor:

public class SomeClass
{
     public SomeClass(Object one, Object two)
     {
        setOne(one);
        setTwo(two);
     }


     public void setOne(Object one)
     {
        if (one == null)
        {
            throw new IllegalArgumentException("one can't be null");
        }           
        //...
     }

     public void setTwo(Object two)
     {
        if (two == null)
        {
            throw new IllegalArgumentException("two can't be null");
        }
        //...
     }
  }

Which of these styles is best?

Or is there an alternative which is more widely accepted?

Answer 1

The second or the third.

Because it tells the user of your API what exactly went wrong.

For less verbosity use Validate.notNull(obj, message) from commons-lang. Thus your constructor will look like:

public SomeClass(Object one, Object two) {
    Validate.notNull(one, "one can't be null");
    Validate.notNull(two, "two can't be null");
    ...
}

Placing the check in the setter is also acceptable, with the same verbosity comment. If your setters also have the role of preserving object consistency, you can choose the third as well.