Question

I have a string, which is read from a database. The string can be just “null”. I need to decide whether it is null or not? Among the following ones, what is the appropriate way to do it?

String a = …;
If  (a == null)

If ( a.length == 0)

I also see something like

If  a.equals(“ “)

How does this work?

Answer 1

if (a == null)

In Java, == compares references, so you can check to see whether the reference a refers to null. a.length and a.equals would throw NullPointerExceptions if a equaled null, since you can’t call methods on null.

When you compare Strings with equals, that:

is true if and only if the
argument is not null and is a String
object that represents the same
sequence of characters as this object.

So even if we could call equals on null (which we can’t, as mentioned before), it’d never return true anyway, as according to the documentation.

String.length would return zero only if a equaled "", an empty string. As according to the method:

Returns the length of this string. The
length is equal to the number of
Unicode code units in the string.

When in doubt, just test it out! 🙂