Any difference between String = null and String.isEmpty?

Question

Is there any difference when using a if-statement to check if the string is empty by using String = null or String.isEmpty() ?

ie:

public String name;

if(name == null)
{
    //do something
}

or

public String name;

if(name.isEmpty())
{
    //do something
}

if there is any different (including performance issues) please let me know.

Answer 1

The empty string is a string with zero length. The null value is not having a string at all.

• The expression s == null will return false if s is an empty string.
• The second version will throw a NullPointerException if the string is null.

Here's a table showing the differences:

+-------+-----------+----------------------+
| s     | s == null | s.isEmpty()          |
+-------+-----------+----------------------+
| null  | true      | NullPointerException |
| ""    | false     | true                 |
| "foo" | false     | false                |
+-------+-----------+----------------------+

Answer 2

The variable name isn't a String. It's a reference to a String.

Hence the null check determines if name actually references a String. If it does, then (and only then) can you perform a further check to see if it's empty. i.e.

String name = null;  // no string
String name = "";    // an 'empty' string

are two different cases. Note that if you don't check for nullness first, then you'll try and call a method on a null reference and that's when you get the dreaded NullPointerException