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. ie

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

Answer 3

用“”赋值的字符串不包含任何值而是空的（长度=0），未实例化的字符串为空。

Answer 4

isEmpty() checks for empty string "" ,

it will throw NullPointerException if you invoke isEmpty() on null instance

Answer 5

If you apply this code:

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

when name is null, you'll get NullPointerException .

null checking shows you whether there is an object generally.
isEmpty checking shows you whether the content of existing String object is empty.

Answer 6

Look at source code of your java version.

For example, in openjdk-7: http://www.docjar.com/html/api/java/lang/String.java.html

  119       /** The count is the number of characters in the String. */
  120       private final int count;

  663       /**
  664        * Returns <tt>true</tt> if, and only if, {@link #length()} is <tt>0</tt>.
  665        *
  666        * @return <tt>true</tt> if {@link #length()} is <tt>0</tt>, otherwise
  667        * <tt>false</tt>
  668        *
  669        * @since 1.6
  670        */
  671       public boolean isEmpty() {
  672           return count == 0;
  673       }

Answer 7

isEmpty checks for String "". Best practise is to check:

if (str != null && !str.isEmpty() {
   // process string
}

Answer 8

I had this issue this week while modifying some old JaVa code and i learn here that i must always make all those check. The answer is indeed correct but i find it hard to remember each time so i decide to make a little function that do it for me in 1 simple call.

whit this, you always get the answer you want :

      public boolean StringIsNull(String pi_sChaine)
        {  boolean bTrueOrFalse = true;

           if (pi_sChaine == null || pi_sChaine.isEmpty())
             { bTrueOrFalse = true; }
           else
             { bTrueOrFalse = false; }

           return bTrueOrFalse;
        }