How to check if my string is equal to null?

Question

I want to perform some action ONLY IF my string has a meaningful value. So, I tried this.

if (!myString.equals("")) {
doSomething
}

and this

if (!myString.equals(null)) {
doSomething
}

and this

if ( (!myString.equals("")) && (!myString.equals(null))) {
doSomething
}

and this

if ( (!myString.equals("")) && (myString!=null)) {
doSomething
}

and this

if ( myString.length()>0) {
doSomething
}

And in all cases my program doSomething in spite on the fact that my string IS EMPTY. It equals to null. So, what is wrong with that?

ADDED:

I found the reason of the problem. The variable was declared as a string and, as a consequence, null assigned to this variable was transformed to "null"! So, if (!myString.equals("null")) works.

Answer 1

if (myString != null && !myString.isEmpty()) {
  // doSomething
}

---

As further comment, you should be aware of this term in the equals contract:

From Object.equals(Object):

For any non-null reference value x, x.equals(null) should return false.

The way to compare with null is to use x == null and x != null.

Moreover, x.field and x.method() throws NullPointerException if x == null.

Answer 2

You misspelled EQUALS in all those examples.

Answer 3

You need to check that the myString object is null

if (myString != null) {
doSomething
}

Answer 4

Try,

myString!=null && myString.length()>0

Answer 5

If myString is null, then calling myString.equals(null) or myString.equals("") will fail with a NullPointerException. You cannot call any instance methods on a null variable.

Check for null first like this:

if (myString != null && !myString.equals("")) {
    //do something
}

This makes use of short-circuit evaluation to not attempt the .equals if myString fails the null check.

Answer 6

If your string is null, calls like this should throw a NullReferenceException:

myString.equals(null)

But anyway, I think a method like this is what you want:

public static class StringUtils
{
    public static bool isNullOrEmpty(String myString)
    {
         return myString == null || "".equals(myString);
    }
}

Then in your code, you can do things like this:

if (!StringUtils.isNullOrEmpty(myString))
{
    doSomething();
}

Answer 7

This should work:

if (myString != null && !myString.equals(""))
    doSomething
}

If not, then myString likely has a value that you are not expecting. Try printing it out like this:

System.out.println("+" + myString + "+");

Using the '+' symbols to surround the string will show you if there is extra whitespace in there that you're not accounting for.

Answer 8

 if (myString != null && myString.length() > 0) {

        // your magic here

 }

Incidently, if you are doing much string manipulation, there's a great Spring class with all sorts of useful methods:

http://static.springsource.org/spring/docs/2.5.x/api/org/springframework/util/StringUtils.html

Answer 9

I'd do something like this:

( myString != null && myString.length() > 0 )
    ? doSomething() : System.out.println("Non valid String");

• Testing for null checks whether myString contains an instance of String.
• length() returns the length and is equivalent to equals("").
• Checking if myString is null first will avoid a NullPointerException.