Skip to main content

Java String NullPointerException safe equals check

If you compare two Strings in Java, normally you would use the equals method to compare them to see if they are similar.

This will be some common use case for input validations. However, if you don't do it properly it may cause you to throw a NullPointerException.

This is pretty much like common sense, but I've seen even more experienced developers make this mistake.

Let's consider the following code.


 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
public class DemoApplication {

 final static String theStringIknow = "Hello";

 public static void myTestMethod(String someString) {
                //do not do this
  if (someString.equals(theStringIknow)) {
   System.out.println("Same same");
  }

 }

 public static void main(String[] args) {
  String testString = "Hello";

  myTestMethod(testString);

 }

}


In myTestMethod, I'm checking if the String in the argument equals to another String that I know.
This code works perfectly fine if you run it, but this is not the best way to do this.

For some reason, if the input String to myTestMethod happens to be null, then this will throw a NullPointerException.


 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
public class DemoApplication {

 final static String theStringIknow = "Hello";

 public static void myTestMethod(String someString) {
                //do not do this
  if (someString.equals(theStringIknow)) {
   System.out.println("Same same");
  }

 }

 public static void main(String[] args) {
  String testString = null;

  myTestMethod(testString);

 }

}


Exception in thread "main" java.lang.NullPointerException
 at com.example.demo.DemoApplication.myTestMethod(DemoApplication.java:7)
 at...

This is because, in the if condition, you are first using the unknown string's equals method to compare the other string. In this case, the unknown string is null, which means it doesn't exist. Thus a NullPointerException.

The better way to do it will be like follows.


  if (theStringIknow.equals(someString)) {
   System.out.println("Same same");
  }

In that case, you are always invoking the equals method of a String that you are pretty sure it exists. Usually, it could be a constant or a new String object.

If you are not really sure either of them exists, so you better to check for null for the String that you are trying to invoke the equals method of.



 if (someString != null && someString .equals(someOtherString)) {
  System.out.println("Same same");
 }


Happy coding!






Labels:

Comments

Post a Comment

Popular posts from this blog

VLC skins

It's been a while from my first blog post, so today I thought to make a blog post about skins in VLC player. I think you all are familiar with VLC player if not, here's the link for the website www.videolan.org/vlc/index.htm l In brief, VLC player is a free and opensource player that can play almost all the types of media formats. Even though it is a such a nice player, it looks kind of old and rusty, like the windows classic look. But you can make it prettier by just adding some new skins to it.  You can download skins for the player from here http://www.videolan.org/vlc/skins.php Then place the downloaded skin file (.vlt file) in the " skin " folder in the installation directory.           eg:- in windows:    C:\Program Files\VideoLAN\VLC\skins           or in linux systems:  ~/.local/share/vlc/skins Then, open the VLC player and go to the preferences. ...
2 comments
Read more

Java, how to create a list with a single element

 I wanted to create a Java List with a single element. Yet, I wanted to add more elements later. So, I was looking for a couple of ways to do this. So far there are multiple elegant ways to create a list with a single element with a one-liner. Not so much for a modifiable list though. Here's what I gathered so far. Followings are a few ways of creating a list with strictly a single entry. Can't add more elements. 1. Collections.singletonList() This returns an immutable list that cannot be modified or add more elements. // An immutable list containing only the specified object. List<String> oneEntryList = Collections. singletonList ( "one" ) ; oneEntryList.add( "two" ) ; // throws UnsupportedOperationException 2.  Arrays.asList() This returns a fixed-size list consisting of the number of elements provided as arguments. The number of elements provided would be an array hence the size is fixed to the length of the array. // Returns a fixed-size list List...
Post a Comment
Read more

Facebook videos are too loud

Not sure if it is only me, but as of this writing, I find some of the Facebook videos are too loud compared with other websites or players on the machine. Whenever I play a video, I usually turn the volume down. Then whenever there's an ad, it screams in my ears. Same when I play the next video, the former adjusted volume level doesn't stick. This could be a bug in FB, so I even suggested a feature improvement.  As a solution, I found this chrome plugin. Sound and video fixes for Facebook Don't know who developed it, but the dude has fixed one of Facebook's problems, and many thanks to the developer. I might as well help him out as it is an open-source project. No more screaming videos! 
3 comments
Read more