Java how to prevent null object exception

Question

I am trying to write less and less code and I am trying to find a way to prevent crashes.

An example what I have encountered is:

public class MyClass
{
   private User user;

   public MyClass()
   {
       // Get user from another class
       // Another thread, user can be null for couple of seconds or minutes
       // Asynchronous call
       user = AnotherClass.getUser();

       // start method
       go();
   }

   private void go()
   {
      // Method 1
      // Program it is crashing if user is null
      if (user.getId() == 155)
      {
         // TO DO
      }
      else
      {
         System.out.println("User is NOT 155 !");
      }

      // Method 2
      // Program still crashes if user is null
      if (user != null && user.getId() == 155)
      {
         // To do
      }
      else
      {
         System.out.println("user is not 155");
      }

      // Method 3
      // Program wont crash, but I write much more code !
      if (user != null)
      {
         if (user.getId() == 155)
         {
            // To do
         }
         else
         {
            System.out.println("User is not 155 !");
         }
      }
      else
      {
          System.out.println("User is not 155 !");
      }
   }
}

As you can see, method 3 it's working, but I am writing much more code... What should I do?

method 2 should work too. recheck it. in java if first part is false second part not evaluated. — Leonidos
– Leonidos, Commented Oct 23, 2013 at 14:04
The point is not that it doesn't work, but that it's relatively verbose — Brian Agnew
– Brian Agnew, Commented Oct 23, 2013 at 14:06

Suresh Atta · Accepted Answer · 2013-10-23 14:11:41Z

2

Prefer the way Short-circuit evaluation, That is method 2.

when the first argument of the AND function evaluates to false, the overall value must be false;

      if (user != null && user.getId() == 155)
      {
         // To do
      }
      else
      {
         System.out.println("user is not 155");
      }

That is the most preferable and readable code.

Your assumtions are wrong that method2 crash and method3 works. In the above code if user != null then only user.getId() == 155 executes.

edited Oct 23, 2013 at 14:11

answered Oct 23, 2013 at 14:04

Suresh Atta

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2 Comments

Zbarcea Christian Over a year ago

What if I swap the two conditions? if (user.getId() == 155 && user != null) ?

Suresh Atta Over a year ago

@ZbarceaChristian You end up with Null pointer exception. If you swap those conditions then its like if (user.getId() == 155) {if(user != null){}, which does'nt make sense.

Brian Agnew · Accepted Answer · 2013-10-23 14:04:04Z

1

Why not use a null object pattern here, so instead of setting user to be null, set it to a special 'null' case (implementation) of the User object ?

e.g.

user = AnotherClass.getUser();
if (user == null) {
   user = new NullUser();
}

(ideally AnotherClass.getUser() would do the null check internally)

In this case

user.getId()

could return a special value (-1 ?) which would never equate to a valid user id. Hence your code will always look like:

if (user.getId() == 155)

The same would apply to other methods on the User object.

answered Oct 23, 2013 at 14:04

Brian Agnew

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2 Comments

Zbarcea Christian Over a year ago

What if I edit the User class and add a field id with a default value of -1 ? private int id = -1; public User(/* params */){}

Brian Agnew Over a year ago

Perhaps that would be suitable too

Mike Thomsen · Accepted Answer · 2013-10-23 14:04:05Z

1

It's got to be something inside the block started by this statement:

if (user != null && user.getId() == 155)

That is logically identical to method 3. When the JVM sees that user is null, it should stop evaluating that.

I will say though that I encountered something like this with JVM 1.3, so if you are using a really old JVM that may be it.

answered Oct 23, 2013 at 14:04

Mike Thomsen

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