| There are various libraries available to check if a variable is null or not. |
| Most notable ones are : |
| |
| * `com.google.common.base.Preconditions.checkNotNull` |
| * `com.google.common.base.Verify.verifyNotNull` |
| * `java.util.Objects.requireNonNull` |
| |
| These methods generally takes two arguments. The first is the reference that |
| should be non-null. The second is the error message to print (usually a string |
| literal). |
| |
| There are some common scenarios where the first argument can't be null but is |
| still checked to be null and is thus redundant. |
| |
| * By specification, a constructor cannot return null. So if the argument is an |
| object creation or array creation expression, checking can be skipped. |
| |
| * Often the order of the two arguments is swapped, and the reference is never |
| actually checked for nullity. This check ensures that the first argument to |
| such methods is not a literal. |
| |
| * When a primitive is passed as the argument to check, the primitive will be |
| [autoboxed](http://docs.oracle.com/javase/7/docs/technotes/guides/language/autoboxing.html) |
| into a boxed object, which is non-null, causing the check to always pass |
| without the condition being evaluated. If the intent was to ensure that the |
| primitive met some criterion (e.g., a boolean that should be non-null), |
| please use `Preconditions.checkState()` or `Preconditions.checkArgument()` |
| instead. |
