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 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.