## com.google.common.collect
Interface Constraint<E>

    @Beta
    @GwtCompatible
    public interface Constraint<E>

A constraint that an element must satisfy in order to be added to a collection. For example, `Constraints.notNull()`, which prevents a collection from including any null elements, could be implemented like this:

       public Object checkElement(Object element) {
         if (element == null) {
           throw new NullPointerException();
         }
         return element;
       }

In order to be effective, constraints should be deterministic; that is, they should not depend on state that can change (such as external state, random variables, and time) and should only depend on the value of the passed-in element. A non-deterministic constraint cannot reliably enforce that all the collection's elements meet the constraint, since the constraint is only enforced when elements are added.

Since:
  3.0
Author:
  Mike Bostock
See Also:
  `Constraints`, `MapConstraint`

Method Summary
`E` `checkElement(E element)` Throws a suitable `RuntimeException` if the specified element is illegal.
`String` `toString()` Returns a brief human readable description of this constraint, such as "Not null" or "Positive number".

Method Detail

### checkElement

    E checkElement(E element)

  Throws a suitable `RuntimeException` if the specified element is illegal. Typically this is either a `NullPointerException`, an `IllegalArgumentException`, or a `ClassCastException`, though an application-specific exception class may be used if appropriate.

  Parameters:
    `element` - the element to check
  Returns:
    the provided element

### toString

    String toString()

  Returns a brief human readable description of this constraint, such as "Not null" or "Positive number".

  Overrides:
    `toString` in class `Object`

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