Package net.sf.saxon.om

## Class ZeroOrOne<T extends Item>

- java.lang.Object
- - net.sf.saxon.om.ZeroOrOne<T>

- All Implemented Interfaces:
    `GroundedValue`, `Sequence`
  Direct Known Subclasses:
    `One`

      public class ZeroOrOne<T extends Item>
      extends java.lang.Object
      implements GroundedValue

  A value that is a sequence containing zero or one items.

- - ### Constructor Summary

    Constructors
    Constructor | Description
    --- | ---
    `ZeroOrOne​(T item)` | Create a sequence containing zero or one items

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `boolean` | `effectiveBooleanValue()` | Get the effective boolean value
    `static <T extends Item> ZeroOrOne<T>` | `empty()` | Return the instance of ZeroOrOne that represents the empty sequence
    `int` | `getLength()` | Get the length of the sequence
    `java.lang.String` | `getStringValue()` | Convert the value to a string, using the serialization rules.
    `java.lang.CharSequence` | `getStringValueCS()` | Get the string value of this sequence.
    `T` | `head()` | Get the first item in the sequence.
    `T` | `itemAt​(int n)` | Get the n'th item in the sequence (starting from 0).
    `ConstrainedIterator<T>` | `iterate()` | Return an iterator over this value.
    `GroundedValue` | `reduce()` | Reduce the sequence to its simplest form.
    `GroundedValue` | `subsequence​(int start, int length)` | Get a subsequence of the value
    `java.lang.String` | `toString()` | Returns a string representation of the object (used only for diagnostics).

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface net.sf.saxon.om.GroundedValue

      `asIterable, concatenate, containsNode, materialize, toShortString`

    - ### Methods inherited from interface net.sf.saxon.om.Sequence

      `makeRepeatable`

- - ### Constructor Detail

    - #### ZeroOrOne

          public ZeroOrOne​(T item)

      Create a sequence containing zero or one items
      Parameters:
        `item` - The item to be contained in the sequence, or null if the sequence is to be empty

  - ### Method Detail

    - #### empty

          public static <T extends Item> ZeroOrOne<T> empty()

      Return the instance of ZeroOrOne that represents the empty sequence
      Returns:
        a representation of the empty sequence that satisfies the type ZeroOrOne

    - #### getStringValueCS

          public java.lang.CharSequence getStringValueCS()

      Get the string value of this sequence. The string value of an item is the result of applying the string() function. The string value of an empty sequence is the zero-length string.
      Specified by:
        `getStringValueCS` in interface `GroundedValue`
      Returns:
        the string value of the sequence.

    - #### getStringValue

          public java.lang.String getStringValue()

      Convert the value to a string, using the serialization rules. For atomic values this is the same as a cast; for sequence values it gives a space-separated list. For QNames and NOTATIONS, or lists containing them, it fails.
      Specified by:
        `getStringValue` in interface `GroundedValue`
      Returns:
        the string value of the sequence.

    - #### head

          public T head()

      Get the first item in the sequence.
      Specified by:
        `head` in interface `GroundedValue`
      Specified by:
        `head` in interface `Sequence`
      Returns:
        the first item in the sequence if there is one, or null if the sequence is empty

    - #### getLength

          public int getLength()

      Get the length of the sequence
      Specified by:
        `getLength` in interface `GroundedValue`
      Returns:
        the number of items in the sequence. Note that for a single item, including a map or array, the result is always 1 (one).

    - #### itemAt

          public T itemAt​(int n)

      Get the n'th item in the sequence (starting from 0). This is defined for all SequenceValues, but its real benefits come for a SequenceValue stored extensionally (or for a MemoClosure, once all the values have been read)
      Specified by:
        `itemAt` in interface `GroundedValue`
      Parameters:
        `n` - the index of the required item, with 0 representing the first item in the sequence
      Returns:
        the n'th item if it exists, or null otherwise

    - #### subsequence

          public GroundedValue subsequence​(int start,
                                           int length)

      Get a subsequence of the value
      Specified by:
        `subsequence` in interface `GroundedValue`
      Parameters:
        `start` - the index of the first item to be included in the result, counting from zero. A negative value is taken as zero. If the value is beyond the end of the sequence, an empty sequence is returned
        `length` - the number of items to be included in the result. Specify Integer.MAX_VALUE to get the subsequence up to the end of the base sequence. If the value is negative, an empty sequence is returned. If the value goes off the end of the sequence, the result returns items up to the end of the sequence
      Returns:
        the required subsequence. If min is

    - #### iterate

          public ConstrainedIterator<T> iterate()

      Return an iterator over this value.
      Specified by:
        `iterate` in interface `GroundedValue`
      Specified by:
        `iterate` in interface `Sequence`
      Returns:
        an iterator (meaning a Saxon `SequenceIterator` rather than a Java `Iterator`) over all the items in this Sequence.

    - #### effectiveBooleanValue

          public boolean effectiveBooleanValue()
                                        throws XPathException

      Get the effective boolean value
      Specified by:
        `effectiveBooleanValue` in interface `GroundedValue`
      Returns:
        the effective boolean value
      Throws:
        `XPathException` - if the sequence has no effective boolean value (for example a sequence of two integers)

    - #### toString

          public java.lang.String toString()

      Returns a string representation of the object (used only for diagnostics).
      Overrides:
        `toString` in class `java.lang.Object`
      Returns:
        a string representation of the object.

    - #### reduce

          public GroundedValue reduce()

      Reduce the sequence to its simplest form. If the value is an empty sequence, the result will be EmptySequence.getInstance(). If the value is a single atomic value, the result will be an instance of AtomicValue. If the value is a single item of any other kind, the result will be an instance of SingletonItem. Otherwise, the result will typically be unchanged.
      Specified by:
        `reduce` in interface `GroundedValue`
      Returns:
        the simplified sequence