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## net.sf.saxon.expr.sort
Class EmptyGreatestComparer

    java.lang.Object
      net.sf.saxon.expr.sort.EmptyGreatestComparer

All Implemented Interfaces:
  Serializable, AtomicComparer

    public class EmptyGreatestComparer

extends Object
implements AtomicComparer, Serializable

A Comparer that modifies a base comparer by sorting empty key values and NaN values last (greatest), as opposed to the default which sorts them first.

Author:
  Michael H. Kay
See Also:
  Serialized Form

Constructor Summary
`EmptyGreatestComparer(AtomicComparer baseComparer)` Create an EmptyGreatestComparer

Method Summary
`int` | `compareAtomicValues(AtomicValue a, AtomicValue b)` Compare two AtomicValue objects according to the rules for their data type.
`boolean` | `comparesEqual(AtomicValue a, AtomicValue b)` Compare two AtomicValue objects for equality according to the rules for their data type.
`AtomicComparer` | `getBaseComparer()` Get the underlying comparer (which compares empty least)
`StringCollator` | `getCollator()` Get the collation used by this AtomicComparer if any
`ComparisonKey` | `getComparisonKey(AtomicValue a)` Get a comparison key for an object.
`AtomicComparer` | `provideContext(XPathContext context)` Supply the dynamic context in case this is needed for the comparison

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### EmptyGreatestComparer

    public EmptyGreatestComparer(AtomicComparer baseComparer)

  Create an EmptyGreatestComparer

  Parameters:
    `baseComparer` - the comparer used to compare non-empty values (which typically sorts empty as least)

Method Detail

### getBaseComparer

    public AtomicComparer getBaseComparer()

  Get the underlying comparer (which compares empty least)

  Returns:
    the base comparer

### getCollator

    public StringCollator getCollator()

  Description copied from interface: `AtomicComparer`
  Get the collation used by this AtomicComparer if any

  Specified by:
    `getCollator` in interface `AtomicComparer`
  Returns:
    the collation used for comparing strings, or null if not applicable

### provideContext

    public AtomicComparer provideContext(XPathContext context)

  Supply the dynamic context in case this is needed for the comparison

  Specified by:
    `provideContext` in interface `AtomicComparer`
  Parameters:
    `context` - the dynamic evaluation context
  Returns:
    either the original AtomicComparer, or a new AtomicComparer in which the context is known. The original AtomicComparer is not modified

### compareAtomicValues

    public int compareAtomicValues(AtomicValue a,
                                   AtomicValue b)
                            throws NoDynamicContextException

  Compare two AtomicValue objects according to the rules for their data type. UntypedAtomic values are compared as if they were strings; if different semantics are wanted, the conversion must be done by the caller.

  Specified by:
    `compareAtomicValues` in interface `AtomicComparer`
  Parameters:
    `a` - the first object to be compared. It is intended that this should normally be an instance of AtomicValue, though this restriction is not enforced. If it is a StringValue, the collator is used to compare the values, otherwise the value must implement the java.util.Comparable interface.
    `b` - the second object to be compared. This must be comparable with the first object: for example, if one is a string, they must both be strings.
  Returns:
    <0 if a 0 if a>b
  Throws:
    `ClassCastException` - if the objects are not comparable
    `NoDynamicContextException` - if this comparer required access to dynamic context information, notably the implicit timezone, and this information is not available. In general this happens if a context-dependent comparison is attempted at compile-time, and it signals the compiler to generate code that tries again at run-time.

### comparesEqual

    public boolean comparesEqual(AtomicValue a,
                                 AtomicValue b)
                          throws NoDynamicContextException

  Compare two AtomicValue objects for equality according to the rules for their data type. UntypedAtomic values are compared by converting to the type of the other operand.

  Specified by:
    `comparesEqual` in interface `AtomicComparer`
  Parameters:
    `a` - the first object to be compared.
    `b` - the second object to be compared.
  Returns:
    true if the values are equal, false if not
  Throws:
    `ClassCastException` - if the objects are not comparable
    `NoDynamicContextException`

### getComparisonKey

    public ComparisonKey getComparisonKey(AtomicValue a)
                                   throws NoDynamicContextException

  Get a comparison key for an object. This must satisfy the rule that if two objects are equal according to the XPath eq operator, then their comparison keys are equal according to the Java equals() method, and vice versa. There is no requirement that the comparison keys should reflect the ordering of the underlying objects.

  Specified by:
    `getComparisonKey` in interface `AtomicComparer`
  Throws:
    `NoDynamicContextException`

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Copyright (c) 2004-2010 Saxonica Limited. All rights reserved.