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

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

All Implemented Interfaces:
  Serializable, AtomicComparer

    public class DescendingComparer

extends Object
implements AtomicComparer, Serializable

A Comparer used for comparing descending keys. This simply returns the inverse of the result delivered by the base comparer.

See Also:
  Serialized Form

Constructor Summary
`DescendingComparer(AtomicComparer base)`

Method Summary
`int` | `compareAtomicValues(AtomicValue a, AtomicValue b)` Compare two objects.
`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 (ascending) comparer
`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

### DescendingComparer

    public DescendingComparer(AtomicComparer base)

Method Detail

### getBaseComparer

    public AtomicComparer getBaseComparer()

  Get the underlying (ascending) comparer

  Returns:
    the underlying (ascending) 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
  Throws:
    `NoDynamicContextException` - if the context is an "early evaluation" (compile-time) context

### compareAtomicValues

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

  Compare two objects.

  Specified by:
    `compareAtomicValues` in interface `AtomicComparer`
  Parameters:
    `a` - the first object to be compared. It is intended that this should 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 of the wrong type for this Comparer
    `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. It is intended that this should 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 equals() method.
    `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:
    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) 2004-2011 Saxonica Limited. All rights reserved.