Package org.apache.lucene.search

# Class FieldComparator<T>

java.lang.Object
org.apache.lucene.search.FieldComparator<T>

Direct Known Subclasses:
  `DocComparator`, `FieldComparator.RelevanceComparator`, `FieldComparator.TermValComparator`, `NumericComparator`, `SimpleFieldComparator`, `TermOrdValComparator`

public abstract class FieldComparator<T> extends Object
Expert: a FieldComparator compares hits so as to determine their sort order when collecting the top results with `TopFieldCollector`. The concrete public FieldComparator classes here correspond to the SortField types.

The document IDs passed to these methods must only move forwards, since they are using doc values iterators to retrieve sort values.

This API is designed to achieve high performance sorting, by exposing a tight interaction with `FieldValueHitQueue` as it visits hits. Whenever a hit is competitive, it's enrolled into a virtual slot, which is an int ranging from 0 to numHits-1. Segment transitions are handled by creating a dedicated per-segment `LeafFieldComparator` which also needs to interact with the `FieldValueHitQueue` but can optimize based on the segment to collect.

The following functions need to be implemented

- `compare(int, int)` Compare a hit at 'slot a' with hit 'slot b'.
- `setTopValue(T)` This method is called by `TopFieldCollector` to notify the FieldComparator of the top most value, which is used by future calls to `LeafFieldComparator.compareTop(int)`.
- `getLeafComparator(org.apache.lucene.index.LeafReaderContext)` Invoked when the search is switching to the next segment. You may need to update internal state of the comparator, for example retrieving new values from DocValues.
- `value(int)` Return the sort value stored in the specified slot. This is only called at the end of the search, in order to populate `FieldDoc.fields` when returning the top results.

See Also:
  - `LeafFieldComparator`
WARNING: This API is experimental and might change in incompatible ways in the next release.

## Nested Class Summary

Nested Classes
Modifier and Type
Class
Description
`static final class`
`FieldComparator.RelevanceComparator`
Sorts by descending relevance.
`static class`
`FieldComparator.TermValComparator`
Sorts by field's natural Term sort order.

## Constructor Summary

Constructors
Constructor
Description
`FieldComparator()`

## Method Summary

Modifier and Type
Method
Description
`abstract int`
`compare(int slot1, int slot2)`
Compare hit at slot1 with hit at slot2.
`int`
`compareValues(T first, T second)`
Returns a negative integer if first is less than second, 0 if they are equal and a positive integer otherwise.
`void`
`disableSkipping()`
Informs the comparator that the skipping of documents should be disabled.
`abstract LeafFieldComparator`
`getLeafComparator(LeafReaderContext context)`
Get a per-segment `LeafFieldComparator` to collect the given `LeafReaderContext`.
`void`
`setSingleSort()`
Informs the comparator that sort is done on this single field.
`abstract void`
`setTopValue(T value)`
Record the top value, for future calls to `LeafFieldComparator.compareTop(int)`.
`abstract T`
`value(int slot)`
Return the actual value in the slot.

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

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

## Constructor Details

### FieldComparator

public FieldComparator()

## Method Details

### compare

public abstract int compare(int slot1, int slot2)
Compare hit at slot1 with hit at slot2.

Parameters:
  `slot1` - first slot to compare
  `slot2` - second slot to compare
Returns:
  any `N < 0` if slot2's value is sorted after slot1, any `N > 0` if the slot2's value is sorted before slot1 and `0` if they are equal

### setTopValue

public abstract void setTopValue(T value)
Record the top value, for future calls to `LeafFieldComparator.compareTop(int)`. This is only called for searches that use searchAfter (deep paging), and is called before any calls to `getLeafComparator(LeafReaderContext)`.

### value

public abstract T value(int slot)
Return the actual value in the slot.

Parameters:
  `slot` - the value
Returns:
  value in this slot

### getLeafComparator

public abstract LeafFieldComparator getLeafComparator(LeafReaderContext context) throws IOException
Get a per-segment `LeafFieldComparator` to collect the given `LeafReaderContext`. All docIDs supplied to this `LeafFieldComparator` are relative to the current reader (you must add docBase if you need to map it to a top-level docID).

Parameters:
  `context` - current reader context
Returns:
  the comparator to use for this segment
Throws:
  `IOException` - if there is a low-level IO error

### compareValues

public int compareValues(T first, T second)
Returns a negative integer if first is less than second, 0 if they are equal and a positive integer otherwise. Default impl to assume the type implements Comparable and invoke .compareTo; be sure to override this method if your FieldComparator's type isn't a Comparable or if your values may sometimes be null

### setSingleSort

public void setSingleSort()
Informs the comparator that sort is done on this single field. This is useful to enable some optimizations for skipping non-competitive documents.

### disableSkipping

public void disableSkipping()
Informs the comparator that the skipping of documents should be disabled. This function is called by TopFieldCollector in cases when the skipping functionality should not be applied or not necessary. An example could be when search sort is a part of the index sort, and can be already efficiently handled by TopFieldCollector, and doing extra work for skipping in the comparator is redundant.