问题
I have used LinkedHashMap because it is important the order in which keys entered in the map.
But now I want to get the value of key in the first place (the first entered entry) or the last.
Should there be a method like first() and last() or something like that?
Do I need to have an iterator to just get the first key entry? That is why I used LinkedHashMap!
Thanks!
回答1:
The semantics of LinkedHashMap are still those of a Map, rather than that of a LinkedList. It retains insertion order, yes, but that's an implementation detail, rather than an aspect of its interface.
The quickest way to get the "first" entry is still entrySet().iterator().next(). Getting the "last" entry is possible, but will entail iterating over the whole entry set by calling .next() until you reach the last. while (iterator.hasNext()) { lastElement = iterator.next() }
edit: However, if you're willing to go beyond the JavaSE API, Apache Commons Collections has its own LinkedMap implementation, which has methods like firstKey and lastKey, which do what you're looking for. The interface is considerably richer.
回答2:
Can you try doing something like (to get the last entry):
linkedHashMap.entrySet().toArray()[linkedHashMap.size() -1];
it's O(N) :)
回答3:
I know that I came too late but I would like to offer some alternatives, not something extraordinary but some cases that none mentioned here. In case that someone doesn't care so much for efficiency but he wants something with more simplicity(perhaps find the last entry value with one line of code), all this will get quite simplified with the arrival of Java 8 . I provide some useful scenarios.
For the sake of the completeness, I compare these alternatives with the solution of arrays that already mentioned in this post by others users. I sum up all the cases and i think they would be useful(when performance does matter or no) especially for new developers, always depends on the matter of each problem
Possible Alternatives
Usage of Array Method
I took it from the previous answer to to make the follow comparisons. This solution belongs @feresr.
public static String FindLasstEntryWithArrayMethod() {
return String.valueOf(linkedmap.entrySet().toArray()[linkedmap.size() - 1]);
}
Usage of ArrayList Method
Similar to the first solution with a little bit different performance
public static String FindLasstEntryWithArrayListMethod() {
List<Entry<Integer, String>> entryList = new ArrayList<Map.Entry<Integer, String>>(linkedmap.entrySet());
return entryList.get(entryList.size() - 1).getValue();
}
Reduce Method
This method will reduce the set of elements until getting the last element of stream. In addition, it will return only deterministic results
public static String FindLasstEntryWithReduceMethod() {
return linkedmap.entrySet().stream().reduce((first, second) -> second).orElse(null).getValue();
}
SkipFunction Method
This method will get the last element of the stream by simply skipping all the elements before it
public static String FindLasstEntryWithSkipFunctionMethod() {
final long count = linkedmap.entrySet().stream().count();
return linkedmap.entrySet().stream().skip(count - 1).findFirst().get().getValue();
}
Iterable Alternative
Iterables.getLast from Google Guava. It has some optimization for Lists and SortedSets too
public static String FindLasstEntryWithGuavaIterable() {
return Iterables.getLast(linkedmap.entrySet()).getValue();
}
Here is the full source code
import com.google.common.collect.Iterables;
import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
public class PerformanceTest {
private static long startTime;
private static long endTime;
private static LinkedHashMap<Integer, String> linkedmap;
public static void main(String[] args) {
linkedmap = new LinkedHashMap<Integer, String>();
linkedmap.put(12, "Chaitanya");
linkedmap.put(2, "Rahul");
linkedmap.put(7, "Singh");
linkedmap.put(49, "Ajeet");
linkedmap.put(76, "Anuj");
//call a useless action so that the caching occurs before the jobs starts.
linkedmap.entrySet().forEach(x -> {});
startTime = System.nanoTime();
FindLasstEntryWithArrayListMethod();
endTime = System.nanoTime();
System.out.println("FindLasstEntryWithArrayListMethod : " + "took " + new BigDecimal((endTime - startTime) / 1000000.000).setScale(3, RoundingMode.CEILING) + " milliseconds");
startTime = System.nanoTime();
FindLasstEntryWithArrayMethod();
endTime = System.nanoTime();
System.out.println("FindLasstEntryWithArrayMethod : " + "took " + new BigDecimal((endTime - startTime) / 1000000.000).setScale(3, RoundingMode.CEILING) + " milliseconds");
startTime = System.nanoTime();
FindLasstEntryWithReduceMethod();
endTime = System.nanoTime();
System.out.println("FindLasstEntryWithReduceMethod : " + "took " + new BigDecimal((endTime - startTime) / 1000000.000).setScale(3, RoundingMode.CEILING) + " milliseconds");
startTime = System.nanoTime();
FindLasstEntryWithSkipFunctionMethod();
endTime = System.nanoTime();
System.out.println("FindLasstEntryWithSkipFunctionMethod : " + "took " + new BigDecimal((endTime - startTime) / 1000000.000).setScale(3, RoundingMode.CEILING) + " milliseconds");
startTime = System.currentTimeMillis();
FindLasstEntryWithGuavaIterable();
endTime = System.currentTimeMillis();
System.out.println("FindLasstEntryWithGuavaIterable : " + "took " + (endTime - startTime) + " milliseconds");
}
public static String FindLasstEntryWithReduceMethod() {
return linkedmap.entrySet().stream().reduce((first, second) -> second).orElse(null).getValue();
}
public static String FindLasstEntryWithSkipFunctionMethod() {
final long count = linkedmap.entrySet().stream().count();
return linkedmap.entrySet().stream().skip(count - 1).findFirst().get().getValue();
}
public static String FindLasstEntryWithGuavaIterable() {
return Iterables.getLast(linkedmap.entrySet()).getValue();
}
public static String FindLasstEntryWithArrayListMethod() {
List<Entry<Integer, String>> entryList = new ArrayList<Map.Entry<Integer, String>>(linkedmap.entrySet());
return entryList.get(entryList.size() - 1).getValue();
}
public static String FindLasstEntryWithArrayMethod() {
return String.valueOf(linkedmap.entrySet().toArray()[linkedmap.size() - 1]);
}
}
Here is the output with performance of each method
FindLasstEntryWithArrayListMethod : took 0.162 milliseconds
FindLasstEntryWithArrayMethod : took 0.025 milliseconds
FindLasstEntryWithReduceMethod : took 2.776 milliseconds
FindLasstEntryWithSkipFunctionMethod : took 3.396 milliseconds
FindLasstEntryWithGuavaIterable : took 11 milliseconds
回答4:
LinkedHashMap current implementation (Java 8) keeps track of its tail. If performance is a concern and/or the map is large in size, you could access that field via reflection.
Because the implementation may change it is probably a good idea to have a fallback strategy too. You may want to log something if an exception is thrown so you know that the implementation has changed.
It could look like:
public static <K, V> Entry<K, V> getFirst(Map<K, V> map) {
if (map.isEmpty()) return null;
return map.entrySet().iterator().next();
}
public static <K, V> Entry<K, V> getLast(Map<K, V> map) {
try {
if (map instanceof LinkedHashMap) return getLastViaReflection(map);
} catch (Exception ignore) { }
return getLastByIterating(map);
}
private static <K, V> Entry<K, V> getLastByIterating(Map<K, V> map) {
Entry<K, V> last = null;
for (Entry<K, V> e : map.entrySet()) last = e;
return last;
}
private static <K, V> Entry<K, V> getLastViaReflection(Map<K, V> map) throws NoSuchFieldException, IllegalAccessException {
Field tail = map.getClass().getDeclaredField("tail");
tail.setAccessible(true);
return (Entry<K, V>) tail.get(map);
}
回答5:
One more way to get first and last entry of a LinkedHashMap is to use "toArray" method of Set interface.
But I think iterating over the entries in the entry set and getting the first and last entry is a better approach.
The usage of array methods leads to warning of the form " ...needs unchecked conversion to conform to ..." which cannot be fixed [but can be only be suppressed by using the annotation @SuppressWarnings("unchecked")].
Here is a small example to demonstrate the usage of "toArray" method:
public static void main(final String[] args) {
final Map<Integer,String> orderMap = new LinkedHashMap<Integer,String>();
orderMap.put(6, "Six");
orderMap.put(7, "Seven");
orderMap.put(3, "Three");
orderMap.put(100, "Hundered");
orderMap.put(10, "Ten");
final Set<Entry<Integer, String>> mapValues = orderMap.entrySet();
final int maplength = mapValues.size();
final Entry<Integer,String>[] test = new Entry[maplength];
mapValues.toArray(test);
System.out.print("First Key:"+test[0].getKey());
System.out.println(" First Value:"+test[0].getValue());
System.out.print("Last Key:"+test[maplength-1].getKey());
System.out.println(" Last Value:"+test[maplength-1].getValue());
}
// the output geneated is :
First Key:6 First Value:Six
Last Key:10 Last Value:Ten
回答6:
It's a bit dirty, but you can override the removeEldestEntry method of LinkedHashMap, which it might suit you to do as a private anonymous member:
private Splat eldest = null;
private LinkedHashMap<Integer, Splat> pastFutures = new LinkedHashMap<Integer, Splat>() {
@Override
protected boolean removeEldestEntry(Map.Entry<Integer, Splat> eldest) {
eldest = eldest.getValue();
return false;
}
};
So you will always be able to get the first entry at your eldest member. It will be updated every time you perform a put.
It should also be easy to override put and set youngest ...
@Override
public Splat put(Integer key, Splat value) {
youngest = value;
return super.put(key, value);
}
It all breaks down when you start removing entries though; haven't figured out a way to kludge that.
It's very annoying that you can't otherwise get access to head or tail in a sensible way ...
回答7:
Perhaps something like this :
LinkedHashMap<Integer, String> myMap;
public String getFirstKey() {
String out = null;
for (int key : myMap.keySet()) {
out = myMap.get(key);
break;
}
return out;
}
public String getLastKey() {
String out = null;
for (int key : myMap.keySet()) {
out = myMap.get(key);
}
return out;
}
回答8:
I would recommend using ConcurrentSkipListMap which has firstKey() and lastKey() methods
回答9:
Suggestion:
map.remove(map.keySet().iterator().next());
回答10:
Though linkedHashMap doesn't provide any method to get first, last or any specific object.
But its pretty trivial to get :
- Map orderMap = new LinkedHashMap();
Set al = orderMap.keySet();
now using iterator on al object ; you can get any object.
回答11:
Yea I came across the same problem, but luckily I only need the first element... - This is what I did for it.
private String getDefaultPlayerType()
{
String defaultPlayerType = "";
for(LinkedHashMap.Entry<String,Integer> entry : getLeagueByName(currentLeague).getStatisticsOrder().entrySet())
{
defaultPlayerType = entry.getKey();
break;
}
return defaultPlayerType;
}
If you need the last element as well - I'd look into how to reverse the order of your map - store it in a temp variable, access the first element in the reversed map(therefore it would be your last element), kill the temp variable.
Here's some good answers on how to reverse order a hashmap:
How to iterate hashmap in reverse order in Java
If you use help from the above link, please give them up-votes :) Hope this can help someone.
回答12:
right, you have to manually enumerate keyset till the end of the linkedlist, then retrieve the entry by key and return this entry.
来源：https://stackoverflow.com/questions/1936462/java-linkedhashmap-get-first-or-last-entry