TreeMap
sorts by the key natural ordering. The keys should implement Comparable
or be compatible with a Comparator
(if you passed one instance to constructor). In you case, Float
already implements Comparable
so you don't have to do anything special.
You can call keySet
to retrieve all the keys in ascending order.
A TreeMap is probably the most straightforward way of doing this. You use it exactly like a normal Map. i.e.
Map<Float,String> mySortedMap = new TreeMap<Float,MyObject>();
// Put some values in it
mySortedMap.put(1.0f,"One");
mySortedMap.put(0.0f,"Zero");
mySortedMap.put(3.0f,"Three");
// Iterate through it and it'll be in order!
for(Map.Entry<Float,String> entry : mySortedMap.entrySet()) {
System.out.println(entry.getValue());
} // outputs Zero One Three
It's worth taking a look at the API docs, http://download.oracle.com/javase/6/docs/api/java/util/TreeMap.html to see what else you can do with it.
Use either of the Map
implementations bundled with Java 6 and later that implement NavigableMap
(the successor to SortedMap
):
TreeMap
if running single-threaded, or if the map is to be read-only across threads after first being populated. ConcurrentSkipListMap
if manipulating the map across threads.NavigableMap
FYI, the SortedMap
interface was succeeded by the NavigableMap
interface.
You would only need to use SortedMap
if using 3rd-party implementations that have not yet declared their support of NavigableMap
. Of the maps bundled with Java, both of the implementations that implement SortedMap
also implement NavigableMap
.
s SortedMap the best answer? TreeMap?
As others mentioned, SortedMap
is an interface while TreeMap
is one of multiple implementations of that interface (and of the more recent NavigableMap
.
Having an interface allows you to write code that uses the map without breaking if you later decide to switch between implementations.
NavigableMap< Employee , Project > currentAssignments = new TreeSet<>() ;
currentAssignments.put( alice , writeAdCopyProject ) ;
currentAssignments.put( bob , setUpNewVendorsProject ) ;
This code still works if later change implementations. Perhaps you later need a map that supports concurrency for use across threads. Change that declaration to:
NavigableMap< Employee , Project > currentAssignments = new ConcurrentSkipListMap<>() ;
…and the rest of your code using that map continues to work.
There are ten implementations of Map
bundled with Java 11. And more implementations provided by 3rd parties such as Google Guava.
Here is a graphic table I made highlighting the various features of each. Notice that two of the bundled implementations keep the keys in sorted order by examining the key’s content. Also, EnumMap
keeps its keys in the order of the objects defined on that enum. Lastly, the LinkedHashMap
remembers original insertion order.
TreeMap, which is an implementation of the SortedMap interface, would work.
How do I use it ?
Map<Float, MyObject> map = new TreeMap<Float, MyObject>();
You can use TreeMap which internally implements the SortedMap below is the example
Sorting by ascending ordering :
Map<Float, String> ascsortedMAP = new TreeMap<Float, String>();
ascsortedMAP.put(8f, "name8");
ascsortedMAP.put(5f, "name5");
ascsortedMAP.put(15f, "name15");
ascsortedMAP.put(35f, "name35");
ascsortedMAP.put(44f, "name44");
ascsortedMAP.put(7f, "name7");
ascsortedMAP.put(6f, "name6");
for (Entry<Float, String> mapData : ascsortedMAP.entrySet()) {
System.out.println("Key : " + mapData.getKey() + "Value : " + mapData.getValue());
}
Sorting by descending ordering :
If you always want this create the map to use descending order in general, if you only need it once create a TreeMap with descending order and put all the data from the original map in.
// Create the map and provide the comparator as a argument
Map<Float, String> dscsortedMAP = new TreeMap<Float, String>(new Comparator<Float>() {
@Override
public int compare(Float o1, Float o2) {
return o2.compareTo(o1);
}
});
dscsortedMAP.putAll(ascsortedMAP);
for further information about SortedMAP read http://examples.javacodegeeks.com/core-java/util/treemap/java-sorted-map-example/
Source: Stackoverflow.com