TreeMap class implements Map similar to HashMap.
Some important points about TreeMap:
- TreeMap implements Map interface and extends HashMap class.
- TreeMap is implemented using Red black tree based NavigableMap.
- TreeMap is ordered collection and store its elements in natural ordering of keys.
- Key which you would like to put in TreeMap must implement Comaparable interface or you can use Comparator for custom sorting
Example:
package org.arpit.java2blog;
import java.util.TreeMap;
public class TreeMapMain {
public static void main(String args[])
{
// TreeMap with Country as key and capital as value
// TreeMap stores elements in natural ordering of keys.
TreeMap countryCapitalMap=new TreeMap();
countryCapitalMap.put("India","Delhi");
countryCapitalMap.put("Japan","Tokyo");
countryCapitalMap.put("France","Paris");
countryCapitalMap.put("Russia","Moscow");
System.out.println("-----------------------------");
// Iterating TreeMap Using keySet() and for each loop
System.out.println("Iterating TreeMap Using keySet() and for each loop");
for (String countryKey:countryCapitalMap.keySet()) {
System.out.println("Country:"+ countryKey +" and Capital:"+countryCapitalMap.get(countryKey));
}
System.out.println("-----------------------------");
}
}
When you run above program, you will get following output:
----------------------------- Iterating TreeMap Using keySet() and for each loop Country:France and Capital:Paris Country:India and Capital:Delhi Country:Japan and Capital:Tokyo Country:Russia and Capital:Moscow -----------------------------
As you can see, it is sorted in ascending order of Key(Country)
What if you want custom sorting rather than natural ordering:
If you want custom sorting , then you can using below TreeMap constructor. You can define your own comparator.
TreeMap countryCapitalMap=new TreeMap(Comparator comp);
Example: Create Country.java as below
package org.arpit.java2blog;
public class Country {
String name;
long population;
public Country(String name, long population) {
super();
this.name = name;
this.population = population;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public long getPopulation() {
return population;
}
public void setPopulation(long population) {
this.population = population;
}
}
Create TreeMapCompMain as below:
package org.arpit.java2blog;
import java.util.Comparator;
import java.util.Iterator;
import java.util.TreeMap;
public class TreeMapCompMain {
public static void main(String[] args)
{
Country india=new Country("India",1000);
Country japan=new Country("Japan",10000);
Country france=new Country("France",2000);
Country russia=new Country("Russia",20000);
Comparator comparator=new Comparator() {
@Override
public int compare(Country o1, Country o2) {
return o2.getName().compareTo(o1.getName());
}
};
System.out.println("Sorting TreeMap in reverse order of country name");
TreeMap countryCapitalMap=new TreeMap(comparator);
countryCapitalMap.put(india,"Delhi");
countryCapitalMap.put(japan,"Tokyo");
countryCapitalMap.put(france,"Paris");
countryCapitalMap.put(russia,"Moscow");
Iterator countryCapitalIter=countryCapitalMap.keySet().iterator();//put debug point at this line
while(countryCapitalIter.hasNext())
{
Country countryObj=countryCapitalIter.next();
String capital=countryCapitalMap.get(countryObj);
System.out.println(countryObj.getName()+"----"+capital);
}
}
}
When you run above program, you will get below output:
Sorting TreeMap in reverse order of country name Russia----Moscow Japan----Tokyo India----Delhi France----Paris
You can pass HashMap to constructor of TreeMap to sort it on Key
TreeMap countryCapitalMap=new TreeMap(Map hm);
By passing HashMap to constructor of TreeMap, you can sort TreeMap.
Example:
Create Country.java as below. It should implement Comparable interface
package org.arpit.java2blog;
public class Country implements Comparable {
String name;
long population;
public Country(String name, long population) {
super();
this.name = name;
this.population = population;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public long getPopulation() {
return population;
}
public void setPopulation(long population) {
this.population = population;
}
@Override
public int compareTo(Object o) {
Country country=(Country) o;
return this.getName().compareTo(country.getName());
}
}
Create TreeMapCompMain.java as below:
package org.arpit.java2blog;
import java.util.HashMap;
import java.util.Iterator;
import java.util.TreeMap;
public class TreeMapCompMain {
public static void main(String[] args)
{
Country india=new Country("India",1000);
Country japan=new Country("Japan",10000);
Country france=new Country("France",2000);
Country russia=new Country("Russia",20000);
HashMap countryCapitalMap=new HashMap();
countryCapitalMap.put(india,"Delhi");
countryCapitalMap.put(japan,"Tokyo");
countryCapitalMap.put(france,"Paris");
countryCapitalMap.put(russia,"Moscow");
System.out.println("Sorting HashMap by passing it to TreeMap constructor");
TreeMap sortedTreeMapCountryCapital=new TreeMap (countryCapitalMap);
Iterator countryCapitalIter=sortedTreeMapCountryCapital.keySet().iterator();//put debug point at this line
while(countryCapitalIter.hasNext())
{
Country countryObj=countryCapitalIter.next();
String capital=countryCapitalMap.get(countryObj);
System.out.println(countryObj.getName()+"----"+capital);
}
}
}
When you run program, you will get below output:
Sorting HashMap by passing it to TreeMap constructor France----Paris India----Delhi Japan----Tokyo Russia----Moscow
