Java.util.TreeMap also contains functions that support retrieval and deletion at both, high and low end of values and hence give a lot of flexibility in applicability and daily use. This function is poll() and has 2 variants discussed in this article.
1. pollFirstEntry() : It removes and retrieves a key-value pair with the least key value in the map and “null” is map is empty.
Syntax : public Map.Entry pollFirstEntry() Parameters: NA Return Value: Retrieves and removes the least key-value if map is filled else returns null. Exception: NA
// Java code to demonstrate the working// of pollFirstEntry()import java.io.*;import java.util.*;public class pollFirstEntry { public static void main(String[] args) { // Declaring the tree map of String and Integer TreeMap<String, Integer> tmp = new TreeMap<String, Integer>(); // Trying to retrieve and remove in empty map // returns null System.out.println ("The smallest key value pair is : " + tmp.pollFirstEntry()); // assigning the values in the tree map // using put() tmp.put("Geeks", 1); tmp.put("for", 4); tmp.put("geeks", 1); // Printing the initial map System.out.println ("The initial Map before deletion is : " + tmp); // Use of pollFirstEntry() // Removes the first entry and returns the least key // lexicographically smallest in case of String // prints Geeks-1 System.out.println ("The smallest key value pair is : " + tmp.pollFirstEntry()); // Printing the map after deletion System.out.println ("The resultant Map after deletion is : " + tmp); }} |
Output:
The smallest key value pair is : null
The initial Map before deletion is : {Geeks=1, for=4, geeks=1}
The smallest key value pair is : Geeks=1
The resultant Map after deletion is : {for=4, geeks=1}
2. pollLastEntry() : It removes and retrieves a key-value pair with the largest key value in the map and “null” is map is empty.
Syntax : public Map.Entry pollLastEntry() Parameters: NA Return Value: Retrieves and removes the largest key-value if map is filled else returns null. Exception: NA
// Java code to demonstrate the working// of pollLastEntry()import java.io.*;import java.util.*;public class pollLastEntry { public static void main(String[] args) { // Declaring the tree map of String and Integer TreeMap<String, Integer> tmp = new TreeMap<String, Integer>(); // Trying to retrieve and remove in empty map // returns null System.out.println ("The largest key value pair is : " + tmp.pollFirstEntry()); // assigning the values in the tree map // using put() tmp.put("Geeks", 1); tmp.put("for", 4); tmp.put("geeks", 1); // Printing the initial map System.out.println ("The initial Map before deletion is : " + tmp); // Use of pollLastEntry() // Removes the last(max) entry and returns the max key // lexicographically greatest in case of String // prints geeks-1 System.out.println ("The largest key value pair is : " + tmp.pollLastEntry()); // Printing the map after deletion System.out.println ("The resultant Map after deletion is : " + tmp); }} |
Output:
The largest key value pair is : null
The initial Map before deletion is : {Geeks=1, for=4, geeks=1}
The largest key value pair is : geeks=1
The resultant Map after deletion is : {Geeks=1, for=4}
Practical Application : There are many applications that can be thought using the concept of deque or priority queueing. One such example is shown in the code below.
// Java code to demonstrate the application// of pollLastEntry() and pollFirstEntry()import java.io.*;import java.util.*;public class pollAppli { public static void main(String[] args) { // Declaring the tree map of Integer and String TreeMap<Integer, String> que = new TreeMap<Integer, String>(); // assigning the values in que // using put() que.put(10, "astha"); que.put(4, "shambhavi"); que.put(7, "manjeet"); que.put(8, "nikhil"); // Defining the priority // takes highest value, if priority is high // else takes lowest value String prio = "high"; // Printing the initial queue System.out.println("The initial queue is : " + que); if (prio == "high") { System.out.println ("The largest valued person is : " + que.pollLastEntry()); System.out.println ("The resultant queue after deletion is : " + que); } else { System.out.println ("The lowest valued person is : " + que.pollFirstEntry()); System.out.println ("The resultant queue after deletion is : " + que); } }} |
Output:
The initial queue is : {4=shambhavi, 7=manjeet, 8=nikhil, 10=astha}
The largest valued person is : 10=astha
The resultant queue after deletion is : {4=shambhavi, 7=manjeet, 8=nikhil}
