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Java.util.TreeMap.pollFirstEntry() and pollLastEntry() in Java

  • Difficulty Level : Basic
  • Last Updated : 20 Nov, 2017

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}

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