Java TreeMap keySet() Method

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Description

TheJava TreeMap keySet() method is used to return a Set view of the keys contained in this map. The set's iterator returns the keys in ascending order. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa.

Declaration

Following is the declaration for java.util.TreeMap.keySet() method.

public Set<K> keySet()

Parameters

NA

Return Value

The method call returns a set view of the keys contained in this map.

Exception

NA

Getting Set of Keys from a TreeMap of Integer, Integer Pairs Example

The following example shows the usage of Java TreeMap keySet() method to get the set view of the keys present in the map. We've created a TreeMap object of Integer,Integer pairs. Then few entries are added, and using keySet() we're printing a keys set in ascending order.

package com.tutorialspoint;

import java.util.Set;
import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating tree map 
      TreeMap<Integer, Integer> treemap = new TreeMap<>();

      // populating tree map
      treemap.put(2, 2);
      treemap.put(1, 1);
      treemap.put(3, 3);
      treemap.put(6, 6);
      treemap.put(5, 5);

      // creating set
      Set set = treemap.keySet();

      // getting set view         
      System.out.println("Checking values of the set");
      System.out.println("Value is: "+set);
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Checking values of the set
Value is: [1, 2, 3, 5, 6]

Getting Set of Keys from a TreeMap of Integer, String Pairs Example

The following example shows the usage of Java TreeMap keySet() method to get the set view of the keys present in the map. We've created a TreeMap object of Integer,String pairs. Then few entries are added, and using keySet() we're printing a keys set in ascending order.

package com.tutorialspoint;

import java.util.Set;
import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating tree map 
      TreeMap<Integer, String> treemap = new TreeMap<>();

      // populating tree map
      treemap.put(2, "two");
      treemap.put(1, "one");
      treemap.put(3, "three");
      treemap.put(6, "six");
      treemap.put(5, "five");

      // creating set
      Set set = treemap.keySet();

      // getting set view         
      System.out.println("Checking values of the set");
      System.out.println("Value is: "+set);
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Checking values of the set
Value is: [1, 2, 3, 5, 6]

Getting Set of Keys from a TreeMap of Integer, Object Pairs Example

The following example shows the usage of Java TreeMap keySet() method to get the set view of the keys present in the map. We've created a TreeMap object of Integer,Student pairs. Then few entries are added, and using keySet() we're printing a keys set in ascending order.

package com.tutorialspoint;

import java.util.Set;
import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating tree map 
      TreeMap<Integer, Student> treemap = new TreeMap<>();

      // populating tree map
      treemap.put(2, new Student(2, "Robert"));
      treemap.put(1, new Student(1, "Julie"));  
      treemap.put(3, new Student(3, "Adam"));
      treemap.put(6, new Student(6, "Julia"));
      treemap.put(5, new Student(5, "Tom"));

      // creating set
      Set set = treemap.keySet();

      // getting set view         
      System.out.println("Checking values of the set");
      System.out.println("Value is: "+set);
   }    
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   @Override
   public boolean equals(Object obj) {
      if(obj == null) return false;
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }
}

Output

Let us compile and run the above program, this will produce the following result.

Checking values of the set
Value is: [1, 2, 3, 5, 6]