What is the TreeMap.lowerKey() method in Java?

In this shot we will learn how to use the TreeMap.lowerKey() method in Java.

Introduction

The TreeMap.lowerKey() method is present in the TreeMap class inside the java.util package.

The TreeMap.lowerKey() method is used to obtain the greatest key lesser than the given key in the parameter present in the map currently. If no such key is present in the TreeMap, it returns null.

Syntax

The syntax of the TreeMap.lowerKey() method is given below:

K lowerKey(K Key);

Parameter

The TreeMap.lowerKey() accepts one parameter:

Key: The key for which we need to determine the greater key from the TreeMap.

Return

The TreeMap.lowerKey() method returns:

Key: It returns the greatest key lesser than the given key in the parameter present in the map currently.

Code

Let’s have a look at the code.

import java.util.*;
class Main 
{
  public static void main(String[] args)
  {
    TreeMap<Integer, String> t1 = new TreeMap<Integer, String>();
    t1.put(1, "Let's");
    t1.put(5, "see");
    t1.put(2, "TreeMap class");
    t1.put(27, "methods");
    t1.put(9, "in java.");
    System.out.println("The greatest key lesser than the given " + 
      "key present in the map is: " + t1.lowerKey(4));
    TreeMap<String, Integer> t2 = new TreeMap<String, Integer>();
    t2.put("apple", 98);
    t2.put("banana", 5);
    t2.put("carrot", 2);
    t2.put("dog", 27);
    t2.put("elephant", 9);
    System.out.println("The greatest key lesser than the given " + 
      "key present in the map is: " + t2.lowerKey("cat"));
  }
}

Explanation

In line 1, we import the required package.
In line 2, we make a Main class.
In line 4, we make a main() function.
In line 6, we declare a TreeMap consisting of keys of type Integer and values of type String.
From lines 8 to 12, we insert values in the TreeMap by using the TreeMap.put() method.
In line 14, we use the TreeMap.lowerKey() method and display the greatest key that is lesser than the given key from the map currently with a message.
From lines 17 to 26, we define another TreeMap object such that the keys are of type String and the values are of type Integer. Now, we can see in the output that the greatest key that is lesser than the $key = cat$ is displayed. It means that for string based keys, the function returns the key based on alphabetical order.

So, this is the way to use the TreeMap.lowerKey() method in Java.

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