Pre-requisite: TreeMap in Java
The lowerKey() method is used to return the greatest key strictly less than to given key, passed as the parameter. In simpler words, this method is used to find the next greatest element after the element passed as the parameter.
Syntax:
public K TreeMap.lowerKey(K key)
Parameters: This method takes a mandatory parameter key which is this is the key to be matched.
Return Value: This method returns the greatest key strictly less than to key, or null if there is no such key.
Exception: This method throws following exceptions:
- ClassCastException: When the specified key cannot be compared with the key available in Map.
- NullPointerException: When the specified key in map is null and it uses natural
ordering which means, comparator does not permit null keys.
Below are examples to illustrate the use of lowerKey() method:
Example 1:
// Java program to demonstrate lowerKey() method  import java.util.TreeMap;  public class FloorKeyDemo {    public static void main(String args[])    {          // create an empty TreeMap        TreeMap<Integer, String>            numMap = new TreeMap<Integer, String>();          // Insert the values        numMap.put(6, "Six");        numMap.put(1, "One");        numMap.put(5, "Five");        numMap.put(3, "Three");        numMap.put(8, "Eight");        numMap.put(10, "Ten");          // Print the Values of TreeMap        System.out.println("TreeMap: " + numMap.toString());          // Get the greatest key mapping of the Map          // As here 9 is not available it returns 8        // because 9 is strictly less than 11, present        System.out.print("Lower Key Entry of Element 9 is: ");        System.out.println(numMap.lowerKey(9));          // Though, 3 is available in the Map        // it returns 1 because this method returns        // strictly less than key according to the specified key        System.out.print("Lower Key Entry of Element 3 is: ");        System.out.println(numMap.lowerKey(3));    }} |
Output:
TreeMap: {1=One, 3=Three, 5=Five, 6=Six, 8=Eight, 10=Ten}
Lower Key Entry of Element 9 is: 8
Lower Key Entry of Element 3 is: 1
Example 2: To demonstrate NullPointerException
// Java program to demonstrate lowerKey() method  import java.util.TreeMap;  public class FloorKeyDemo {    public static void main(String args[])    {          // create an empty TreeMap        TreeMap<Integer, String>            numMap = new TreeMap<Integer, String>();          // Insert the values        numMap.put(6, "Six");        numMap.put(1, "One");        numMap.put(5, "Five");        numMap.put(3, "Three");        numMap.put(8, "Eight");        numMap.put(10, "Ten");          // Print the Values of TreeMap        System.out.println("TreeMap: " + numMap.toString());          try {            // Passing null as parameter to lowerKey()            // This will throw exception            System.out.println(numMap.lowerKey(null));        }        catch (Exception e) {            System.out.println("Exception: " + e);        }    }} |
Output:
TreeMap: {1=One, 3=Three, 5=Five, 6=Six, 8=Eight, 10=Ten}
Exception: java.lang.NullPointerException
