The following code empties the collection  TreeSet by calling a single statement which is the clear() of the TreeSet.

import java.util.HashSet; 
import java.util.TreeSet; 
import java.util.Set; 

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

 Set treeset = new TreeSet(); 

 treeset.add("Elm1"); 
 treeset.add("Elm2"); 
 treeset.add("Elm3"); 
 
 System.out.println("before: "+ treeset); 

 treeset.clear(); 
 
 System.out.println("after: " + treeset); 
 }
}

Result:

before: [E2, E1, E3]
after: []