The pollFirst() method of TreeSet in Java is used to retrieves and removes the first (lowest) element, or returns null if this TreeSet is empty.
Syntax:
E pollFirst()
Where, E is the type of elements maintained by this TreeSet container.
Parameters: This function does not accepts any parameter.
Return Type:It returns the first element of the Treeset if TreeSet is not empty else returns null.
Below programs illustrate the pollFirst() method in Java:
Example 1
// A Java program to demonstrate // pollFirst() method of TreeSet import java.util.TreeSet;
public class GFG {
public static void main(String[] args)
{
// creating TreeSet
TreeSet<Integer> ts = new TreeSet<Integer>();
// adding element to the TreeSet
ts.add( 0 );
ts.add( 1 );
ts.add( 2 );
ts.add( 3 );
ts.add( 4 );
ts.add( 5 );
ts.add( 6 );
// before removing element
System.out.println( "Before removing " +
"element from TreeSet: " + ts);
// first element is removed
System.out.println( "First lowest element " +
"removed is : " + ts.pollFirst());
System.out.println( "After removing element" +
" from TreeSet: " + ts);
}
} |
Output:
Before removing element from TreeSet: [0, 1, 2, 3, 4, 5, 6] First lowest element removed is : 0 After removing element from TreeSet: [1, 2, 3, 4, 5, 6]
Example 2
// A Java program to demonstrate // pollFirst() method of TreeSet import java.util.TreeSet;
public class GFG {
public static void main(String[] args)
{
// creating TreeSet
TreeSet<String> ts = new TreeSet<String>();
// removing element when TreeSet
// is empty
System.out.println( "pollFirst() method when " +
"TreeSet is empty : " + ts.pollFirst());
// adding element to the TreeSet
ts.add( "Geeks" );
ts.add( "for" );
ts.add( "Geek" );
// before removing element
System.out.println( "Before removing element from" +
" TreeSet: " + ts);
// first element is removed
System.out.println( "First lowest element removed is : "
+ ts.pollFirst());
System.out.println( "After removing element " +
"from TreeSet: " + ts);
}
} |
Output:
pollFirst() method when TreeSet is empty : null Before removing element from TreeSet: [Geek, Geeks, for] First lowest element removed is : Geek After removing element from TreeSet: [Geeks, for]
If TreeSet is empty, then this method returns null.
Example 3
// A Java program to demonstrate // pollFirst() method of TreeSet import java.util.TreeSet;
public class GFG {
public static void main(String[] args)
{
// creating TreeSet
TreeSet<Integer> ts = new TreeSet<Integer>();
// before removing element
System.out.println( "Before removing " +
"element from TreeSet: " + ts);
// first element is removed
System.out.println( "First lowest element " +
"removed is : " + ts.pollFirst());
System.out.println( "After removing element" +
" from TreeSet: " + ts);
}
} |
Output:
Before removing element from TreeSet: [] First lowest element removed is : null After removing element from TreeSet: []