How to Loop Over TreeSet in Java?

TreeSet is one of the most important implementations of the SortedSet interface in Java that uses a Tree for storage. The ordering of the elements is maintained by a set using their natural ordering whether or not an explicit comparator is provided. This must be consistent with equals if it is to correctly implement the Set interface. 

Now the task is to explore out how many ways are there in order to loop over TreeSet. As we all know TreeSet provides an implementation of the SortedSet Interface and SortedSet extends Set Interface. It behaves like a simple set with the exception that it stores elements in a sorted format.

Following are some traits associated with TreeSet are as follows:

• TreeSet uses a Tree data structure for storage.
• Objects are stored in sorted, ascending order. But we can iterate in descending order using the method TreeSet.descendingIterator().
• Access and retrieval times are very fast which make TreeSet an excellent choice for the storage of large volume of data in a sorted format.
• TreeSet doesn’t use hashCode() and equals() methods to compare it’s elements. It uses compare() (or compareTo()) method to determine the equality of two elements.

Methods:

Below we  have listed various ways to iterate over the TreeSet in java which we are going to discuss further and will provide a clean java program for each of the following methods as follows:

1. Using Enhanced For loop
2. Using Iterator
3. Using streams (from Java8 onwards)

Method 1: Using Enhanced For loop

Enhanced For loop can be used to loop over the TreeSet in the below manner.

Syntax:

for (Integer value : ts)
{
    System.out.print(value);
}

Example

Java

// Java Program to Loop over TreeSet
// Using Enhanced For loop

// Importing required classes
import java.util.Iterator;
import java.util.TreeSet;

// Main class
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty TreeSet by
        // declaring object of TreeSet class of Integer type
        TreeSet<Integer> ts = new TreeSet<Integer>();

        // Adding elements to above TreeSet object
        ts.add(10);
        ts.add(61);
        ts.add(87);
        ts.add(39);

        // Display message for better readability
        System.out.print("TreeSet: ");

        // Looping over the TreeSet values
        for (Integer value : ts)

            // Print the values
            System.out.print(value + ", ");

        System.out.println();
    }
}

Output

TreeSet: 10, 39, 61, 87, 

Method 2: Using Iterator

Iterator can be created over the TreeSet objects. Hence this iterator can be used to traverse or loop through the TreeSet.

Syntax:

Iterator iterator = ts.iterator();

while (iterator.hasNext())
{
System.out.print(iterator.next());
}

Example

Java

// Java program to loop over TreeSet
// Using Iterator

// Importing required classes
import java.util.Iterator;
import java.util.TreeSet;

// Main class
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty TreeSet by
        // declaring an object of TreeSet class
        TreeSet<Integer> ts = new TreeSet<Integer>();

        // Adding elements to above object of TreeSet class
        // Using add() method
        ts.add(10);
        ts.add(61);
        ts.add(87);
        ts.add(39);

        // Create an Iterator over the TreeSet
        Iterator<Integer> iterator = ts.iterator();

        // Display message for better readability
        System.out.print("TreeSet: ");

        // Looping over the TreeSet values
        while (iterator.hasNext())

            // Print all the values inside TreeSet object
            System.out.print(iterator.next() + ", ");

        System.out.println();
    }
}

Output

TreeSet: 10, 39, 61, 87, 

Method 3: Using Java 8 forEach / stream: 

Java 8 forEach / stream can be used to loop over the TreeSet in the below manner.

Syntax:

Tree_Set.forEach(iterator -> System.out.print(i + " "));
// Using forEach 

Tree_Set.stream().map(iterator -> String.valueOf(i)).collect(Collectors.joining(", "))
// Using stream  

Example

Java

// Java program to loop over TreeSet
// Using For-each and Stream in Java8

// Importing required classes
import java.util.Arrays;
import java.util.Iterator;
import java.util.TreeSet;
import java.util.stream.Collectors;

// Main class
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty TreeSet if integer type
        TreeSet<Integer> ts = new TreeSet<Integer>();

        // Adding elements to the TreeSet
        // using addAll() method
        ts.addAll(Arrays.asList(10, 61, 87, 39));

        // Looping over the TreeSet & print values
        System.out.print("TreeSet without Comma: ");

        // Iterating through the TreeSet
        // using forEach
        ts.forEach(i -> System.out.print(i + " "));
        System.out.println();

        // Looping over the TreeSet & print values
        System.out.print("TreeSet with Comma: ");

        // Iterating through the TreeSet
        // Using stream concept introduced in Java8
        System.out.print(
            ts.stream()
                .map(i -> String.valueOf(i))
                .collect(Collectors.joining(", ")));
    }
}

Output

TreeSet without Comma: 10 39 61 87 
TreeSet with Comma: 10, 39, 61, 87