How to Add Elements to Java TreeSet Example

This example shows how to add elements to TreeSet in Java (including custom class objects). This example also shows how to add elements using the add method and addAll method.

How to add elements to TreeSet in Java using the add method?

The add method of the TreeSet class adds the specified element to the TreeSet object.

1	public boolean add(E e)

The add method adds the specified element to the TreeSet object if it was not already present in the set and returns true. If the element was already present in the set, the set remains unchanged and it returns false.

import java.util.TreeSet;

public class TreeSetAddElementsExample {

public static void main(String[] args) {

TreeSet<Integer> tSetOddNumbers = new TreeSet<Integer>();

* Use the TreeSet add method to add elements

* to TreeSet object

//this will add element "1" to the TreeSet and return true

System.out.println( tSetOddNumbers.add(1) );

//this will return false as element "1" already exists

System.out.println( tSetOddNumbers.add(1) );

System.out.println("TreeSet contains: " + tSetOddNumbers);

}

Output

true

false

TreeSet contains: [1]

How to add custom class objects to the TreeSet?

Let’s create a Book custom class and try to add its objects to a TreeSet using the add method.

import java.util.TreeSet;

class Book{

private int bookId;

public Book(int bookId){

this.bookId = bookId;

}

public int getBookId(){

return this.bookId;

}

public String toString(){

return "Book -> " + getBookId();

}

public class TreeSetAddElementsExample {

public static void main(String[] args) {

TreeSet<Book> tSetBooks = new TreeSet<Book>();

tSetBooks.add( new Book(1) );

}

Output

Exception in thread "main" java.lang.ClassCastException: com.javacodeexamples.collections.treeset.Book cannot be cast to java.lang.Comparable

at java.util.TreeMap.compare(Unknown Source)

at java.util.TreeMap.put(Unknown Source)

at java.util.TreeSet.add(Unknown Source)

The code throws “java.lang.ClassCastException: com.javacodeexamples.collections.treeset.Book cannot be cast to java.lang.Comparable” when we run it.

The TreeSet class sorts the elements according to the natural ordering of the elements or by the specified Comparator object. In other words, all the elements of the TreeSet either must implement the Comparable interface or a custom comparator must be provided in the TreeSet constructor.

Let’s implement the Comparable interface in the Book class and try again.

import java.util.TreeSet;

* Implement the Comparable interface

class Book implements Comparable<Book>{

private int bookId;

public Book(int bookId){

this.bookId = bookId;

}

public int getBookId(){

return this.bookId;

}

public String toString(){

return "Book -> " + getBookId();

}

* define the compareTo method

public int compareTo(Book otherBook) {

return getBookId() - otherBook.getBookId();

}

public class TreeSetAddElementsExample {

public static void main(String[] args) {

TreeSet<Book> tSetBooks = new TreeSet<Book>();

//this will add Book with id 1 and return true

System.out.println( tSetBooks.add( new Book(1) ) );

//this will return false as book with id 1 already exists

System.out.println( tSetBooks.add( new Book(1) ) );

System.out.println("TreeSet contains: " + tSetBooks);

}

Output

true

false

TreeSet contains: [Book -> 1]

How to add all elements of other collections to the TreeSet using the addAll method?

The addAll method of the TreeSet class adds all the elements of the specified collection to this set.

1	public boolean addAll(Collection<? extends E> collection)

The addAll method returns true if the set is changed as a result of this method call. The addAll method can be used to convert a collection like HashSet to TreeSet as given below.