Does TreeSet allow null key?

Null values in TreeSet − The TreeSet object doesn't allows null values but, If you try to add them, a runtime exception will be generated at.

tutorialspoint.com - Can we add null elements to a Set in Java? - Tutorialspoint

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A Set is a collection that cannot contain duplicate elements. It models the mathematical set abstraction. It does not allow duplicate elements and allow one null value at most. Set also adds a stronger contract on the behavior of the equals and hashCode operations, allowing Set instances to be compared meaningfully even if their implementation types differ.

There are three classes implementing this interface −

HashSet − Set implementation based on hash table.

− Set implementation based on hash table. LinkedHashSet − HashSet implementation based on linked list. − HashSet implementation based on linked list. TreeSet − Set implementation based on trees.

Null values in a Set object

As per the definition a set object does not allow duplicate values but it does allow at most one null value. Null values in HashSet − The HashSet object allows null values but, you can add only one null element to it. Though you add more null values if you try to print its contents, it displays only one null.

Example

import java.util.HashSet; import java.util.Set; public class HashSetExample { public static void main(String args[]) { Set hashSet = new HashSet(); //Populating the HashSet hashSet.add(1124); hashSet.add(3654); hashSet.add(7854); hashSet.add(9945); System.out.println(hashSet); //Adding null elements hashSet.add(null); hashSet.add(null); hashSet.add(null); System.out.println(hashSet); } }

Output

[1124, 3654, 9945, 7854] [null, 1124, 3654, 9945, 7854]

Null values in LinkedHashSet: Just like the HashSet object, this also allows null values but, you can add only one null element to it. Though you add more null values if you try to print its contents, it displays only one null.

Example

import java.util.LinkedHashSet; import java.util.Set; public class LinkedHashSetExample { public static void main(String args[]) { Set linkedHashSet = new LinkedHashSet(); //Populating the HashSet linkedHashSet.add(1124); linkedHashSet.add(3654); linkedHashSet.add(7854); linkedHashSet.add(9945); System.out.println(linkedHashSet); //Adding null elements linkedHashSet.add(null); linkedHashSet.add(null); linkedHashSet.add(null); System.out.println(linkedHashSet); } }

Output

[1124, 3654, 9945, 7854] [null, 1124, 3654, 9945, 7854]

Null values in TreeSet − The TreeSet object doesn’t allows null values but, If you try to add them, a runtime exception will be generated at.

Example

import java.util.Set; import java.util.TreeSet; public class TreeSetExample { public static void main(String args[]) { Set treeSet = new TreeSet(); //Populating the HashSet treeSet.add(1124); treeSet.add(3654); treeSet.add(7854); treeSet.add(9945); System.out.println(treeSet); //Adding null elements treeSet.add(null); treeSet.add(null); treeSet.add(null); System.out.println(treeSet); } }

Run time exception

[1124, 3654, 7854, 9945] Exception in thread "main" java.lang.NullPointerException at java.util.TreeMap.put(Unknown Source) at java.util.TreeSet.add(Unknown Source) at MyPackage.TreeSetExample.main(TreeSetExample.java:16)

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What is better than ArrayList?

An array is faster and that is because ArrayList uses a fixed amount of array. However when you add an element to the ArrayList and it overflows. It creates a new Array and copies every element from the old one to the new one. List over arrays.

edureka.co - Which is faster amongst an Array and an ArrayList | Edureka Community

Array Over List:

The capacity of an Array is fixed.

Whereas ArrayList can increase and decrease size dynamically.

An Array is a collection of similar items.

Whereas ArrayList can hold item of different types.

List over arrays.

If you do not exceed the capacity it is going to be as fast as an array.

Better handling and much easier way of doing things.

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